2293

Committed 02 May 2024 08:09PM UTC coverage: 90.759% (+0.01%) from 90.747%

Build # 2293

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push

Evergreen

Committed by

web-flow

Commit Message

Fix a deadlock when accessing current user from inside an App listener (#7671)

App::switch_user() emitted changes without first releasing the lock on
m_user_mutex, leading to a deadlock if anyone inside the listener tried to
acquire the mutex. The rest of the places where we emitted changes were
correct.

The newly added wrapper catches this error when building with clang.

Run Details

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67 existing lines in 15 files now uncovered.

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74.11

/src/realm/sync/noinst/server/server.cpp

#include <realm/sync/noinst/server/server.hpp>

#include <realm/binary_data.hpp>
#include <realm/impl/simulated_failure.hpp>
#include <realm/object_id.hpp>
#include <realm/string_data.hpp>
#include <realm/sync/changeset.hpp>
#include <realm/sync/trigger.hpp>
#include <realm/sync/impl/clamped_hex_dump.hpp>
#include <realm/sync/impl/clock.hpp>
#include <realm/sync/network/http.hpp>
#include <realm/sync/network/network_ssl.hpp>
#include <realm/sync/network/websocket.hpp>
#include <realm/sync/noinst/client_history_impl.hpp>
#include <realm/sync/noinst/protocol_codec.hpp>
#include <realm/sync/noinst/server/access_control.hpp>
#include <realm/sync/noinst/server/server_dir.hpp>
#include <realm/sync/noinst/server/server_file_access_cache.hpp>
#include <realm/sync/noinst/server/server_impl_base.hpp>
#include <realm/sync/transform.hpp>
#include <realm/util/base64.hpp>
#include <realm/util/bind_ptr.hpp>
#include <realm/util/buffer_stream.hpp>
#include <realm/util/circular_buffer.hpp>
#include <realm/util/compression.hpp>
#include <realm/util/file.hpp>
#include <realm/util/json_parser.hpp>
#include <realm/util/load_file.hpp>
#include <realm/util/memory_stream.hpp>
#include <realm/util/optional.hpp>
#include <realm/util/platform_info.hpp>
#include <realm/util/random.hpp>
#include <realm/util/safe_int_ops.hpp>
#include <realm/util/scope_exit.hpp>
#include <realm/util/scratch_allocator.hpp>
#include <realm/util/thread.hpp>
#include <realm/util/thread_exec_guard.hpp>
#include <realm/util/value_reset_guard.hpp>
#include <realm/version.hpp>

#include <algorithm>
#include <atomic>
#include <cctype>
#include <chrono>
#include <cmath>
#include <condition_variable>
#include <cstdint>
#include <cstdio>
#include <cstring>
#include <functional>
#include <locale>
#include <map>
#include <memory>
#include <queue>
#include <sstream>
#include <stdexcept>
#include <thread>
#include <vector>

// NOTE: The protocol specification is in `/doc/protocol.md`


// FIXME: Verify that session identifier spoofing cannot be used to get access
// to sessions belonging to other network conections in any way.
// FIXME: Seems that server must close connection with zero sessions after a
// certain timeout.


using namespace realm;
using namespace realm::sync;
using namespace realm::util;

// clang-format off
using ServerHistory         = _impl::ServerHistory;
using ServerProtocol        = _impl::ServerProtocol;
using ServerFileAccessCache = _impl::ServerFileAccessCache;
using ServerImplBase = _impl::ServerImplBase;

using IntegratableChangeset = ServerHistory::IntegratableChangeset;
using IntegratableChangesetList = ServerHistory::IntegratableChangesetList;
using IntegratableChangesets = ServerHistory::IntegratableChangesets;
using IntegrationResult = ServerHistory::IntegrationResult;
using BootstrapError = ServerHistory::BootstrapError;
using ExtendedIntegrationError = ServerHistory::ExtendedIntegrationError;
using ClientType = ServerHistory::ClientType;
using FileIdentAllocSlot = ServerHistory::FileIdentAllocSlot;
using FileIdentAllocSlots = ServerHistory::FileIdentAllocSlots;

using UploadChangeset = ServerProtocol::UploadChangeset;
// clang-format on


using UploadChangesets = std::vector<UploadChangeset>;

using EventLoopMetricsHandler = network::Service::EventLoopMetricsHandler;


static_assert(std::numeric_limits<session_ident_type>::digits >= 63, "Bad session identifier type");
static_assert(std::numeric_limits<file_ident_type>::digits >= 63, "Bad file identifier type");
static_assert(std::numeric_limits<version_type>::digits >= 63, "Bad version type");
static_assert(std::numeric_limits<timestamp_type>::digits >= 63, "Bad timestamp type");


namespace {

enum class SchedStatus { done = 0, pending, in_progress };

// Only used by the Sync Server to support older pbs sync clients (prior to protocol v8)
constexpr std::string_view get_old_pbs_websocket_protocol_prefix() noexcept
{
    return "com.mongodb.realm-sync/";
}

std::string short_token_fmt(const std::string& str, size_t cutoff = 30)
{
    if (str.size() > cutoff) {
        return "..." + str.substr(str.size() - cutoff);
    }
    else {
        return str;
    }
}


class HttpListHeaderValueParser {
public:
    HttpListHeaderValueParser(std::string_view string) noexcept
        : m_string{string}
    {
    }
    bool next(std::string_view& elem) noexcept
    {
        while (m_pos < m_string.size()) {
            size_type i = m_pos;
            size_type j = m_string.find(',', i);
            if (j != std::string_view::npos) {
                m_pos = j + 1;
            }
            else {
                j = m_string.size();
                m_pos = j;
            }

            // Exclude leading and trailing white space
            while (i < j && is_http_lws(m_string[i]))
                ++i;
            while (j > i && is_http_lws(m_string[j - 1]))
                --j;

            if (i != j) {
                elem = m_string.substr(i, j - i);
                return true;
            }
        }
        return false;
    }

private:
    using size_type = std::string_view::size_type;
    const std::string_view m_string;
    size_type m_pos = 0;
    static bool is_http_lws(char ch) noexcept
    {
        return (ch == '\t' || ch == '\n' || ch == '\r' || ch == ' ');
    }
};


using SteadyClock = std::conditional<std::chrono::high_resolution_clock::is_steady,
                                     std::chrono::high_resolution_clock, std::chrono::steady_clock>::type;
using SteadyTimePoint = SteadyClock::time_point;

SteadyTimePoint steady_clock_now() noexcept
{
    return SteadyClock::now();
}

milliseconds_type steady_duration(SteadyTimePoint start_time, SteadyTimePoint end_time = steady_clock_now()) noexcept
{
    auto duration = end_time - start_time;
    auto millis_duration = std::chrono::duration_cast<std::chrono::milliseconds>(duration).count();
    return milliseconds_type(millis_duration);
}


bool determine_try_again(ProtocolError error_code) noexcept
{
    return (error_code == ProtocolError::connection_closed);
}


class ServerFile;
class ServerImpl;
class HTTPConnection;
class SyncConnection;
class Session;


using Formatter = util::ResettableExpandableBufferOutputStream;
using OutputBuffer = util::ResettableExpandableBufferOutputStream;

using ProtocolVersionRange = std::pair<int, int>;

class MiscBuffers {
public:
    Formatter formatter;
    OutputBuffer download_message;

    using ProtocolVersionRanges = std::vector<ProtocolVersionRange>;
    ProtocolVersionRanges protocol_version_ranges;

    std::vector<char> compress;

    MiscBuffers()
    {
        formatter.imbue(std::locale::classic());
        download_message.imbue(std::locale::classic());
    }
};


struct DownloadCache {
    std::unique_ptr<char[]> body;
    std::size_t uncompressed_body_size;
    std::size_t compressed_body_size;
    bool body_is_compressed;
    version_type end_version;
    DownloadCursor download_progress;
    std::uint_fast64_t downloadable_bytes;
    std::size_t num_changesets;
    std::size_t accum_original_size;
    std::size_t accum_compacted_size;
};


// An unblocked work unit is comprised of one Work object for each of the files
// that contribute work to the work unit, generally one reference file and a
// number of partial files.
class Work {
public:
    // In general, primary work is all forms of modifying work, including file
    // deletion.
    bool has_primary_work = false;

    // Only for reference files
    bool might_produce_new_sync_version = false;

    bool produced_new_realm_version = false;
    bool produced_new_sync_version = false;
    bool expired_reference_version = false;

    // True if, and only if changesets_from_downstream contains at least one
    // changeset.
    bool have_changesets_from_downstream = false;

    FileIdentAllocSlots file_ident_alloc_slots;
    std::vector<std::unique_ptr<char[]>> changeset_buffers;
    IntegratableChangesets changesets_from_downstream;

    VersionInfo version_info;

    // Result of integration of changesets from downstream clients
    IntegrationResult integration_result;

    void reset() noexcept
    {
        has_primary_work = false;

        might_produce_new_sync_version = false;

        produced_new_realm_version = false;
        produced_new_sync_version = false;
        expired_reference_version = false;
        have_changesets_from_downstream = false;

        file_ident_alloc_slots.clear();
        changeset_buffers.clear();
        changesets_from_downstream.clear();

        version_info = {};
        integration_result = {};
    }
};


class WorkerState {
public:
    FileIdentAllocSlots file_ident_alloc_slots;
    util::ScratchMemory scratch_memory;
    bool use_file_cache = true;
    std::unique_ptr<ServerHistory> reference_hist;
    DBRef reference_sg;
};


// ============================ SessionQueue ============================

class SessionQueue {
public:
    void push_back(Session*) noexcept;
    Session* pop_front() noexcept;
    void clear() noexcept;

private:
    Session* m_back = nullptr;
};


// ============================ FileIdentReceiver ============================

class FileIdentReceiver {
public:
    virtual void receive_file_ident(SaltedFileIdent) = 0;

protected:
    ~FileIdentReceiver() {}
};


// ============================ WorkerBox =============================

class WorkerBox {
public:
    using JobType = util::UniqueFunction<void(WorkerState&)>;
    void add_work(WorkerState& state, JobType job)
    {
        std::unique_lock<std::mutex> lock(m_mutex);
        if (m_jobs.size() >= m_queue_limit) {
            // Once we have many queued jobs, it is better to use this thread to run a new job
            // than to queue it.
            run_a_job(lock, state, job);
        }
        else {
            // Create worker threads on demand (if all existing threads are active):
            if (m_threads.size() < m_max_num_threads && m_active >= m_threads.size()) {
                m_threads.emplace_back([this]() {
                    WorkerState state;
                    state.use_file_cache = false;
                    JobType the_job;
                    std::unique_lock<std::mutex> lock(m_mutex);
                    for (;;) {
                        while (m_jobs.empty() && !m_finish_up)
                            m_changes.wait(lock);
                        if (m_finish_up)
                            break; // terminate thread
                        the_job = std::move(m_jobs.back());
                        m_jobs.pop_back();
                        run_a_job(lock, state, the_job);
                        m_changes.notify_all();
                    }
                });
            }

            // Submit the job for execution:
            m_jobs.emplace_back(std::move(job));
            m_changes.notify_all();
        }
    }

    // You should call wait_completion() before trying to destroy a WorkerBox to get proper
    // propagation of exceptions.
    void wait_completion(WorkerState& state)
    {
        std::unique_lock<std::mutex> lock(m_mutex);
        while (!m_jobs.empty() || m_active > 0) {
            if (!m_jobs.empty()) { // if possible, make this thread participate in running m_jobs
                JobType the_job = std::move(m_jobs.back());
                m_jobs.pop_back();
                run_a_job(lock, state, the_job);
            }
            else {
                m_changes.wait(lock);
            }
        }
        if (m_epr) {
            std::rethrow_exception(m_epr);
        }
    }

    WorkerBox(unsigned int num_threads)
    {
        m_queue_limit = num_threads * 10; // fudge factor for job size variation
        m_max_num_threads = num_threads;
    }

    ~WorkerBox()
    {
        {
            std::unique_lock<std::mutex> lock(m_mutex);
            m_finish_up = true;
            m_changes.notify_all();
        }
        for (auto& e : m_threads)
            e.join();
    }

private:
    std::mutex m_mutex;
    std::condition_variable m_changes;
    std::vector<std::thread> m_threads;
    std::vector<JobType> m_jobs;
    unsigned int m_active = 0;
    bool m_finish_up = false;
    unsigned int m_queue_limit = 0;
    unsigned int m_max_num_threads = 0;
    std::exception_ptr m_epr;

    void run_a_job(std::unique_lock<std::mutex>& lock, WorkerState& state, JobType& job)
    {
        ++m_active;
        lock.unlock();
        try {
            job(state);
            lock.lock();
        }
        catch (...) {
            lock.lock();
            if (!m_epr)
                m_epr = std::current_exception();
        }
        --m_active;
    }
};


// ============================ ServerFile ============================

class ServerFile : public util::RefCountBase {
public:
    util::PrefixLogger logger;

    // Logger to be used by the worker thread
    util::PrefixLogger wlogger;

    ServerFile(ServerImpl& server, ServerFileAccessCache& cache, const std::string& virt_path, std::string real_path,
               bool disable_sync_to_disk);
    ~ServerFile() noexcept;

    void initialize();
    void activate();

    ServerImpl& get_server() noexcept
    {
        return m_server;
    }

    const std::string& get_real_path() const noexcept
    {
        return m_file.realm_path;
    }

    const std::string& get_virt_path() const noexcept
    {
        return m_file.virt_path;
    }

    ServerFileAccessCache::File& access()
    {
        return m_file.access(); // Throws
    }

    ServerFileAccessCache::File& worker_access()
    {
        return m_worker_file.access(); // Throws
    }

    version_type get_realm_version() const noexcept
    {
        return m_version_info.realm_version;
    }

    version_type get_sync_version() const noexcept
    {
        return m_version_info.sync_version.version;
    }

    SaltedVersion get_salted_sync_version() const noexcept
    {
        return m_version_info.sync_version;
    }

    DownloadCache& get_download_cache() noexcept;

    void register_client_access(file_ident_type client_file_ident);

    using file_ident_request_type = std::int_fast64_t;

    // Initiate a request for a new client file identifier.
    //
    // Unless the request is cancelled, the identifier will be delivered to the
    // receiver by way of an invocation of
    // FileIdentReceiver::receive_file_ident().
    //
    // FileIdentReceiver::receive_file_ident() is guaranteed to not be called
    // until after request_file_ident() has returned (no callback reentrance).
    //
    // New client file identifiers will be delivered to receivers in the order
    // that they were requested.
    //
    // The returned value is a nonzero integer that can be used to cancel the
    // request before the file identifier is delivered using
    // cancel_file_ident_request().
    auto request_file_ident(FileIdentReceiver&, file_ident_type proxy_file, ClientType) -> file_ident_request_type;

    // Cancel the specified file identifier request.
    //
    // It is an error to call this function after the identifier has been
    // delivered.
    void cancel_file_ident_request(file_ident_request_type) noexcept;

    void add_unidentified_session(Session*);
    void identify_session(Session*, file_ident_type client_file_ident);

    void remove_unidentified_session(Session*) noexcept;
    void remove_identified_session(file_ident_type client_file_ident) noexcept;

    Session* get_identified_session(file_ident_type client_file_ident) noexcept;

    bool can_add_changesets_from_downstream() const noexcept;
    void add_changesets_from_downstream(file_ident_type client_file_ident, UploadCursor upload_progress,
                                        version_type locked_server_version, const UploadChangeset*,
                                        std::size_t num_changesets);

    // bootstrap_client_session calls the function of same name in server_history
    // but corrects the upload_progress with information from pending
    // integratable changesets. A situation can occur where a client terminates
    // a session and starts a new session and re-uploads changesets that are known
    // by the ServerFile object but not by the ServerHistory.
    BootstrapError bootstrap_client_session(SaltedFileIdent client_file_ident, DownloadCursor download_progress,
                                            SaltedVersion server_version, ClientType client_type,
                                            UploadCursor& upload_progress, version_type& locked_server_version,
                                            Logger&);

    // NOTE: This function is executed by the worker thread
    void worker_process_work_unit(WorkerState&);

    void recognize_external_change();

private:
    ServerImpl& m_server;
    ServerFileAccessCache::Slot m_file;

    // In general, `m_version_info` refers to the last snapshot of the Realm
    // file that is supposed to be visible to remote peers engaging in regular
    // Realm file synchronization.
    VersionInfo m_version_info;

    file_ident_request_type m_last_file_ident_request = 0;

    // The set of sessions whose client file identifier is not yet known, i.e.,
    // those for which an IDENT message has not yet been received,
    std::set<Session*> m_unidentified_sessions;

    // A map of the sessions whose client file identifier is known, i.e, those
    // for which an IDENT message has been received.
    std::map<file_ident_type, Session*> m_identified_sessions;

    // Used when a file used as partial view wants to allocate a client file
    // identifier from the reference Realm.
    file_ident_request_type m_file_ident_request = 0;

    struct FileIdentRequestInfo {
        FileIdentReceiver* receiver;
        file_ident_type proxy_file;
        ClientType client_type;
    };

    // When nonempty, it counts towards outstanding blocked work (see
    // `m_has_blocked_work`).
    std::map<file_ident_request_type, FileIdentRequestInfo> m_file_ident_requests;

    // Changesets received from the downstream clients, and waiting to be
    // integrated, as well as information about the clients progress in terms of
    // integrating changesets received from the server. When nonempty, it counts
    // towards outstanding blocked work (see `m_has_blocked_work`).
    //
    // At any given time, the set of changesets from a particular client-side
    // file may be comprised of changesets received via distinct sessions.
    //
    // See also `m_num_changesets_from_downstream`.
    IntegratableChangesets m_changesets_from_downstream;

    // Keeps track of the number of changesets in `m_changesets_from_downstream`.
    //
    // Its purpose is also to initialize
    // `Work::have_changesets_from_downstream`.
    std::size_t m_num_changesets_from_downstream = 0;

    // The total size, in bytes, of the changesets that were received from
    // clients, are targeting this file, and are currently part of the blocked
    // work unit.
    //
    // Together with `m_unblocked_changesets_from_downstream_byte_size`, its
    // purpose is to allow the server to keep track of the accumulated size of
    // changesets being processed, or waiting to be processed (metric
    // `upload.pending.bytes`) (see
    // ServerImpl::inc_byte_size_for_pending_downstream_changesets()).
    //
    // Its purpose is also to enable the "very poor man's" backpressure solution
    // (see can_add_changesets_from_downstream()).
    std::size_t m_blocked_changesets_from_downstream_byte_size = 0;

    // Same as `m_blocked_changesets_from_downstream_byte_size` but for the
    // currently unblocked work unit.
    std::size_t m_unblocked_changesets_from_downstream_byte_size = 0;

    // When nonempty, it counts towards outstanding blocked work (see
    // `m_has_blocked_work`).
    std::vector<std::string> m_permission_changes;

    // True iff this file, or any of its associated partial files (when
    // applicable), has a nonzero amount of outstanding work that is currently
    // held back from being passed to the worker thread because a previously
    // accumulated chunk of work related to this file is currently in progress.
    bool m_has_blocked_work = false;

    // A file, that is not a partial file, is considered *exposed to the worker
    // thread* from the point in time where it is submitted to the worker
    // (Worker::enqueue()) and up until the point in time where
    // group_postprocess_stage_1() starts to execute. A partial file is
    // considered *exposed to the worker thread* precisely when the associated
    // reference file is exposed to the worker thread, but only if it was in
    // `m_reference_file->m_work.partial_files` at the point in time where the
    // reference file was passed to the worker.
    //
    // While this file is exposed to the worker thread, all members of `m_work`
    // other than `changesets_from_downstream` may be accessed and modified by
    // the worker thread only.
    //
    // While this file is exposed to the worker thread,
    // `m_work.changesets_from_downstream` may be accessed by all threads, but
    // must not be modified by any thread. This special status of
    // `m_work.changesets_from_downstream` is required to allow
    // ServerFile::bootstrap_client_session() to read from it at any time.
    Work m_work;

    // For reference files, set to true when work is unblocked, and reset back
    // to false when the work finalization process completes
    // (group_postprocess_stage_3()). Always zero for partial files.
    bool m_has_work_in_progress = 0;

    // This one must only be accessed by the worker thread.
    //
    // More specifically, `m_worker_file.access()` must only be called by the
    // worker thread, and if it was ever called, it must be closed by the worker
    // thread before the ServerFile object is destroyed, if destruction happens
    // before the destruction of the server object itself.
    ServerFileAccessCache::Slot m_worker_file;

    std::vector<std::int_fast64_t> m_deleting_connections;

    DownloadCache m_download_cache;

    void on_changesets_from_downstream_added(std::size_t num_changesets, std::size_t num_bytes);
    void on_work_added();
    void group_unblock_work();
    void unblock_work();

    /// Resume history scanning in all sessions bound to this file. To be called
    /// after a successfull integration of a changeset.
    void resume_download() noexcept;

    // NOTE: These functions are executed by the worker thread
    void worker_allocate_file_identifiers();
    bool worker_integrate_changes_from_downstream(WorkerState&);
    ServerHistory& get_client_file_history(WorkerState& state, std::unique_ptr<ServerHistory>& hist_ptr,
                                           DBRef& sg_ptr);
    ServerHistory& get_reference_file_history(WorkerState& state);
    void group_postprocess_stage_1();
    void group_postprocess_stage_2();
    void group_postprocess_stage_3();
    void group_finalize_work_stage_1();
    void group_finalize_work_stage_2();
    void finalize_work_stage_1();
    void finalize_work_stage_2();
};


inline DownloadCache& ServerFile::get_download_cache() noexcept
{
    return m_download_cache;
}

inline void ServerFile::group_finalize_work_stage_1()
{
    finalize_work_stage_1(); // Throws
}

inline void ServerFile::group_finalize_work_stage_2()
{
    finalize_work_stage_2(); // Throws
}


// ============================ Worker ============================

// All write transaction on server-side Realm files performed on behalf of the
// server, must be performed by the worker thread, not the network event loop
// thread. This is to ensure that the network event loop thread never gets
// blocked waiting for the worker thread to end a long running write
// transaction.
//
// FIXME: Currently, the event loop thread does perform a number of write
// transactions, but only on subtier nodes of a star topology server cluster.
class Worker : public ServerHistory::Context {
public:
    std::shared_ptr<util::Logger> logger_ptr;
    util::Logger& logger;

    explicit Worker(ServerImpl&);

    ServerFileAccessCache& get_file_access_cache() noexcept;

    void enqueue(ServerFile*);

    // Overriding members of ServerHistory::Context
    std::mt19937_64& server_history_get_random() noexcept override final;

private:
    ServerImpl& m_server;
    std::mt19937_64 m_random;
    ServerFileAccessCache m_file_access_cache;

    util::Mutex m_mutex;
    util::CondVar m_cond; // Protected by `m_mutex`

    bool m_stop = false; // Protected by `m_mutex`

    util::CircularBuffer<ServerFile*> m_queue; // Protected by `m_mutex`

    WorkerState m_state;

    void run();
    void stop() noexcept;

    friend class util::ThreadExecGuardWithParent<Worker, ServerImpl>;
};


inline ServerFileAccessCache& Worker::get_file_access_cache() noexcept
{
    return m_file_access_cache;
}


// ============================ ServerImpl ============================

class ServerImpl : public ServerImplBase, public ServerHistory::Context {
public:
    std::uint_fast64_t errors_seen = 0;

    std::atomic<milliseconds_type> m_par_time;
    std::atomic<milliseconds_type> m_seq_time;

    util::Mutex last_client_accesses_mutex;

    const std::shared_ptr<util::Logger> logger_ptr;
    util::Logger& logger;

    network::Service& get_service() noexcept
    {
        return m_service;
    }

    const network::Service& get_service() const noexcept
    {
        return m_service;
    }

    std::mt19937_64& get_random() noexcept
    {
        return m_random;
    }

    const Server::Config& get_config() const noexcept
    {
        return m_config;
    }

    std::size_t get_max_upload_backlog() const noexcept
    {
        return m_max_upload_backlog;
    }

    const std::string& get_root_dir() const noexcept
    {
        return m_root_dir;
    }

    network::ssl::Context& get_ssl_context() noexcept
    {
        return *m_ssl_context;
    }

    const AccessControl& get_access_control() const noexcept
    {
        return m_access_control;
    }

    ProtocolVersionRange get_protocol_version_range() const noexcept
    {
        return m_protocol_version_range;
    }

    ServerProtocol& get_server_protocol() noexcept
    {
        return m_server_protocol;
    }

    compression::CompressMemoryArena& get_compress_memory_arena() noexcept
    {
        return m_compress_memory_arena;
    }

    MiscBuffers& get_misc_buffers() noexcept
    {
        return m_misc_buffers;
    }

    int_fast64_t get_current_server_session_ident() const noexcept
    {
        return m_current_server_session_ident;
    }

    util::ScratchMemory& get_scratch_memory() noexcept
    {
        return m_scratch_memory;
    }

    Worker& get_worker() noexcept
    {
        return m_worker;
    }

    void get_workunit_timers(milliseconds_type& parallel_section, milliseconds_type& sequential_section)
    {
        parallel_section = m_par_time;
        sequential_section = m_seq_time;
    }

    ServerImpl(const std::string& root_dir, util::Optional<sync::PKey>, Server::Config);
    ~ServerImpl() noexcept;

    void start();

    void start(std::string listen_address, std::string listen_port, bool reuse_address)
    {
        m_config.listen_address = listen_address;
        m_config.listen_port = listen_port;
        m_config.reuse_address = reuse_address;

        start(); // Throws
    }

    network::Endpoint listen_endpoint() const
    {
        return m_acceptor.local_endpoint();
    }

    void run();
    void stop() noexcept;

    void remove_http_connection(std::int_fast64_t conn_id) noexcept;

    void add_sync_connection(int_fast64_t connection_id, std::unique_ptr<SyncConnection>&& sync_conn);
    void remove_sync_connection(int_fast64_t connection_id);

    size_t get_number_of_http_connections()
    {
        return m_http_connections.size();
    }

    size_t get_number_of_sync_connections()
    {
        return m_sync_connections.size();
    }

    bool is_sync_stopped()
    {
        return m_sync_stopped;
    }

    const std::set<std::string>& get_realm_names() const noexcept
    {
        return m_realm_names;
    }

    // virt_path must be valid when get_or_create_file() is called.
    util::bind_ptr<ServerFile> get_or_create_file(const std::string& virt_path)
    {
        util::bind_ptr<ServerFile> file = get_file(virt_path);
        if (REALM_LIKELY(file))
            return file;

        _impl::VirtualPathComponents virt_path_components =
            _impl::parse_virtual_path(m_root_dir, virt_path); // Throws
        REALM_ASSERT(virt_path_components.is_valid);

        _impl::make_dirs(m_root_dir, virt_path); // Throws
        m_realm_names.insert(virt_path);         // Throws
        {
            bool disable_sync_to_disk = m_config.disable_sync_to_disk;
            file.reset(new ServerFile(*this, m_file_access_cache, virt_path, virt_path_components.real_realm_path,
                                      disable_sync_to_disk)); // Throws
        }

        file->initialize();
        m_files[virt_path] = file; // Throws
        file->activate();          // Throws
        return file;
    }

    std::unique_ptr<ServerHistory> make_history_for_path()
    {
        return std::make_unique<ServerHistory>(*this);
    }

    util::bind_ptr<ServerFile> get_file(const std::string& virt_path) noexcept
    {
        auto i = m_files.find(virt_path);
        if (REALM_LIKELY(i != m_files.end()))
            return i->second;
        return {};
    }

    // Returns the number of seconds since the Epoch of
    // std::chrono::system_clock.
    std::chrono::system_clock::time_point token_expiration_clock_now() const noexcept
    {
        if (REALM_UNLIKELY(m_config.token_expiration_clock))
            return m_config.token_expiration_clock->now();
        return std::chrono::system_clock::now();
    }

    void set_connection_reaper_timeout(milliseconds_type);

    void close_connections();
    bool map_virtual_to_real_path(const std::string& virt_path, std::string& real_path);

    void recognize_external_change(const std::string& virt_path);

    void stop_sync_and_wait_for_backup_completion(util::UniqueFunction<void(bool did_backup)> completion_handler,
                                                  milliseconds_type timeout);

    // Server global outputbuffers that can be reused.
    // The server is single threaded, so there are no
    // synchronization issues.
    // output_buffers_count is equal to the
    // maximum number of buffers needed at any point.
    static constexpr int output_buffers_count = 1;
    OutputBuffer output_buffers[output_buffers_count];

    bool is_load_balancing_allowed() const
    {
        return m_allow_load_balancing;
    }

    // inc_byte_size_for_pending_downstream_changesets() must be called by
    // ServerFile objects when changesets from downstream clients have been
    // received.
    //
    // dec_byte_size_for_pending_downstream_changesets() must be called by
    // ServerFile objects when changesets from downstream clients have been
    // processed or discarded.
    //
    // ServerImpl uses this information to keep a running tally (metric
    // `upload.pending.bytes`) of the total byte size of pending changesets from
    // downstream clients.
    //
    // These functions must be called on the network thread.
    void inc_byte_size_for_pending_downstream_changesets(std::size_t byte_size);
    void dec_byte_size_for_pending_downstream_changesets(std::size_t byte_size);

    // Overriding member functions in _impl::ServerHistory::Context
    std::mt19937_64& server_history_get_random() noexcept override final;

private:
    Server::Config m_config;
    network::Service m_service;
    std::mt19937_64 m_random;
    const std::size_t m_max_upload_backlog;
    const std::string m_root_dir;
    const AccessControl m_access_control;
    const ProtocolVersionRange m_protocol_version_range;

    // The reserved files will be closed in situations where the server
    // runs out of file descriptors.
    std::unique_ptr<File> m_reserved_files[5];

    // The set of all Realm files known to this server, represented by their
    // virtual path.
    //
    // INVARIANT: If a Realm file is in the servers directory (i.e., it would be
    // reported by an invocation of _impl::get_realm_names()), then the
    // corresponding virtual path is in `m_realm_names`, assuming no external
    // file-system level intervention.
    std::set<std::string> m_realm_names;

    std::unique_ptr<network::ssl::Context> m_ssl_context;
    ServerFileAccessCache m_file_access_cache;
    Worker m_worker;
    std::map<std::string, util::bind_ptr<ServerFile>> m_files; // Key is virtual path
    network::Acceptor m_acceptor;
    std::int_fast64_t m_next_conn_id = 0;
    std::unique_ptr<HTTPConnection> m_next_http_conn;
    network::Endpoint m_next_http_conn_endpoint;
    std::map<std::int_fast64_t, std::unique_ptr<HTTPConnection>> m_http_connections;
    std::map<std::int_fast64_t, std::unique_ptr<SyncConnection>> m_sync_connections;
    ServerProtocol m_server_protocol;
    compression::CompressMemoryArena m_compress_memory_arena;
    MiscBuffers m_misc_buffers;
    int_fast64_t m_current_server_session_ident;
    Optional<network::DeadlineTimer> m_connection_reaper_timer;
    bool m_allow_load_balancing = false;

    util::Mutex m_mutex;

    bool m_stopped = false; // Protected by `m_mutex`

    // m_sync_stopped is used by stop_sync_and_wait_for_backup_completion().
    // When m_sync_stopped is true, the server does not perform any sync.
    bool m_sync_stopped = false;

    std::atomic<bool> m_running{false}; // Debugging facility

    std::size_t m_pending_changesets_from_downstream_byte_size = 0;

    util::CondVar m_wait_or_service_stopped_cond; // Protected by `m_mutex`

    util::ScratchMemory m_scratch_memory;

    void listen();
    void initiate_accept();
    void handle_accept(std::error_code);

    void reap_connections();
    void initiate_connection_reaper_timer(milliseconds_type timeout);
    void do_close_connections();

    static std::size_t determine_max_upload_backlog(Server::Config& config) noexcept
    {
        if (config.max_upload_backlog == 0)
            return 4294967295; // 4GiB - 1 (largest allowable number on a 32-bit platform)
        return config.max_upload_backlog;
    }

    static ProtocolVersionRange determine_protocol_version_range(Server::Config& config)
    {
        const int actual_min = ServerImplBase::get_oldest_supported_protocol_version();
        const int actual_max = get_current_protocol_version();
        static_assert(actual_min <= actual_max, "");
        int min = actual_min;
        int max = actual_max;
        if (config.max_protocol_version != 0 && config.max_protocol_version < max) {
            if (config.max_protocol_version < min)
                throw Server::NoSupportedProtocolVersions();
            max = config.max_protocol_version;
        }
        return {min, max};
    }

    void do_recognize_external_change(const std::string& virt_path);

    void do_stop_sync_and_wait_for_backup_completion(util::UniqueFunction<void(bool did_complete)> completion_handler,
                                                     milliseconds_type timeout);
};

// ============================ SyncConnection ============================

class SyncConnection : public websocket::Config {
public:
    const std::shared_ptr<util::Logger> logger_ptr;
    util::Logger& logger;

    // Clients with sync protocol version 8 or greater support pbs->flx migration
    static constexpr int PBS_FLX_MIGRATION_PROTOCOL_VERSION = 8;
    // Clients with sync protocol version less than 10 do not support log messages
    static constexpr int SERVER_LOG_PROTOCOL_VERSION = 10;

    SyncConnection(ServerImpl& serv, std::int_fast64_t id, std::unique_ptr<network::Socket>&& socket,
                   std::unique_ptr<network::ssl::Stream>&& ssl_stream,
                   std::unique_ptr<network::ReadAheadBuffer>&& read_ahead_buffer, int client_protocol_version,
                   std::string client_user_agent, std::string remote_endpoint, std::string appservices_request_id)
        : logger_ptr{std::make_shared<util::PrefixLogger>(util::LogCategory::server, make_logger_prefix(id),
                                                          serv.logger_ptr)} // Throws
        , logger{*logger_ptr}
        , m_server{serv}
        , m_id{id}
        , m_socket{std::move(socket)}
        , m_ssl_stream{std::move(ssl_stream)}
        , m_read_ahead_buffer{std::move(read_ahead_buffer)}
        , m_websocket{*this}
        , m_client_protocol_version{client_protocol_version}
        , m_client_user_agent{std::move(client_user_agent)}
        , m_remote_endpoint{std::move(remote_endpoint)}
        , m_appservices_request_id{std::move(appservices_request_id)}
    {
        // Make the output buffer stream throw std::bad_alloc if it fails to
        // expand the buffer
        m_output_buffer.exceptions(std::ios_base::badbit | std::ios_base::failbit);

        network::Service& service = m_server.get_service();
        auto handler = [this](Status status) {
            if (!status.is_ok())
                return;
            if (!m_is_sending)
                send_next_message(); // Throws
        };
        m_send_trigger = std::make_unique<Trigger<network::Service>>(&service, std::move(handler)); // Throws
    }

    ~SyncConnection() noexcept;

    ServerImpl& get_server() noexcept
    {
        return m_server;
    }

    ServerProtocol& get_server_protocol() noexcept
    {
        return m_server.get_server_protocol();
    }

    int get_client_protocol_version()
    {
        return m_client_protocol_version;
    }

    const std::string& get_client_user_agent() const noexcept
    {
        return m_client_user_agent;
    }

    const std::string& get_remote_endpoint() const noexcept
    {
        return m_remote_endpoint;
    }

    const std::shared_ptr<util::Logger>& websocket_get_logger() noexcept final
    {
        return logger_ptr;
    }

    std::mt19937_64& websocket_get_random() noexcept final override
    {
        return m_server.get_random();
    }

    bool websocket_binary_message_received(const char* data, size_t size) final override
    {
        using sf = _impl::SimulatedFailure;
        if (sf::check_trigger(sf::sync_server__read_head)) {
            // Suicide
            read_error(sf::sync_server__read_head);
            return false;
        }
        // After a connection level error has occurred, all incoming messages
        // will be ignored. By continuing to read until end of input, the server
        // is able to know when the client closes the connection, which in
        // general means that is has received the ERROR message.
        if (REALM_LIKELY(!m_is_closing)) {
            m_last_activity_at = steady_clock_now();
            handle_message_received(data, size);
        }
        return true;
    }

    bool websocket_ping_message_received(const char* data, size_t size) final override
    {
        if (REALM_LIKELY(!m_is_closing)) {
            m_last_activity_at = steady_clock_now();
            handle_ping_received(data, size);
        }
        return true;
    }

    void async_write(const char* data, size_t size, websocket::WriteCompletionHandler handler) final override
    {
        if (m_ssl_stream) {
            m_ssl_stream->async_write(data, size, std::move(handler)); // Throws
        }
        else {
            m_socket->async_write(data, size, std::move(handler)); // Throws
        }
    }

    void async_read(char* buffer, size_t size, websocket::ReadCompletionHandler handler) final override
    {
        if (m_ssl_stream) {
            m_ssl_stream->async_read(buffer, size, *m_read_ahead_buffer, std::move(handler)); // Throws
        }
        else {
            m_socket->async_read(buffer, size, *m_read_ahead_buffer, std::move(handler)); // Throws
        }
    }

    void async_read_until(char* buffer, size_t size, char delim,
                          websocket::ReadCompletionHandler handler) final override
    {
        if (m_ssl_stream) {
            m_ssl_stream->async_read_until(buffer, size, delim, *m_read_ahead_buffer,
                                           std::move(handler)); // Throws
        }
        else {
            m_socket->async_read_until(buffer, size, delim, *m_read_ahead_buffer,
                                       std::move(handler)); // Throws
        }
    }

    void websocket_read_error_handler(std::error_code ec) final override
    {
        read_error(ec);
    }

    void websocket_write_error_handler(std::error_code ec) final override
    {
        write_error(ec);
    }

    void websocket_handshake_error_handler(std::error_code ec, const HTTPHeaders*, std::string_view) final override
    {
        // WebSocket class has already logged a message for this error
        close_due_to_error(ec); // Throws
    }

    void websocket_protocol_error_handler(std::error_code ec) final override
    {
        logger.error("WebSocket protocol error (%1): %2", ec, ec.message()); // Throws
        close_due_to_error(ec);                                              // Throws
    }

    void websocket_handshake_completion_handler(const HTTPHeaders&) final override
    {
        // This is not called since we handle HTTP request in handle_request_for_sync()
        REALM_TERMINATE("websocket_handshake_completion_handler should not have been called");
    }

    int_fast64_t get_id() const noexcept
    {
        return m_id;
    }

    network::Socket& get_socket() noexcept
    {
        return *m_socket;
    }

    void initiate();

    // Commits suicide
    template <class... Params>
    void terminate(Logger::Level, const char* log_message, Params... log_params);

    // Commits suicide
    void terminate_if_dead(SteadyTimePoint now);

    void enlist_to_send(Session*) noexcept;

    // Sessions should get the output_buffer and insert a message, after which
    // they call initiate_write_output_buffer().
    OutputBuffer& get_output_buffer()
    {
        m_output_buffer.reset();
        return m_output_buffer;
    }

    // More advanced memory strategies can be implemented if needed.
    void release_output_buffer() {}

    // When this function is called, the connection will initiate a write with
    // its output_buffer. Sessions use this method.
    void initiate_write_output_buffer();

    void initiate_pong_output_buffer();

    void handle_protocol_error(Status status);

    void receive_bind_message(session_ident_type, std::string path, std::string signed_user_token,
                              bool need_client_file_ident, bool is_subserver);

    void receive_ident_message(session_ident_type, file_ident_type client_file_ident,
                               salt_type client_file_ident_salt, version_type scan_server_version,
                               version_type scan_client_version, version_type latest_server_version,
                               salt_type latest_server_version_salt);

    void receive_upload_message(session_ident_type, version_type progress_client_version,
                                version_type progress_server_version, version_type locked_server_version,
                                const UploadChangesets&);

    void receive_mark_message(session_ident_type, request_ident_type);

    void receive_unbind_message(session_ident_type);

    void receive_ping(milliseconds_type timestamp, milliseconds_type rtt);

    void receive_error_message(session_ident_type, int error_code, std::string_view error_body);

    void protocol_error(ProtocolError, Session* = nullptr);

    void initiate_soft_close();

    void discard_session(session_ident_type) noexcept;

    void send_log_message(util::Logger::Level level, const std::string&& message, session_ident_type sess_ident = 0,
                          std::optional<std::string> co_id = std::nullopt);

private:
    ServerImpl& m_server;
    const int_fast64_t m_id;
    std::unique_ptr<network::Socket> m_socket;
    std::unique_ptr<network::ssl::Stream> m_ssl_stream;
    std::unique_ptr<network::ReadAheadBuffer> m_read_ahead_buffer;

    websocket::Socket m_websocket;
    std::unique_ptr<char[]> m_input_body_buffer;
    OutputBuffer m_output_buffer;
    std::map<session_ident_type, std::unique_ptr<Session>> m_sessions;

    // The protocol version in use by the connected client.
    const int m_client_protocol_version;

    // The user agent description passed by the client.
    const std::string m_client_user_agent;

    const std::string m_remote_endpoint;

    const std::string m_appservices_request_id;

    // A queue of sessions that have enlisted for an opportunity to send a
    // message. Sessions will be served in the order that they enlist. A session
    // can only occur once in this queue (linked list). If the queue is not
    // empty, and no message is currently being written to the socket, the first
    // session is taken out of the queue, and then granted an opportunity to
    // send a message.
    //
    // Sessions will never be destroyed while in this queue. This is ensured
    // because the connection owns the sessions that are associated with it, and
    // the connection only removes a session from m_sessions at points in time
    // where that session is guaranteed to not be in m_sessions_enlisted_to_send
    // (Connection::send_next_message() and Connection::~Connection()).
    SessionQueue m_sessions_enlisted_to_send;

    Session* m_receiving_session = nullptr;

    bool m_is_sending = false;
    bool m_is_closing = false;

    bool m_send_pong = false;
    bool m_sending_pong = false;

    std::unique_ptr<Trigger<network::Service>> m_send_trigger;

    milliseconds_type m_last_ping_timestamp = 0;

    // If `m_is_closing` is true, this is the time at which `m_is_closing` was
    // set to true (initiation of soft close). Otherwise, if no messages have
    // been received from the client, this is the time at which the connection
    // object was initiated (completion of WebSocket handshake). Otherwise this
    // is the time at which the last message was received from the client.
    SteadyTimePoint m_last_activity_at;

    // These are initialized by do_initiate_soft_close().
    //
    // With recent versions of the protocol (when the version is greater than,
    // or equal to 23), `m_error_session_ident` is always zero.
    ProtocolError m_error_code = {};
    session_ident_type m_error_session_ident = 0;

    struct LogMessage {
        session_ident_type sess_ident;
        util::Logger::Level level;
        std::string message;
        std::optional<std::string> co_id;
    };

    std::mutex m_log_mutex;
    std::queue<LogMessage> m_log_messages;

    static std::string make_logger_prefix(int_fast64_t id)
    {
        std::ostringstream out;
        out.imbue(std::locale::classic());
        out << "Sync Connection[" << id << "]: "; // Throws
        return out.str();                         // Throws
    }

    // The return value of handle_message_received() designates whether
    // message processing should continue. If the connection object is
    // destroyed during execution of handle_message_received(), the return
    // value must be false.
    void handle_message_received(const char* data, size_t size);

    void handle_ping_received(const char* data, size_t size);

    void send_next_message();
    void send_pong(milliseconds_type timestamp);
    void send_log_message(const LogMessage& log_msg);

    void handle_write_output_buffer();
    void handle_pong_output_buffer();

    void initiate_write_error(ProtocolError, session_ident_type);
    void handle_write_error(std::error_code ec);

    void do_initiate_soft_close(ProtocolError, session_ident_type);
    void read_error(std::error_code);
    void write_error(std::error_code);

    void close_due_to_close_by_client(std::error_code);
    void close_due_to_error(std::error_code);

    void terminate_sessions();

    void bad_session_ident(const char* message_type, session_ident_type);
    void message_after_unbind(const char* message_type, session_ident_type);
    void message_before_ident(const char* message_type, session_ident_type);
};


inline void SyncConnection::read_error(std::error_code ec)
{
    REALM_ASSERT(ec != util::error::operation_aborted);
    if (ec == util::MiscExtErrors::end_of_input || ec == util::error::connection_reset) {
        // Suicide
        close_due_to_close_by_client(ec); // Throws
        return;
    }
    if (ec == util::MiscExtErrors::delim_not_found) {
        logger.error("Input message head delimited not found"); // Throws
        protocol_error(ProtocolError::limits_exceeded);         // Throws
        return;
    }

    logger.error("Reading failed: %1", ec.message()); // Throws

    // Suicide
    close_due_to_error(ec); // Throws
}

inline void SyncConnection::write_error(std::error_code ec)
{
    REALM_ASSERT(ec != util::error::operation_aborted);
    if (ec == util::error::broken_pipe || ec == util::error::connection_reset) {
        // Suicide
        close_due_to_close_by_client(ec); // Throws
        return;
    }
    logger.error("Writing failed: %1", ec.message()); // Throws

    // Suicide
    close_due_to_error(ec); // Throws
}


// ============================ HTTPConnection ============================

std::string g_user_agent = "User-Agent";

class HTTPConnection {
public:
    const std::shared_ptr<Logger> logger_ptr;
    util::Logger& logger;

    HTTPConnection(ServerImpl& serv, int_fast64_t id, bool is_ssl)
        : logger_ptr{std::make_shared<PrefixLogger>(util::LogCategory::server, make_logger_prefix(id),
                                                    serv.logger_ptr)} // Throws
        , logger{*logger_ptr}
        , m_server{serv}
        , m_id{id}
        , m_socket{new network::Socket{serv.get_service()}} // Throws
        , m_read_ahead_buffer{new network::ReadAheadBuffer} // Throws
        , m_http_server{*this, logger_ptr}
    {
        // Make the output buffer stream throw std::bad_alloc if it fails to
        // expand the buffer
        m_output_buffer.exceptions(std::ios_base::badbit | std::ios_base::failbit);

        if (is_ssl) {
            using namespace network::ssl;
            Context& ssl_context = serv.get_ssl_context();
            m_ssl_stream = std::make_unique<Stream>(*m_socket, ssl_context,
                                                    Stream::server); // Throws
        }
    }

    ServerImpl& get_server() noexcept
    {
        return m_server;
    }

    int_fast64_t get_id() const noexcept
    {
        return m_id;
    }

    network::Socket& get_socket() noexcept
    {
        return *m_socket;
    }

    template <class H>
    void async_write(const char* data, size_t size, H handler)
    {
        if (m_ssl_stream) {
            m_ssl_stream->async_write(data, size, std::move(handler)); // Throws
        }
        else {
            m_socket->async_write(data, size, std::move(handler)); // Throws
        }
    }

    template <class H>
    void async_read(char* buffer, size_t size, H handler)
    {
        if (m_ssl_stream) {
            m_ssl_stream->async_read(buffer, size, *m_read_ahead_buffer,
                                     std::move(handler)); // Throws
        }
        else {
            m_socket->async_read(buffer, size, *m_read_ahead_buffer,
                                 std::move(handler)); // Throws
        }
    }

    template <class H>
    void async_read_until(char* buffer, size_t size, char delim, H handler)
    {
        if (m_ssl_stream) {
            m_ssl_stream->async_read_until(buffer, size, delim, *m_read_ahead_buffer,
                                           std::move(handler)); // Throws
        }
        else {
            m_socket->async_read_until(buffer, size, delim, *m_read_ahead_buffer,
                                       std::move(handler)); // Throws
        }
    }

    void initiate(std::string remote_endpoint)
    {
        m_last_activity_at = steady_clock_now();
        m_remote_endpoint = std::move(remote_endpoint);

        logger.detail("Connection from %1", m_remote_endpoint); // Throws

        if (m_ssl_stream) {
            initiate_ssl_handshake(); // Throws
        }
        else {
            initiate_http(); // Throws
        }
    }

    void respond_200_ok()
    {
        handle_text_response(HTTPStatus::Ok, "OK"); // Throws
    }

    void respond_404_not_found()
    {
        handle_text_response(HTTPStatus::NotFound, "Not found"); // Throws
    }

    void respond_503_service_unavailable()
    {
        handle_text_response(HTTPStatus::ServiceUnavailable, "Service unavailable"); // Throws
    }

    // Commits suicide
    template <class... Params>
    void terminate(Logger::Level log_level, const char* log_message, Params... log_params)
    {
        logger.log(log_level, log_message, log_params...); // Throws
        m_ssl_stream.reset();
        m_socket.reset();
        m_server.remove_http_connection(m_id); // Suicide
    }

    // Commits suicide
    void terminate_if_dead(SteadyTimePoint now)
    {
        milliseconds_type time = steady_duration(m_last_activity_at, now);
        const Server::Config& config = m_server.get_config();
        if (m_is_sending) {
            if (time >= config.http_response_timeout) {
                // Suicide
                terminate(Logger::Level::detail,
                          "HTTP connection closed (request timeout)"); // Throws
            }
        }
        else {
            if (time >= config.http_request_timeout) {
                // Suicide
                terminate(Logger::Level::detail,
                          "HTTP connection closed (response timeout)"); // Throws
            }
        }
    }

    std::string get_appservices_request_id() const
    {
        return m_appservices_request_id.to_string();
    }

private:
    ServerImpl& m_server;
    const int_fast64_t m_id;
    const ObjectId m_appservices_request_id = ObjectId::gen();
    std::unique_ptr<network::Socket> m_socket;
    std::unique_ptr<network::ssl::Stream> m_ssl_stream;
    std::unique_ptr<network::ReadAheadBuffer> m_read_ahead_buffer;
    HTTPServer<HTTPConnection> m_http_server;
    OutputBuffer m_output_buffer;
    bool m_is_sending = false;
    SteadyTimePoint m_last_activity_at;
    std::string m_remote_endpoint;
    int m_negotiated_protocol_version = 0;

    void initiate_ssl_handshake()
    {
        auto handler = [this](std::error_code ec) {
            if (ec != util::error::operation_aborted)
                handle_ssl_handshake(ec); // Throws
        };
        m_ssl_stream->async_handshake(std::move(handler)); // Throws
    }

    void handle_ssl_handshake(std::error_code ec)
    {
        if (ec) {
            logger.error("SSL handshake error (%1): %2", ec, ec.message()); // Throws
            close_due_to_error(ec);                                         // Throws
            return;
        }
        initiate_http(); // Throws
    }

    void initiate_http()
    {
        logger.debug("Connection initiates HTTP receipt");

        auto handler = [this](HTTPRequest request, std::error_code ec) {
            if (REALM_UNLIKELY(ec == util::error::operation_aborted))
                return;
            if (REALM_UNLIKELY(ec == HTTPParserError::MalformedRequest)) {
                logger.error("Malformed HTTP request");
                close_due_to_error(ec); // Throws
                return;
            }
            if (REALM_UNLIKELY(ec == HTTPParserError::BadRequest)) {
                logger.error("Bad HTTP request");
                const char* body = "The HTTP request was corrupted";
                handle_400_bad_request(body); // Throws
                return;
            }
            if (REALM_UNLIKELY(ec)) {
                read_error(ec); // Throws
                return;
            }
            handle_http_request(std::move(request)); // Throws
        };
        m_http_server.async_receive_request(std::move(handler)); // Throws
    }

    void handle_http_request(const HTTPRequest& request)
    {
        StringData path = request.path;

        logger.debug("HTTP request received, request = %1", request);

        m_is_sending = true;
        m_last_activity_at = steady_clock_now();

        // FIXME: When thinking of this function as a switching device, it seem
        // wrong that it requires a `%2F` after `/realm-sync/`. If `%2F` is
        // supposed to be mandatory, then that check ought to be delegated to
        // handle_request_for_sync(), as that will yield a sharper separation of
        // concerns.
        if (path == "/realm-sync" || path.begins_with("/realm-sync?") || path.begins_with("/realm-sync/%2F")) {
            handle_request_for_sync(request); // Throws
        }
        else {
            handle_404_not_found(request); // Throws
        }
    }

    void handle_request_for_sync(const HTTPRequest& request)
    {
        if (m_server.is_sync_stopped()) {
            logger.debug("Attempt to create a sync connection to a server that has been "
                         "stopped"); // Throws
            handle_503_service_unavailable(request, "The server does not accept sync "
                                                    "connections"); // Throws
            return;
        }

        util::Optional<std::string> sec_websocket_protocol = websocket::read_sec_websocket_protocol(request);

        // Figure out whether there are any protocol versions supported by both
        // the client and the server, and if so, choose the newest one of them.
        MiscBuffers& misc_buffers = m_server.get_misc_buffers();
        using ProtocolVersionRanges = MiscBuffers::ProtocolVersionRanges;
        ProtocolVersionRanges& protocol_version_ranges = misc_buffers.protocol_version_ranges;
        {
            protocol_version_ranges.clear();
            util::MemoryInputStream in;
            in.imbue(std::locale::classic());
            in.unsetf(std::ios_base::skipws);
            std::string_view value;
            if (sec_websocket_protocol)
                value = *sec_websocket_protocol;
            HttpListHeaderValueParser parser{value};
            std::string_view elem;
            while (parser.next(elem)) {
                // FIXME: Use std::string_view::begins_with() in C++20.
                const StringData protocol{elem};
                std::string_view prefix;
                if (protocol.begins_with(get_pbs_websocket_protocol_prefix()))
                    prefix = get_pbs_websocket_protocol_prefix();
                else if (protocol.begins_with(get_old_pbs_websocket_protocol_prefix()))
                    prefix = get_old_pbs_websocket_protocol_prefix();
                if (!prefix.empty()) {
                    auto parse_version = [&](std::string_view str) {
                        in.set_buffer(str.data(), str.data() + str.size());
                        int version = 0;
                        in >> version;
                        if (REALM_LIKELY(in && in.eof() && version >= 0))
                            return version;
                        return -1;
                    };
                    int min, max;
                    std::string_view range = elem.substr(prefix.size());
                    auto i = range.find('-');
                    if (i != std::string_view::npos) {
                        min = parse_version(range.substr(0, i));
                        max = parse_version(range.substr(i + 1));
                    }
                    else {
                        min = parse_version(range);
                        max = min;
                    }
                    if (REALM_LIKELY(min >= 0 && max >= 0 && min <= max)) {
                        protocol_version_ranges.emplace_back(min, max); // Throws
                        continue;
                    }
                    logger.error("Protocol version negotiation failed: Client sent malformed "
                                 "specification of supported protocol versions: '%1'",
                                 elem); // Throws
                    handle_400_bad_request("Protocol version negotiation failed: Malformed "
                                           "specification of supported protocol "
                                           "versions\n"); // Throws
                    return;
                }
                logger.warn("Unrecognized protocol token in HTTP response header "
                            "Sec-WebSocket-Protocol: '%1'",
                            elem); // Throws
            }
            if (protocol_version_ranges.empty()) {
                logger.error("Protocol version negotiation failed: Client did not send a "
                             "specification of supported protocol versions"); // Throws
                handle_400_bad_request("Protocol version negotiation failed: Missing specification "
                                       "of supported protocol versions\n"); // Throws
                return;
            }
        }
        {
            ProtocolVersionRange server_range = m_server.get_protocol_version_range();
            int server_min = server_range.first;
            int server_max = server_range.second;
            int best_match = 0;
            int overall_client_min = std::numeric_limits<int>::max();
            int overall_client_max = std::numeric_limits<int>::min();
            for (const auto& range : protocol_version_ranges) {
                int client_min = range.first;
                int client_max = range.second;
                if (client_max >= server_min && client_min <= server_max) {
                    // Overlap
                    int version = std::min(client_max, server_max);
                    if (version > best_match) {
                        best_match = version;
                    }
                }
                if (client_min < overall_client_min)
                    overall_client_min = client_min;
                if (client_max > overall_client_max)
                    overall_client_max = client_max;
            }
            Formatter& formatter = misc_buffers.formatter;
            if (REALM_UNLIKELY(best_match == 0)) {
                const char* elaboration = "No version supported by both client and server";
                const char* identifier_hint = nullptr;
                if (overall_client_max < server_min) {
                    // Client is too old
                    elaboration = "Client is too old for server";
                    identifier_hint = "CLIENT_TOO_OLD";
                }
                else if (overall_client_min > server_max) {
                    // Client is too new
                    elaboration = "Client is too new for server";
                    identifier_hint = "CLIENT_TOO_NEW";
                }
                auto format_ranges = [&](const auto& list) {
                    bool nonfirst = false;
                    for (auto range : list) {
                        if (nonfirst)
                            formatter << ", "; // Throws
                        int min = range.first, max = range.second;
                        REALM_ASSERT(min <= max);
                        formatter << min;
                        if (max != min)
                            formatter << "-" << max;
                        nonfirst = true;
                    }
                };
                using Range = ProtocolVersionRange;
                formatter.reset();
                format_ranges(protocol_version_ranges); // Throws
                logger.error("Protocol version negotiation failed: %1 "
                             "(client supports: %2)",
                             elaboration, std::string_view(formatter.data(), formatter.size())); // Throws
                formatter.reset();
                formatter << "Protocol version negotiation failed: "
                             ""
                          << elaboration << ".\n\n";                                   // Throws
                formatter << "Server supports: ";                                      // Throws
                format_ranges(std::initializer_list<Range>{{server_min, server_max}}); // Throws
                formatter << "\n";                                                     // Throws
                formatter << "Client supports: ";                                      // Throws
                format_ranges(protocol_version_ranges);                                // Throws
                formatter << "\n\n";                                                   // Throws
                formatter << "REALM_SYNC_PROTOCOL_MISMATCH";                           // Throws
                if (identifier_hint)
                    formatter << ":" << identifier_hint;                      // Throws
                formatter << "\n";                                            // Throws
                handle_400_bad_request({formatter.data(), formatter.size()}); // Throws
                return;
            }
            m_negotiated_protocol_version = best_match;
            logger.debug("Received: Sync HTTP request (negotiated_protocol_version=%1)",
                         m_negotiated_protocol_version); // Throws
            formatter.reset();
        }

        std::string sec_websocket_protocol_2;
        {
            std::string_view prefix =
                m_negotiated_protocol_version < SyncConnection::PBS_FLX_MIGRATION_PROTOCOL_VERSION
                    ? get_old_pbs_websocket_protocol_prefix()
                    : get_pbs_websocket_protocol_prefix();
            std::ostringstream out;
            out.imbue(std::locale::classic());
            out << prefix << m_negotiated_protocol_version; // Throws
            sec_websocket_protocol_2 = std::move(out).str();
        }

        std::error_code ec;
        util::Optional<HTTPResponse> response =
            websocket::make_http_response(request, sec_websocket_protocol_2, ec); // Throws

        if (ec) {
            if (ec == websocket::HttpError::bad_request_header_upgrade) {
                logger.error("There must be a header of the form 'Upgrade: websocket'");
            }
            else if (ec == websocket::HttpError::bad_request_header_connection) {
                logger.error("There must be a header of the form 'Connection: Upgrade'");
            }
            else if (ec == websocket::HttpError::bad_request_header_websocket_version) {
                logger.error("There must be a header of the form 'Sec-WebSocket-Version: 13'");
            }
            else if (ec == websocket::HttpError::bad_request_header_websocket_key) {
                logger.error("The header Sec-WebSocket-Key is missing");
            }

            logger.error("The HTTP request with the error is:\n%1", request);
            logger.error("Check the proxy configuration and make sure that the "
                         "HTTP request is a valid Websocket request.");
            close_due_to_error(ec);
            return;
        }
        REALM_ASSERT(response);
        add_common_http_response_headers(*response);

        std::string user_agent;
        {
            auto i = request.headers.find(g_user_agent);
            if (i != request.headers.end())
                user_agent = i->second; // Throws (copy)
        }

        auto handler = [protocol_version = m_negotiated_protocol_version, user_agent = std::move(user_agent),
                        this](std::error_code ec) {
            // If the operation is aborted, the socket object may have been destroyed.
            if (ec != util::error::operation_aborted) {
                if (ec) {
                    write_error(ec);
                    return;
                }

                std::unique_ptr<SyncConnection> sync_conn = std::make_unique<SyncConnection>(
                    m_server, m_id, std::move(m_socket), std::move(m_ssl_stream), std::move(m_read_ahead_buffer),
                    protocol_version, std::move(user_agent), std::move(m_remote_endpoint),
                    get_appservices_request_id()); // Throws
                SyncConnection& sync_conn_ref = *sync_conn;
                m_server.add_sync_connection(m_id, std::move(sync_conn));
                m_server.remove_http_connection(m_id);
                sync_conn_ref.initiate();
            }
        };
        m_http_server.async_send_response(*response, std::move(handler));
    }

    void handle_text_response(HTTPStatus http_status, std::string_view body)
    {
        std::string body_2 = std::string(body); // Throws

        HTTPResponse response;
        response.status = http_status;
        add_common_http_response_headers(response);
        response.headers["Connection"] = "close";

        if (!body_2.empty()) {
            response.headers["Content-Length"] = util::to_string(body_2.size());
            response.body = std::move(body_2);
        }

        auto handler = [this](std::error_code ec) {
            if (REALM_UNLIKELY(ec == util::error::operation_aborted))
                return;
            if (REALM_UNLIKELY(ec)) {
                write_error(ec);
                return;
            }
            terminate(Logger::Level::detail, "HTTP connection closed"); // Throws
        };
        m_http_server.async_send_response(response, std::move(handler));
    }

    void handle_400_bad_request(std::string_view body)
    {
        logger.detail("400 Bad Request");
        handle_text_response(HTTPStatus::BadRequest, body); // Throws
    }

    void handle_404_not_found(const HTTPRequest&)
    {
        logger.detail("404 Not Found"); // Throws
        handle_text_response(HTTPStatus::NotFound,
                             "Realm sync server\n\nPage not found\n"); // Throws
    }

    void handle_503_service_unavailable(const HTTPRequest&, std::string_view message)
    {
        logger.debug("503 Service Unavailable");                       // Throws
        handle_text_response(HTTPStatus::ServiceUnavailable, message); // Throws
    }

    void add_common_http_response_headers(HTTPResponse& response)
    {
        response.headers["Server"] = "RealmSync/" REALM_VERSION_STRING; // Throws
        if (m_negotiated_protocol_version < SyncConnection::SERVER_LOG_PROTOCOL_VERSION) {
            // This isn't a real X-Appservices-Request-Id, but it should be enough to test with
            response.headers["X-Appservices-Request-Id"] = get_appservices_request_id();
        }
    }

    void read_error(std::error_code ec)
    {
        REALM_ASSERT(ec != util::error::operation_aborted);
        if (ec == util::MiscExtErrors::end_of_input || ec == util::error::connection_reset) {
            // Suicide
            close_due_to_close_by_client(ec); // Throws
            return;
        }
        if (ec == util::MiscExtErrors::delim_not_found) {
            logger.error("Input message head delimited not found"); // Throws
            close_due_to_error(ec);                                 // Throws
            return;
        }

        logger.error("Reading failed: %1", ec.message()); // Throws

        // Suicide
        close_due_to_error(ec); // Throws
    }

    void write_error(std::error_code ec)
    {
        REALM_ASSERT(ec != util::error::operation_aborted);
        if (ec == util::error::broken_pipe || ec == util::error::connection_reset) {
            // Suicide
            close_due_to_close_by_client(ec); // Throws
            return;
        }
        logger.error("Writing failed: %1", ec.message()); // Throws

        // Suicide
        close_due_to_error(ec); // Throws
    }

    void close_due_to_close_by_client(std::error_code ec)
    {
        auto log_level = (ec == util::MiscExtErrors::end_of_input ? Logger::Level::detail : Logger::Level::info);
        // Suicide
        terminate(log_level, "HTTP connection closed by client: %1", ec.message()); // Throws
    }

    void close_due_to_error(std::error_code ec)
    {
        // Suicide
        terminate(Logger::Level::error, "HTTP connection closed due to error: %1",
                  ec.message()); // Throws
    }

    static std::string make_logger_prefix(int_fast64_t id)
    {
        std::ostringstream out;
        out.imbue(std::locale::classic());
        out << "HTTP Connection[" << id << "]: "; // Throws
        return out.str();                         // Throws
    }
};


class DownloadHistoryEntryHandler : public ServerHistory::HistoryEntryHandler {
public:
    std::size_t num_changesets = 0;
    std::size_t accum_original_size = 0;
    std::size_t accum_compacted_size = 0;

    DownloadHistoryEntryHandler(ServerProtocol& protocol, OutputBuffer& buffer, util::Logger& logger) noexcept
        : m_protocol{protocol}
        , m_buffer{buffer}
        , m_logger{logger}
    {
    }

    void handle(version_type server_version, const HistoryEntry& entry, size_t original_size) override
    {
        version_type client_version = entry.remote_version;
        ServerProtocol::ChangesetInfo info{server_version, client_version, entry, original_size};
        m_protocol.insert_single_changeset_download_message(m_buffer, info, m_logger); // Throws
        ++num_changesets;
        accum_original_size += original_size;
        accum_compacted_size += entry.changeset.size();
    }

private:
    ServerProtocol& m_protocol;
    OutputBuffer& m_buffer;
    util::Logger& m_logger;
};


// ============================ Session ============================

//                        Need cli-   Send     IDENT     UNBIND              ERROR
//   Protocol             ent file    IDENT    message   message   Error     message
//   state                identifier  message  received  received  occurred  sent
// ---------------------------------------------------------------------------------
//   AllocatingIdent      yes         yes      no        no        no        no
//   SendIdent            no          yes      no        no        no        no
//   WaitForIdent         no          no       no        no        no        no
//   WaitForUnbind        maybe       no       yes       no        no        no
//   SendError            maybe       maybe    maybe     no        yes       no
//   WaitForUnbindErr     maybe       maybe    maybe     no        yes       yes
//   SendUnbound          maybe       maybe    maybe     yes       maybe     no
//
//
//   Condition                      Expression
// ----------------------------------------------------------
//   Need client file identifier    need_client_file_ident()
//   Send IDENT message             must_send_ident_message()
//   IDENT message received         ident_message_received()
//   UNBIND message received        unbind_message_received()
//   Error occurred                 error_occurred()
//   ERROR message sent             m_error_message_sent
//
//
//   Protocol
//   state                Will send              Can receive
// -----------------------------------------------------------------------
//   AllocatingIdent      none                   UNBIND
//   SendIdent            IDENT                  UNBIND
//   WaitForIdent         none                   IDENT, UNBIND
//   WaitForUnbind        DOWNLOAD, TRANSACT,    UPLOAD, TRANSACT, MARK,
//                        MARK, ALLOC            ALLOC, UNBIND
//   SendError            ERROR                  any
//   WaitForUnbindErr     none                   any
//   SendUnbound          UNBOUND                none
//
class Session final : private FileIdentReceiver {
public:
    util::PrefixLogger logger;

    Session(SyncConnection& conn, session_ident_type session_ident)
        : logger{util::LogCategory::server, make_logger_prefix(session_ident), conn.logger_ptr} // Throws
        , m_connection{conn}
        , m_session_ident{session_ident}
    {
    }

    ~Session() noexcept
    {
        REALM_ASSERT(!is_enlisted_to_send());
        detach_from_server_file();
    }

    SyncConnection& get_connection() noexcept
    {
        return m_connection;
    }

    const Optional<std::array<char, 64>>& get_encryption_key()
    {
        return m_connection.get_server().get_config().encryption_key;
    }

    session_ident_type get_session_ident() const noexcept
    {
        return m_session_ident;
    }

    ServerProtocol& get_server_protocol() noexcept
    {
        return m_connection.get_server_protocol();
    }

    bool need_client_file_ident() const noexcept
    {
        return (m_file_ident_request != 0);
    }

    bool must_send_ident_message() const noexcept
    {
        return m_send_ident_message;
    }

    bool ident_message_received() const noexcept
    {
        return m_client_file_ident != 0;
    }

    bool unbind_message_received() const noexcept
    {
        return m_unbind_message_received;
    }

    bool error_occurred() const noexcept
    {
        return int(m_error_code) != 0;
    }

    bool relayed_alloc_request_in_progress() const noexcept
    {
        return (need_client_file_ident() || m_allocated_file_ident.ident != 0);
    }

    // Returns the file identifier (always a nonzero value) of the client side
    // file if ident_message_received() returns true. Otherwise it returns zero.
    file_ident_type get_client_file_ident() const noexcept
    {
        return m_client_file_ident;
    }

    void initiate()
    {
        logger.detail("Session initiated", m_session_ident); // Throws
    }

    void terminate()
    {
        logger.detail("Session terminated", m_session_ident); // Throws
    }

    // Initiate the deactivation process, if it has not been initiated already
    // by the client.
    //
    // IMPORTANT: This function must not be called with protocol versions
    // earlier than 23.
    //
    // The deactivation process will eventually lead to termination of the
    // session.
    //
    // The session will detach itself from the server file when the deactivation
    // process is initiated, regardless of whether it is initiated by the
    // client, or by calling this function.
    void initiate_deactivation(ProtocolError error_code)
    {
        REALM_ASSERT(is_session_level_error(error_code));
        REALM_ASSERT(!error_occurred()); // Must only be called once

        // If the UNBIND message has been received, then the client has
        // initiated the deactivation process already.
        if (REALM_LIKELY(!unbind_message_received())) {
            detach_from_server_file();
            m_error_code = error_code;
            // Protocol state is now SendError
            ensure_enlisted_to_send();
            return;
        }
        // Protocol state was SendUnbound, and remains unchanged
    }

    bool is_enlisted_to_send() const noexcept
    {
        return m_next != nullptr;
    }

    void ensure_enlisted_to_send() noexcept
    {
        if (!is_enlisted_to_send())
            enlist_to_send();
    }

    void enlist_to_send() noexcept
    {
        m_connection.enlist_to_send(this);
    }

    // Overriding memeber function in FileIdentReceiver
    void receive_file_ident(SaltedFileIdent file_ident) override final
    {
        // Protocol state must be AllocatingIdent or WaitForUnbind
        if (!ident_message_received()) {
            REALM_ASSERT(need_client_file_ident());
            REALM_ASSERT(m_send_ident_message);
        }
        else {
            REALM_ASSERT(!m_send_ident_message);
        }
        REALM_ASSERT(!unbind_message_received());
        REALM_ASSERT(!error_occurred());
        REALM_ASSERT(!m_error_message_sent);

        m_file_ident_request = 0;
        m_allocated_file_ident = file_ident;

        // If the protocol state was AllocatingIdent, it is now SendIdent,
        // otherwise it continues to be WaitForUnbind.

        logger.debug("Acquired outbound salted file identifier (%1, %2)", file_ident.ident,
                     file_ident.salt); // Throws

        ensure_enlisted_to_send();
    }

    // Called by the associated connection object when this session is granted
    // an opportunity to initiate the sending of a message.
    //
    // This function may lead to the destruction of the session object
    // (suicide).
    void send_message()
    {
        if (REALM_LIKELY(!unbind_message_received())) {
            if (REALM_LIKELY(!error_occurred())) {
                if (REALM_LIKELY(ident_message_received())) {
                    // State is WaitForUnbind.
                    bool relayed_alloc = (m_allocated_file_ident.ident != 0);
                    if (REALM_LIKELY(!relayed_alloc)) {
                        // Send DOWNLOAD or MARK.
                        continue_history_scan(); // Throws
                        // Session object may have been
                        // destroyed at this point (suicide)
                        return;
                    }
                    send_alloc_message(); // Throws
                    return;
                }
                // State is SendIdent
                send_ident_message(); // Throws
                return;
            }
            // State is SendError
            send_error_message(); // Throws
            return;
        }
        // State is SendUnbound
        send_unbound_message(); // Throws
        terminate();            // Throws
        m_connection.discard_session(m_session_ident);
        // This session is now destroyed!
    }

    bool receive_bind_message(std::string path, std::string signed_user_token, bool need_client_file_ident,
                              bool is_subserver, ProtocolError& error)
    {
        if (logger.would_log(util::Logger::Level::info)) {
            logger.detail("Received: BIND(server_path=%1, signed_user_token='%2', "
                          "need_client_file_ident=%3, is_subserver=%4)",
                          path, short_token_fmt(signed_user_token), int(need_client_file_ident),
                          int(is_subserver)); // Throws
        }

        ServerImpl& server = m_connection.get_server();
        _impl::VirtualPathComponents virt_path_components =
            _impl::parse_virtual_path(server.get_root_dir(), path); // Throws

        if (!virt_path_components.is_valid) {
            logger.error("Bad virtual path (message_type='bind', path='%1', "
                         "signed_user_token='%2')",
                         path,
                         short_token_fmt(signed_user_token)); // Throws
            error = ProtocolError::illegal_realm_path;
            return false;
        }

        // The user has proper permissions at this stage.

        m_server_file = server.get_or_create_file(path); // Throws

        m_server_file->add_unidentified_session(this); // Throws

        logger.info("Client info: (path='%1', from=%2, protocol=%3) %4", path, m_connection.get_remote_endpoint(),
                    m_connection.get_client_protocol_version(),
                    m_connection.get_client_user_agent()); // Throws

        m_is_subserver = is_subserver;
        if (REALM_LIKELY(!need_client_file_ident)) {
            // Protocol state is now WaitForUnbind
            return true;
        }

        // FIXME: We must make a choice about client file ident for read only
        // sessions. They should have a special read-only client file ident.
        file_ident_type proxy_file = 0; // No proxy
        ClientType client_type = (is_subserver ? ClientType::subserver : ClientType::regular);
        m_file_ident_request = m_server_file->request_file_ident(*this, proxy_file, client_type); // Throws
        m_send_ident_message = true;
        // Protocol state is now AllocatingIdent

        return true;
    }

    bool receive_ident_message(file_ident_type client_file_ident, salt_type client_file_ident_salt,
                               version_type scan_server_version, version_type scan_client_version,
                               version_type latest_server_version, salt_type latest_server_version_salt,
                               ProtocolError& error)
    {
        // Protocol state must be WaitForIdent
        REALM_ASSERT(!need_client_file_ident());
        REALM_ASSERT(!m_send_ident_message);
        REALM_ASSERT(!ident_message_received());
        REALM_ASSERT(!unbind_message_received());
        REALM_ASSERT(!error_occurred());
        REALM_ASSERT(!m_error_message_sent);

        logger.debug("Received: IDENT(client_file_ident=%1, client_file_ident_salt=%2, "
                     "scan_server_version=%3, scan_client_version=%4, latest_server_version=%5, "
                     "latest_server_version_salt=%6)",
                     client_file_ident, client_file_ident_salt, scan_server_version, scan_client_version,
                     latest_server_version, latest_server_version_salt); // Throws

        SaltedFileIdent client_file_ident_2 = {client_file_ident, client_file_ident_salt};
        DownloadCursor download_progress = {scan_server_version, scan_client_version};
        SaltedVersion server_version_2 = {latest_server_version, latest_server_version_salt};
        ClientType client_type = (m_is_subserver ? ClientType::subserver : ClientType::regular);
        UploadCursor upload_threshold = {0, 0};
        version_type locked_server_version = 0;
        BootstrapError error_2 =
            m_server_file->bootstrap_client_session(client_file_ident_2, download_progress, server_version_2,
                                                    client_type, upload_threshold, locked_server_version,
                                                    logger); // Throws
        switch (error_2) {
            case BootstrapError::no_error:
                break;
            case BootstrapError::client_file_expired:
                logger.warn("Client (%1) expired", client_file_ident); // Throws
                error = ProtocolError::client_file_expired;
                return false;
            case BootstrapError::bad_client_file_ident:
                logger.error("Bad client file ident (%1) in IDENT message",
                             client_file_ident); // Throws
                error = ProtocolError::bad_client_file_ident;
                return false;
            case BootstrapError::bad_client_file_ident_salt:
                logger.error("Bad client file identifier salt (%1) in IDENT message",
                             client_file_ident_salt); // Throws
                error = ProtocolError::diverging_histories;
                return false;
            case BootstrapError::bad_download_server_version:
                logger.error("Bad download progress server version in IDENT message"); // Throws
                error = ProtocolError::bad_server_version;
                return false;
            case BootstrapError::bad_download_client_version:
                logger.error("Bad download progress client version in IDENT message"); // Throws
                error = ProtocolError::bad_client_version;
                return false;
            case BootstrapError::bad_server_version:
                logger.error("Bad server version (message_type='ident')"); // Throws
                error = ProtocolError::bad_server_version;
                return false;
            case BootstrapError::bad_server_version_salt:
                logger.error("Bad server version salt in IDENT message"); // Throws
                error = ProtocolError::diverging_histories;
                return false;
            case BootstrapError::bad_client_type:
                logger.error("Bad client type (%1) in IDENT message", int(client_type)); // Throws
                error = ProtocolError::bad_client_file_ident; // FIXME: Introduce new protocol-level error
                                                              // `bad_client_type`.
                return false;
        }

        // Make sure there is no other session currently associcated with the
        // same client-side file
        if (Session* other_sess = m_server_file->get_identified_session(client_file_ident)) {
            SyncConnection& other_conn = other_sess->get_connection();
            // It is a protocol violation if the other session is associated
            // with the same connection
            if (&other_conn == &m_connection) {
                logger.error("Client file already bound in other session associated with "
                             "the same connection"); // Throws
                error = ProtocolError::bound_in_other_session;
                return false;
            }
            // When the other session is associated with a different connection
            // (`other_conn`), the clash may be due to the server not yet having
            // realized that the other connection has been closed by the
            // client. If so, the other connention is a "zombie". In the
            // interest of getting rid of zombie connections as fast as
            // possible, we shall assume that a clash with a session in another
            // connection is always due to that other connection being a
            // zombie. And when such a situation is detected, we want to close
            // the zombie connection immediately.
            auto log_level = Logger::Level::detail;
            other_conn.terminate(log_level,
                                 "Sync connection closed (superseded session)"); // Throws
        }

        logger.info("Bound to client file (client_file_ident=%1)", client_file_ident); // Throws

        send_log_message(util::Logger::Level::debug, util::format("Session %1 bound to client file ident %2",
                                                                  m_session_ident, client_file_ident));

        m_server_file->identify_session(this, client_file_ident); // Throws

        m_client_file_ident = client_file_ident;
        m_download_progress = download_progress;
        m_upload_threshold = upload_threshold;
        m_locked_server_version = locked_server_version;

        ServerImpl& server = m_connection.get_server();
        const Server::Config& config = server.get_config();
        m_disable_download = (config.disable_download_for.count(client_file_ident) != 0);

        if (REALM_UNLIKELY(config.session_bootstrap_callback)) {
            config.session_bootstrap_callback(m_server_file->get_virt_path(),
                                              client_file_ident); // Throws
        }

        // Protocol  state is now WaitForUnbind
        enlist_to_send();
        return true;
    }

    bool receive_upload_message(version_type progress_client_version, version_type progress_server_version,
                                version_type locked_server_version, const UploadChangesets& upload_changesets,
                                ProtocolError& error)
    {
        // Protocol state must be WaitForUnbind
        REALM_ASSERT(!m_send_ident_message);
        REALM_ASSERT(ident_message_received());
        REALM_ASSERT(!unbind_message_received());
        REALM_ASSERT(!error_occurred());
        REALM_ASSERT(!m_error_message_sent);

        logger.detail("Received: UPLOAD(progress_client_version=%1, progress_server_version=%2, "
                      "locked_server_version=%3, num_changesets=%4)",
                      progress_client_version, progress_server_version, locked_server_version,
                      upload_changesets.size()); // Throws

        // We are unable to reproduce the cursor object for the upload progress
        // when the protocol version is less than 29, because the client does
        // not provide the required information. When the protocol version is
        // less than 25, we can always get a consistent cursor by taking it from
        // the changeset that was uploaded last, but in protocol versions 25,
        // 26, 27, and 28, things are more complicated. Here, we receive new
        // values for `last_integrated_server_version` which we cannot afford to
        // ignore, but we do not know what client versions they correspond
        // to. Fortunately, we can produce a cursor that works, and is mutually
        // consistent with previous cursors, by simply bumping
        // `upload_progress.client_version` when
        // `upload_progress.last_intgerated_server_version` grows.
        //
        // To see that this scheme works, consider the last changeset, A, that
        // will have already been uploaded and integrated at the beginning of
        // the next session, and the first changeset, B, that follows A in the
        // client side history, and is not upload skippable (of local origin and
        // nonempty). We then need to show that A will be skipped, if uploaded
        // in the next session, but B will not.
        //
        // Let V be the client version produced by A, and let T be the value of
        // `upload_progress.client_version` as determined in this session, which
        // is used as threshold in the next session. Then we know that A is
        // skipped during the next session if V is less than, or equal to T. If
        // the protocol version is at least 29, the protocol requires that T is
        // greater than, or equal to V. If the protocol version is less than 25,
        // T will be equal to V. Finally, if the protocol version is 25, 26, 27,
        // or 28, we construct T such that it is always greater than, or equal
        // to V, so in all cases, A will be skipped during the next session.
        //
        // Let W be the client version on which B is based. We then know that B
        // will be retained if, and only if W is greater than, or equalto T. If
        // the protocol version is at least 29, we know that T is less than, or
        // equal to W, since B is not integrated until the next session. If the
        // protocol version is less tahn 25, we know that T is V. Since V must
        // be less than, or equal to W, we again know that T is less than, or
        // equal to W. Finally, if the protocol version is 25, 26, 27, or 28, we
        // construct T such that it is equal to V + N, where N is the number of
        // observed increments in `last_integrated_server_version` since the
        // client version prodiced by A. For each of these observed increments,
        // there must have been a distinct new client version, but all these
        // client versions must be less than, or equal to W, since B is not
        // integrated until the next session. Therefore, we know that T = V + N
        // is less than, or qual to W. So, in all cases, B will not skipped
        // during the next session.
        int protocol_version = m_connection.get_client_protocol_version();
        static_cast<void>(protocol_version); // No protocol diversion (yet)

        UploadCursor upload_progress;
        upload_progress = {progress_client_version, progress_server_version};

        // `upload_progress.client_version` must be nondecreasing across the
        // session.
        bool good_1 = (upload_progress.client_version >= m_upload_progress.client_version);
        if (REALM_UNLIKELY(!good_1)) {
            logger.error("Decreasing client version in upload progress (%1 < %2)", upload_progress.client_version,
                         m_upload_progress.client_version); // Throws
            error = ProtocolError::bad_client_version;
            return false;
        }
        // `upload_progress.last_integrated_server_version` must be a version
        // that the client can have heard about.
        bool good_2 = (upload_progress.last_integrated_server_version <= m_download_progress.server_version);
        if (REALM_UNLIKELY(!good_2)) {
            logger.error("Bad last integrated server version in upload progress (%1 > %2)",
                         upload_progress.last_integrated_server_version,
                         m_download_progress.server_version); // Throws
            error = ProtocolError::bad_server_version;
            return false;
        }

        // `upload_progress` must be consistent.
        if (REALM_UNLIKELY(!is_consistent(upload_progress))) {
            logger.error("Upload progress is inconsistent (%1, %2)", upload_progress.client_version,
                         upload_progress.last_integrated_server_version); // Throws
            error = ProtocolError::bad_server_version;
            return false;
        }
        // `upload_progress` and `m_upload_threshold` must be mutually
        // consistent.
        if (REALM_UNLIKELY(!are_mutually_consistent(upload_progress, m_upload_threshold))) {
            logger.error("Upload progress (%1, %2) is mutually inconsistent with "
                         "threshold (%3, %4)",
                         upload_progress.client_version, upload_progress.last_integrated_server_version,
                         m_upload_threshold.client_version,
                         m_upload_threshold.last_integrated_server_version); // Throws
            error = ProtocolError::bad_server_version;
            return false;
        }
        // `upload_progress` and `m_upload_progress` must be mutually
        // consistent.
        if (REALM_UNLIKELY(!are_mutually_consistent(upload_progress, m_upload_progress))) {
            logger.error("Upload progress (%1, %2) is mutually inconsistent with previous "
                         "upload progress (%3, %4)",
                         upload_progress.client_version, upload_progress.last_integrated_server_version,
                         m_upload_progress.client_version,
                         m_upload_progress.last_integrated_server_version); // Throws
            error = ProtocolError::bad_server_version;
            return false;
        }

        version_type locked_server_version_2 = locked_server_version;

        // `locked_server_version_2` must be nondecreasing over the lifetime of
        // the client-side file.
        if (REALM_UNLIKELY(locked_server_version_2 < m_locked_server_version)) {
            logger.error("Decreasing locked server version (%1 < %2)", locked_server_version_2,
                         m_locked_server_version); // Throws
            error = ProtocolError::bad_server_version;
            return false;
        }
        // `locked_server_version_2` must be a version that the client can have
        // heard about.
        if (REALM_UNLIKELY(locked_server_version_2 > m_download_progress.server_version)) {
            logger.error("Bad locked server version (%1 > %2)", locked_server_version_2,
                         m_download_progress.server_version); // Throws
            error = ProtocolError::bad_server_version;
            return false;
        }

        std::size_t num_previously_integrated_changesets = 0;
        if (!upload_changesets.empty()) {
            UploadCursor up = m_upload_progress;
            for (const ServerProtocol::UploadChangeset& uc : upload_changesets) {
                // `uc.upload_cursor.client_version` must be increasing across
                // all the changesets in this UPLOAD message, and all must be
                // greater than upload_progress.client_version of previous
                // UPLOAD message.
                if (REALM_UNLIKELY(uc.upload_cursor.client_version <= up.client_version)) {
                    logger.error("Nonincreasing client version in upload cursor of uploaded "
                                 "changeset (%1 <= %2)",
                                 uc.upload_cursor.client_version,
                                 up.client_version); // Throws
                    error = ProtocolError::bad_client_version;
                    return false;
                }
                // `uc.upload_progress` must be consistent.
                if (REALM_UNLIKELY(!is_consistent(uc.upload_cursor))) {
                    logger.error("Upload cursor of uploaded changeset is inconsistent (%1, %2)",
                                 uc.upload_cursor.client_version,
                                 uc.upload_cursor.last_integrated_server_version); // Throws
                    error = ProtocolError::bad_server_version;
                    return false;
                }
                // `uc.upload_progress` must be mutually consistent with
                // previous upload cursor.
                if (REALM_UNLIKELY(!are_mutually_consistent(uc.upload_cursor, up))) {
                    logger.error("Upload cursor of uploaded changeset (%1, %2) is mutually "
                                 "inconsistent with previous upload cursor (%3, %4)",
                                 uc.upload_cursor.client_version, uc.upload_cursor.last_integrated_server_version,
                                 up.client_version, up.last_integrated_server_version); // Throws
                    error = ProtocolError::bad_server_version;
                    return false;
                }
                // `uc.upload_progress` must be mutually consistent with
                // threshold, that is, for changesets that have not previously
                // been integrated, it is important that the specified value of
                // `last_integrated_server_version` is greater than, or equal to
                // the reciprocal history base version.
                bool consistent_with_threshold = are_mutually_consistent(uc.upload_cursor, m_upload_threshold);
                if (REALM_UNLIKELY(!consistent_with_threshold)) {
                    logger.error("Upload cursor of uploaded changeset (%1, %2) is mutually "
                                 "inconsistent with threshold (%3, %4)",
                                 uc.upload_cursor.client_version, uc.upload_cursor.last_integrated_server_version,
                                 m_upload_threshold.client_version,
                                 m_upload_threshold.last_integrated_server_version); // Throws
                    error = ProtocolError::bad_server_version;
                    return false;
                }
                bool previously_integrated = (uc.upload_cursor.client_version <= m_upload_threshold.client_version);
                if (previously_integrated)
                    ++num_previously_integrated_changesets;
                up = uc.upload_cursor;
            }
            // `upload_progress.client_version` must be greater than, or equal
            // to client versions produced by each of the changesets in this
            // UPLOAD message.
            if (REALM_UNLIKELY(up.client_version > upload_progress.client_version)) {
                logger.error("Upload progress less than client version produced by uploaded "
                             "changeset (%1 > %2)",
                             up.client_version,
                             upload_progress.client_version); // Throws
                error = ProtocolError::bad_client_version;
                return false;
            }
            // The upload cursor of last uploaded changeset must be mutually
            // consistent with the reported upload progress.
            if (REALM_UNLIKELY(!are_mutually_consistent(up, upload_progress))) {
                logger.error("Upload cursor (%1, %2) of last uploaded changeset is mutually "
                             "inconsistent with upload progress (%3, %4)",
                             up.client_version, up.last_integrated_server_version, upload_progress.client_version,
                             upload_progress.last_integrated_server_version); // Throws
                error = ProtocolError::bad_server_version;
                return false;
            }
        }

        // FIXME: Part of a very poor man's substitute for a proper backpressure
        // scheme.
        if (REALM_UNLIKELY(!m_server_file->can_add_changesets_from_downstream())) {
            logger.debug("Terminating uploading session because buffer is full"); // Throws
            // Using this exact error code, because it causes `try_again` flag
            // to be set to true, which causes the client to wait for about 5
            // minuites before trying to connect again.
            error = ProtocolError::connection_closed;
            return false;
        }

        m_upload_progress = upload_progress;

        bool have_real_upload_progress = (upload_progress.client_version > m_upload_threshold.client_version);
        bool bump_locked_server_version = (locked_server_version_2 > m_locked_server_version);

        std::size_t num_changesets_to_integrate = upload_changesets.size() - num_previously_integrated_changesets;
        REALM_ASSERT(have_real_upload_progress || num_changesets_to_integrate == 0);

        bool have_anything_to_do = (have_real_upload_progress || bump_locked_server_version);
        if (!have_anything_to_do)
            return true;

        if (!have_real_upload_progress)
            upload_progress = m_upload_threshold;

        if (num_previously_integrated_changesets > 0) {
            logger.detail("Ignoring %1 previously integrated changesets",
                          num_previously_integrated_changesets); // Throws
        }
        if (num_changesets_to_integrate > 0) {
            logger.detail("Initiate integration of %1 remote changesets",
                          num_changesets_to_integrate); // Throws
        }

        REALM_ASSERT(m_server_file);
        ServerFile& file = *m_server_file;
        std::size_t offset = num_previously_integrated_changesets;
        file.add_changesets_from_downstream(m_client_file_ident, upload_progress, locked_server_version_2,
                                            upload_changesets.data() + offset, num_changesets_to_integrate); // Throws

        m_locked_server_version = locked_server_version_2;
        return true;
    }

    bool receive_mark_message(request_ident_type request_ident, ProtocolError&)
    {
        // Protocol state must be WaitForUnbind
        REALM_ASSERT(!m_send_ident_message);
        REALM_ASSERT(ident_message_received());
        REALM_ASSERT(!unbind_message_received());
        REALM_ASSERT(!error_occurred());
        REALM_ASSERT(!m_error_message_sent);

        logger.debug("Received: MARK(request_ident=%1)", request_ident); // Throws

        m_download_completion_request = request_ident;

        ensure_enlisted_to_send();
        return true;
    }

    // Returns true if the deactivation process has been completed, at which
    // point the caller (SyncConnection::receive_unbind_message()) should
    // terminate the session.
    //
    // CAUTION: This function may commit suicide!
    void receive_unbind_message()
    {
        // Protocol state may be anything but SendUnbound
        REALM_ASSERT(!m_unbind_message_received);

        logger.detail("Received: UNBIND"); // Throws

        detach_from_server_file();
        m_unbind_message_received = true;

        // Detect completion of the deactivation process
        if (m_error_message_sent) {
            // Deactivation process completed
            terminate(); // Throws
            m_connection.discard_session(m_session_ident);
            // This session is now destroyed!
            return;
        }

        // Protocol state is now SendUnbound
        ensure_enlisted_to_send();
    }

    void receive_error_message(session_ident_type, int, std::string_view)
    {
        REALM_ASSERT(!m_unbind_message_received);

        logger.detail("Received: ERROR"); // Throws
    }

private:
    SyncConnection& m_connection;

    const session_ident_type m_session_ident;

    // Not null if, and only if this session is in
    // m_connection.m_sessions_enlisted_to_send.
    Session* m_next = nullptr;

    // Becomes nonnull when the BIND message is received, if no error occurs. Is
    // reset to null when the deactivation process is initiated, either when the
    // UNBIND message is recieved, or when initiate_deactivation() is called.
    util::bind_ptr<ServerFile> m_server_file;

    bool m_disable_download = false;
    bool m_is_subserver = false;

    using file_ident_request_type = ServerFile::file_ident_request_type;

    // When nonzero, this session has an outstanding request for a client file
    // identifier.
    file_ident_request_type m_file_ident_request = 0;

    // Payload for next outgoing ALLOC message.
    SaltedFileIdent m_allocated_file_ident = {0, 0};

    // Zero until the session receives an IDENT message from the client.
    file_ident_type m_client_file_ident = 0;

    // Zero until initiate_deactivation() is called.
    ProtocolError m_error_code = {};

    // The current point of progression of the download process. Set to (<server
    // version>, <client version>) of the IDENT message when the IDENT message
    // is received. At the time of return from continue_history_scan(), it
    // points to the latest server version such that all preceding changesets in
    // the server-side history have been downloaded, are currently being
    // downloaded, or are *download excluded*.
    DownloadCursor m_download_progress = {0, 0};

    request_ident_type m_download_completion_request = 0;

    // Records the progress of the upload process. Used to check that the client
    // uploads changesets in order. Also, when m_upload_progress >
    // m_upload_threshold, m_upload_progress works as a cache of the persisted
    // version of the upload progress.
    UploadCursor m_upload_progress = {0, 0};

    // Initialized on reception of the IDENT message. Specifies the actual
    // upload progress (as recorded on the server-side) at the beginning of the
    // session, and it remains fixed throughout the session.
    //
    // m_upload_threshold includes the progress resulting from the received
    // changesets that have not yet been integrated (only relevant for
    // synchronous backup).
    UploadCursor m_upload_threshold = {0, 0};

    // Works partially as a cache of the persisted value, and partially as a way
    // of checking that the client respects that it can never decrease.
    version_type m_locked_server_version = 0;

    bool m_send_ident_message = false;
    bool m_unbind_message_received = false;
    bool m_error_message_sent = false;

    /// m_one_download_message_sent denotes whether at least one DOWNLOAD message
    /// has been sent in the current session. The variable is used to ensure
    /// that a DOWNLOAD message is always sent in a session. The received
    /// DOWNLOAD message is needed by the client to ensure that its current
    /// download progress is up to date.
    bool m_one_download_message_sent = false;

    static std::string make_logger_prefix(session_ident_type session_ident)
    {
        std::ostringstream out;
        out.imbue(std::locale::classic());
        out << "Session[" << session_ident << "]: "; // Throws
        return out.str();                            // Throws
    }

    // Scan the history for changesets to be downloaded.
    // If the history is longer than the end point of the previous scan,
    // a DOWNLOAD message will be sent.
    // A MARK message is sent if no DOWNLOAD message is sent, and the client has
    // requested to be notified about download completion.
    // In case neither a DOWNLOAD nor a MARK is sent, no message is sent.
    //
    // This function may lead to the destruction of the session object
    // (suicide).
    void continue_history_scan()
    {
        // Protocol state must be WaitForUnbind
        REALM_ASSERT(!m_send_ident_message);
        REALM_ASSERT(ident_message_received());
        REALM_ASSERT(!unbind_message_received());
        REALM_ASSERT(!error_occurred());
        REALM_ASSERT(!m_error_message_sent);
        REALM_ASSERT(!is_enlisted_to_send());

        SaltedVersion last_server_version = m_server_file->get_salted_sync_version();
        REALM_ASSERT(last_server_version.version >= m_download_progress.server_version);

        ServerImpl& server = m_connection.get_server();
        const Server::Config& config = server.get_config();
        if (REALM_UNLIKELY(m_disable_download))
            return;

        bool have_more_to_scan =
            (last_server_version.version > m_download_progress.server_version || !m_one_download_message_sent);
        if (have_more_to_scan) {
            m_server_file->register_client_access(m_client_file_ident);     // Throws
            const ServerHistory& history = m_server_file->access().history; // Throws
            const char* body;
            std::size_t uncompressed_body_size;
            std::size_t compressed_body_size = 0;
            bool body_is_compressed = false;
            version_type end_version = last_server_version.version;
            DownloadCursor download_progress;
            UploadCursor upload_progress = {0, 0};
            std::uint_fast64_t downloadable_bytes = 0;
            std::size_t num_changesets;
            std::size_t accum_original_size;
            std::size_t accum_compacted_size;
            ServerProtocol& protocol = get_server_protocol();
            bool disable_download_compaction = config.disable_download_compaction;
            bool enable_cache = (config.enable_download_bootstrap_cache && m_download_progress.server_version == 0 &&
                                 m_upload_progress.client_version == 0 && m_upload_threshold.client_version == 0);
            DownloadCache& cache = m_server_file->get_download_cache();
            bool fetch_from_cache = (enable_cache && cache.body && end_version == cache.end_version);
            if (fetch_from_cache) {
                body = cache.body.get();
                uncompressed_body_size = cache.uncompressed_body_size;
                compressed_body_size = cache.compressed_body_size;
                body_is_compressed = cache.body_is_compressed;
                download_progress = cache.download_progress;
                downloadable_bytes = cache.downloadable_bytes;
                num_changesets = cache.num_changesets;
                accum_original_size = cache.accum_original_size;
                accum_compacted_size = cache.accum_compacted_size;
            }
            else {
                // Discard the old cached DOWNLOAD body before generating a new
                // one to be cached. This can make a big difference because the
                // size of that body can be very large (10GiB has been seen in a
                // real-world case).
                if (enable_cache)
                    cache.body = {};

                OutputBuffer& out = server.get_misc_buffers().download_message;
                out.reset();
                download_progress = m_download_progress;
                auto fetch_and_compress = [&](std::size_t max_download_size) {
                    DownloadHistoryEntryHandler handler{protocol, out, logger};
                    std::uint_fast64_t cumulative_byte_size_current;
                    std::uint_fast64_t cumulative_byte_size_total;
                    bool not_expired = history.fetch_download_info(
                        m_client_file_ident, download_progress, end_version, upload_progress, handler,
                        cumulative_byte_size_current, cumulative_byte_size_total, disable_download_compaction,
                        max_download_size); // Throws
                    REALM_ASSERT(upload_progress.client_version >= download_progress.last_integrated_client_version);
                    SyncConnection& conn = get_connection();
                    if (REALM_UNLIKELY(!not_expired)) {
                        logger.debug("History scanning failed: Client file entry "
                                     "expired during session"); // Throws
                        conn.protocol_error(ProtocolError::client_file_expired, this);
                        // Session object may have been destroyed at this point
                        // (suicide).
                        return false;
                    }

                    downloadable_bytes = cumulative_byte_size_total - cumulative_byte_size_current;
                    uncompressed_body_size = out.size();
                    BinaryData uncompressed = {out.data(), uncompressed_body_size};
                    body = uncompressed.data();
                    std::size_t max_uncompressed = 1024;
                    if (uncompressed.size() > max_uncompressed) {
                        compression::CompressMemoryArena& arena = server.get_compress_memory_arena();
                        std::vector<char>& buffer = server.get_misc_buffers().compress;
                        compression::allocate_and_compress(arena, uncompressed, buffer); // Throws
                        if (buffer.size() < uncompressed.size()) {
                            body = buffer.data();
                            compressed_body_size = buffer.size();
                            body_is_compressed = true;
                        }
                    }
                    num_changesets = handler.num_changesets;
                    accum_original_size = handler.accum_original_size;
                    accum_compacted_size = handler.accum_compacted_size;
                    return true;
                };
                if (enable_cache) {
                    std::size_t max_download_size = std::numeric_limits<size_t>::max();
                    if (!fetch_and_compress(max_download_size)) { // Throws
                        // Session object may have been destroyed at this point
                        // (suicide).
                        return;
                    }
                    REALM_ASSERT(upload_progress.client_version == 0);
                    std::size_t body_size = (body_is_compressed ? compressed_body_size : uncompressed_body_size);
                    cache.body = std::make_unique<char[]>(body_size); // Throws
                    std::copy(body, body + body_size, cache.body.get());
                    cache.uncompressed_body_size = uncompressed_body_size;
                    cache.compressed_body_size = compressed_body_size;
                    cache.body_is_compressed = body_is_compressed;
                    cache.end_version = end_version;
                    cache.download_progress = download_progress;
                    cache.downloadable_bytes = downloadable_bytes;
                    cache.num_changesets = num_changesets;
                    cache.accum_original_size = accum_original_size;
                    cache.accum_compacted_size = accum_compacted_size;
                }
                else {
                    std::size_t max_download_size = config.max_download_size;
                    if (!fetch_and_compress(max_download_size)) { // Throws
                        // Session object may have been destroyed at this point
                        // (suicide).
                        return;
                    }
                }
            }

            OutputBuffer& out = m_connection.get_output_buffer();
            protocol.make_download_message(
                m_connection.get_client_protocol_version(), out, m_session_ident, download_progress.server_version,
                download_progress.last_integrated_client_version, last_server_version.version,
                last_server_version.salt, upload_progress.client_version,
                upload_progress.last_integrated_server_version, downloadable_bytes, num_changesets, body,
                uncompressed_body_size, compressed_body_size, body_is_compressed, logger); // Throws

            if (!disable_download_compaction) {
                std::size_t saved = accum_original_size - accum_compacted_size;
                double saved_2 = (accum_original_size == 0 ? 0 : std::round(saved * 100.0 / accum_original_size));
                logger.detail("Download compaction: Saved %1 bytes (%2%%)", saved, saved_2); // Throws
            }

            m_download_progress = download_progress;
            logger.debug("Setting of m_download_progress.server_version = %1",
                         m_download_progress.server_version); // Throws
            send_download_message();
            m_one_download_message_sent = true;

            enlist_to_send();
        }
        else if (m_download_completion_request) {
            // Send a MARK message
            request_ident_type request_ident = m_download_completion_request;
            send_mark_message(request_ident);  // Throws
            m_download_completion_request = 0; // Request handled
            enlist_to_send();
        }
    }

    void send_ident_message()
    {
        // Protocol state must be SendIdent
        REALM_ASSERT(!need_client_file_ident());
        REALM_ASSERT(m_send_ident_message);
        REALM_ASSERT(!ident_message_received());
        REALM_ASSERT(!unbind_message_received());
        REALM_ASSERT(!error_occurred());
        REALM_ASSERT(!m_error_message_sent);

        REALM_ASSERT(m_allocated_file_ident.ident != 0);

        file_ident_type client_file_ident = m_allocated_file_ident.ident;
        salt_type client_file_ident_salt = m_allocated_file_ident.salt;

        logger.debug("Sending: IDENT(client_file_ident=%1, client_file_ident_salt=%2)", client_file_ident,
                     client_file_ident_salt); // Throws

        ServerProtocol& protocol = get_server_protocol();
        OutputBuffer& out = m_connection.get_output_buffer();
        int protocol_version = m_connection.get_client_protocol_version();
        protocol.make_ident_message(protocol_version, out, m_session_ident, client_file_ident,
                                    client_file_ident_salt); // Throws
        m_connection.initiate_write_output_buffer();         // Throws

        m_allocated_file_ident.ident = 0; // Consumed
        m_send_ident_message = false;
        // Protocol state is now WaitForStateRequest or WaitForIdent
    }

    void send_download_message()
    {
        m_connection.initiate_write_output_buffer(); // Throws
    }

    void send_mark_message(request_ident_type request_ident)
    {
        logger.debug("Sending: MARK(request_ident=%1)", request_ident); // Throws

        ServerProtocol& protocol = get_server_protocol();
        OutputBuffer& out = m_connection.get_output_buffer();
        protocol.make_mark_message(out, m_session_ident, request_ident); // Throws
        m_connection.initiate_write_output_buffer();                     // Throws
    }

    void send_alloc_message()
    {
        // Protocol state must be WaitForUnbind
        REALM_ASSERT(!m_send_ident_message);
        REALM_ASSERT(ident_message_received());
        REALM_ASSERT(!unbind_message_received());
        REALM_ASSERT(!error_occurred());
        REALM_ASSERT(!m_error_message_sent);

        REALM_ASSERT(m_allocated_file_ident.ident != 0);

        // Relayed allocations are only allowed from protocol version 23 (old protocol).
        REALM_ASSERT(false);

        file_ident_type file_ident = m_allocated_file_ident.ident;

        logger.debug("Sending: ALLOC(file_ident=%1)", file_ident); // Throws

        ServerProtocol& protocol = get_server_protocol();
        OutputBuffer& out = m_connection.get_output_buffer();
        protocol.make_alloc_message(out, m_session_ident, file_ident); // Throws
        m_connection.initiate_write_output_buffer();                   // Throws

        m_allocated_file_ident.ident = 0; // Consumed

        // Other messages may be waiting to be sent.
        enlist_to_send();
    }

    void send_unbound_message()
    {
        // Protocol state must be SendUnbound
        REALM_ASSERT(unbind_message_received());
        REALM_ASSERT(!m_error_message_sent);

        logger.debug("Sending: UNBOUND"); // Throws

        ServerProtocol& protocol = get_server_protocol();
        OutputBuffer& out = m_connection.get_output_buffer();
        protocol.make_unbound_message(out, m_session_ident); // Throws
        m_connection.initiate_write_output_buffer();         // Throws
    }

    void send_error_message()
    {
        // Protocol state must be SendError
        REALM_ASSERT(!unbind_message_received());
        REALM_ASSERT(error_occurred());
        REALM_ASSERT(!m_error_message_sent);

        REALM_ASSERT(is_session_level_error(m_error_code));

        ProtocolError error_code = m_error_code;
        const char* message = get_protocol_error_message(int(error_code));
        std::size_t message_size = std::strlen(message);
        bool try_again = determine_try_again(error_code);

        logger.detail("Sending: ERROR(error_code=%1, message_size=%2, try_again=%3)", int(error_code), message_size,
                      try_again); // Throws

        ServerProtocol& protocol = get_server_protocol();
        OutputBuffer& out = m_connection.get_output_buffer();
        int protocol_version = m_connection.get_client_protocol_version();
        protocol.make_error_message(protocol_version, out, error_code, message, message_size, try_again,
                                    m_session_ident); // Throws
        m_connection.initiate_write_output_buffer();  // Throws

        m_error_message_sent = true;
        // Protocol state is now WaitForUnbindErr
    }

    void send_log_message(util::Logger::Level level, const std::string&& message)
    {
        if (m_connection.get_client_protocol_version() < SyncConnection::SERVER_LOG_PROTOCOL_VERSION) {
            return logger.log(level, message.c_str());
        }

        m_connection.send_log_message(level, std::move(message), m_session_ident);
    }

    // Idempotent
    void detach_from_server_file() noexcept
    {
        if (!m_server_file)
            return;
        ServerFile& file = *m_server_file;
        if (ident_message_received()) {
            file.remove_identified_session(m_client_file_ident);
        }
        else {
            file.remove_unidentified_session(this);
        }
        if (m_file_ident_request != 0)
            file.cancel_file_ident_request(m_file_ident_request);
        m_server_file.reset();
    }

    friend class SessionQueue;
};


// ============================ SessionQueue implementation ============================

void SessionQueue::push_back(Session* sess) noexcept
{
    REALM_ASSERT(!sess->m_next);
    if (m_back) {
        sess->m_next = m_back->m_next;
        m_back->m_next = sess;
    }
    else {
        sess->m_next = sess;
    }
    m_back = sess;
}


Session* SessionQueue::pop_front() noexcept
{
    Session* sess = nullptr;
    if (m_back) {
        sess = m_back->m_next;
        if (sess != m_back) {
            m_back->m_next = sess->m_next;
        }
        else {
            m_back = nullptr;
        }
        sess->m_next = nullptr;
    }
    return sess;
}


void SessionQueue::clear() noexcept
{
    if (m_back) {
        Session* sess = m_back;
        for (;;) {
            Session* next = sess->m_next;
            sess->m_next = nullptr;
            if (next == m_back)
                break;
            sess = next;
        }
        m_back = nullptr;
    }
}


// ============================ ServerFile implementation ============================

ServerFile::ServerFile(ServerImpl& server, ServerFileAccessCache& cache, const std::string& virt_path,
                       std::string real_path, bool disable_sync_to_disk)
    : logger{util::LogCategory::server, "ServerFile[" + virt_path + "]: ", server.logger_ptr}               // Throws
    , wlogger{util::LogCategory::server, "ServerFile[" + virt_path + "]: ", server.get_worker().logger_ptr} // Throws
    , m_server{server}
    , m_file{cache, real_path, virt_path, false, disable_sync_to_disk} // Throws
    , m_worker_file{server.get_worker().get_file_access_cache(), real_path, virt_path, true, disable_sync_to_disk}
{
}


ServerFile::~ServerFile() noexcept
{
    REALM_ASSERT(m_unidentified_sessions.empty());
    REALM_ASSERT(m_identified_sessions.empty());
    REALM_ASSERT(m_file_ident_request == 0);
}


void ServerFile::initialize()
{
    const ServerHistory& history = access().history; // Throws
    file_ident_type partial_file_ident = 0;
    version_type partial_progress_reference_version = 0;
    bool has_upstream_sync_status;
    history.get_status(m_version_info, has_upstream_sync_status, partial_file_ident,
                       partial_progress_reference_version); // Throws
    REALM_ASSERT(!has_upstream_sync_status);
    REALM_ASSERT(partial_file_ident == 0);
}


void ServerFile::activate() {}


// This function must be called only after a completed invocation of
// initialize(). Both functinos must only ever be called by the network event
// loop thread.
void ServerFile::register_client_access(file_ident_type) {}


auto ServerFile::request_file_ident(FileIdentReceiver& receiver, file_ident_type proxy_file, ClientType client_type)
    -> file_ident_request_type
{
    auto request = ++m_last_file_ident_request;
    m_file_ident_requests[request] = {&receiver, proxy_file, client_type}; // Throws

    on_work_added(); // Throws
    return request;
}


void ServerFile::cancel_file_ident_request(file_ident_request_type request) noexcept
{
    auto i = m_file_ident_requests.find(request);
    REALM_ASSERT(i != m_file_ident_requests.end());
    FileIdentRequestInfo& info = i->second;
    REALM_ASSERT(info.receiver);
    info.receiver = nullptr;
}


void ServerFile::add_unidentified_session(Session* sess)
{
    REALM_ASSERT(m_unidentified_sessions.count(sess) == 0);
    m_unidentified_sessions.insert(sess); // Throws
}


void ServerFile::identify_session(Session* sess, file_ident_type client_file_ident)
{
    REALM_ASSERT(m_unidentified_sessions.count(sess) == 1);
    REALM_ASSERT(m_identified_sessions.count(client_file_ident) == 0);

    m_identified_sessions[client_file_ident] = sess; // Throws
    m_unidentified_sessions.erase(sess);
}


void ServerFile::remove_unidentified_session(Session* sess) noexcept
{
    REALM_ASSERT(m_unidentified_sessions.count(sess) == 1);
    m_unidentified_sessions.erase(sess);
}


void ServerFile::remove_identified_session(file_ident_type client_file_ident) noexcept
{
    REALM_ASSERT(m_identified_sessions.count(client_file_ident) == 1);
    m_identified_sessions.erase(client_file_ident);
}


Session* ServerFile::get_identified_session(file_ident_type client_file_ident) noexcept
{
    auto i = m_identified_sessions.find(client_file_ident);
    if (i == m_identified_sessions.end())
        return nullptr;
    return i->second;
}

bool ServerFile::can_add_changesets_from_downstream() const noexcept
{
    return (m_blocked_changesets_from_downstream_byte_size < m_server.get_max_upload_backlog());
}


void ServerFile::add_changesets_from_downstream(file_ident_type client_file_ident, UploadCursor upload_progress,
                                                version_type locked_server_version, const UploadChangeset* changesets,
                                                std::size_t num_changesets)
{
    register_client_access(client_file_ident); // Throws

    bool dirty = false;

    IntegratableChangesetList& list = m_changesets_from_downstream[client_file_ident]; // Throws
    std::size_t num_bytes = 0;
    for (std::size_t i = 0; i < num_changesets; ++i) {
        const UploadChangeset& uc = changesets[i];
        auto& changesets = list.changesets;
        changesets.emplace_back(client_file_ident, uc.origin_timestamp, uc.origin_file_ident, uc.upload_cursor,
                                uc.changeset); // Throws
        num_bytes += uc.changeset.size();
        dirty = true;
    }

    REALM_ASSERT(upload_progress.client_version >= list.upload_progress.client_version);
    REALM_ASSERT(are_mutually_consistent(upload_progress, list.upload_progress));
    if (upload_progress.client_version > list.upload_progress.client_version) {
        list.upload_progress = upload_progress;
        dirty = true;
    }

    REALM_ASSERT(locked_server_version >= list.locked_server_version);
    if (locked_server_version > list.locked_server_version) {
        list.locked_server_version = locked_server_version;
        dirty = true;
    }

    if (REALM_LIKELY(dirty)) {
        if (num_changesets > 0) {
            on_changesets_from_downstream_added(num_changesets, num_bytes); // Throws
        }
        else {
            on_work_added(); // Throws
        }
    }
}


BootstrapError ServerFile::bootstrap_client_session(SaltedFileIdent client_file_ident,
                                                    DownloadCursor download_progress, SaltedVersion server_version,
                                                    ClientType client_type, UploadCursor& upload_progress,
                                                    version_type& locked_server_version, Logger& logger)
{
    // The Realm file may contain a later snapshot than the one reflected by
    // `m_sync_version`, but if so, the client cannot "legally" know about it.
    if (server_version.version > m_version_info.sync_version.version)
        return BootstrapError::bad_server_version;

    const ServerHistory& hist = access().history; // Throws
    BootstrapError error = hist.bootstrap_client_session(client_file_ident, download_progress, server_version,
                                                         client_type, upload_progress, locked_server_version,
                                                         logger); // Throws

    // FIXME: Rather than taking previously buffered changesets from the same
    // client file into account when determining the upload progress, and then
    // allowing for an error during the integration of those changesets to be
    // reported to, and terminate the new session, consider to instead postpone
    // the bootstrapping of the new session until all previously buffered
    // changesets from same client file have been fully processed.

    if (error == BootstrapError::no_error) {
        register_client_access(client_file_ident.ident); // Throws

        // If upload, or releaseing of server versions progressed further during
        // previous sessions than the persisted points, take that into account
        auto i = m_work.changesets_from_downstream.find(client_file_ident.ident);
        if (i != m_work.changesets_from_downstream.end()) {
            const IntegratableChangesetList& list = i->second;
            REALM_ASSERT(list.upload_progress.client_version >= upload_progress.client_version);
            upload_progress = list.upload_progress;
            REALM_ASSERT(list.locked_server_version >= locked_server_version);
            locked_server_version = list.locked_server_version;
        }
        auto j = m_changesets_from_downstream.find(client_file_ident.ident);
        if (j != m_changesets_from_downstream.end()) {
            const IntegratableChangesetList& list = j->second;
            REALM_ASSERT(list.upload_progress.client_version >= upload_progress.client_version);
            upload_progress = list.upload_progress;
            REALM_ASSERT(list.locked_server_version >= locked_server_version);
            locked_server_version = list.locked_server_version;
        }
    }

    return error;
}

// NOTE: This function is executed by the worker thread
void ServerFile::worker_process_work_unit(WorkerState& state)
{
    SteadyTimePoint start_time = steady_clock_now();
    milliseconds_type parallel_time = 0;

    Work& work = m_work;
    wlogger.debug("Work unit execution started"); // Throws

    if (work.has_primary_work) {
        if (REALM_UNLIKELY(!m_work.file_ident_alloc_slots.empty()))
            worker_allocate_file_identifiers(); // Throws

        if (!m_work.changesets_from_downstream.empty())
            worker_integrate_changes_from_downstream(state); // Throws
    }

    wlogger.debug("Work unit execution completed"); // Throws

    milliseconds_type time = steady_duration(start_time);
    milliseconds_type seq_time = time - parallel_time;
    m_server.m_seq_time.fetch_add(seq_time, std::memory_order_relaxed);
    m_server.m_par_time.fetch_add(parallel_time, std::memory_order_relaxed);

    // Pass control back to the network event loop thread
    network::Service& service = m_server.get_service();
    service.post([this](Status) {
        // FIXME: The safety of capturing `this` here, relies on the fact
        // that ServerFile objects currently are not destroyed until the
        // server object is destroyed.
        group_postprocess_stage_1(); // Throws
        // Suicide may have happened at this point
    }); // Throws
}


void ServerFile::on_changesets_from_downstream_added(std::size_t num_changesets, std::size_t num_bytes)
{
    m_num_changesets_from_downstream += num_changesets;

    if (num_bytes > 0) {
        m_blocked_changesets_from_downstream_byte_size += num_bytes;
        get_server().inc_byte_size_for_pending_downstream_changesets(num_bytes); // Throws
    }

    on_work_added(); // Throws
}


void ServerFile::on_work_added()
{
    if (m_has_blocked_work)
        return;
    m_has_blocked_work = true;
    // Reference file
    if (m_has_work_in_progress)
        return;
    group_unblock_work(); // Throws
}


void ServerFile::group_unblock_work()
{
    REALM_ASSERT(!m_has_work_in_progress);
    if (REALM_LIKELY(!m_server.is_sync_stopped())) {
        unblock_work(); // Throws
        const Work& work = m_work;
        if (REALM_LIKELY(work.has_primary_work)) {
            logger.trace("Work unit unblocked"); // Throws
            m_has_work_in_progress = true;
            Worker& worker = m_server.get_worker();
            worker.enqueue(this); // Throws
        }
    }
}


void ServerFile::unblock_work()
{
    REALM_ASSERT(m_has_blocked_work);

    m_work.reset();

    // Discard requests for file identifiers whose receiver is no longer
    // waiting.
    {
        auto i = m_file_ident_requests.begin();
        auto end = m_file_ident_requests.end();
        while (i != end) {
            auto j = i++;
            const FileIdentRequestInfo& info = j->second;
            if (!info.receiver)
                m_file_ident_requests.erase(j);
        }
    }
    std::size_t n = m_file_ident_requests.size();
    if (n > 0) {
        m_work.file_ident_alloc_slots.resize(n); // Throws
        std::size_t i = 0;
        for (const auto& pair : m_file_ident_requests) {
            const FileIdentRequestInfo& info = pair.second;
            FileIdentAllocSlot& slot = m_work.file_ident_alloc_slots[i];
            slot.proxy_file = info.proxy_file;
            slot.client_type = info.client_type;
            ++i;
        }
        m_work.has_primary_work = true;
    }

    // FIXME: `ServerFile::m_changesets_from_downstream` and
    // `Work::changesets_from_downstream` should be renamed to something else,
    // as it may contain kinds of data other than changesets.

    using std::swap;
    swap(m_changesets_from_downstream, m_work.changesets_from_downstream);
    m_work.have_changesets_from_downstream = (m_num_changesets_from_downstream > 0);
    bool has_changesets = !m_work.changesets_from_downstream.empty();
    if (has_changesets) {
        m_work.has_primary_work = true;
    }

    // Keep track of the size of pending changesets
    REALM_ASSERT(m_unblocked_changesets_from_downstream_byte_size == 0);
    m_unblocked_changesets_from_downstream_byte_size = m_blocked_changesets_from_downstream_byte_size;
    m_blocked_changesets_from_downstream_byte_size = 0;

    m_num_changesets_from_downstream = 0;
    m_has_blocked_work = false;
}


void ServerFile::resume_download() noexcept
{
    for (const auto& entry : m_identified_sessions) {
        Session& sess = *entry.second;
        sess.ensure_enlisted_to_send();
    }
}


void ServerFile::recognize_external_change()
{
    VersionInfo prev_version_info = m_version_info;
    const ServerHistory& history = access().history;       // Throws
    bool has_upstream_status;                              // Dummy
    sync::file_ident_type partial_file_ident;              // Dummy
    sync::version_type partial_progress_reference_version; // Dummy
    history.get_status(m_version_info, has_upstream_status, partial_file_ident,
                       partial_progress_reference_version); // Throws

    REALM_ASSERT(m_version_info.realm_version >= prev_version_info.realm_version);
    REALM_ASSERT(m_version_info.sync_version.version >= prev_version_info.sync_version.version);
    if (m_version_info.sync_version.version > prev_version_info.sync_version.version) {
        REALM_ASSERT(m_version_info.realm_version > prev_version_info.realm_version);
        resume_download();
    }
}


// NOTE: This function is executed by the worker thread
void ServerFile::worker_allocate_file_identifiers()
{
    Work& work = m_work;
    REALM_ASSERT(!work.file_ident_alloc_slots.empty());
    ServerHistory& hist = worker_access().history;                                      // Throws
    hist.allocate_file_identifiers(m_work.file_ident_alloc_slots, m_work.version_info); // Throws
    m_work.produced_new_realm_version = true;
}


// Returns true when, and only when this function produces a new sync version
// (adds a new entry to the sync history).
//
// NOTE: This function is executed by the worker thread
bool ServerFile::worker_integrate_changes_from_downstream(WorkerState& state)
{
    REALM_ASSERT(!m_work.changesets_from_downstream.empty());

    std::unique_ptr<ServerHistory> hist_ptr;
    DBRef sg_ptr;
    ServerHistory& hist = get_client_file_history(state, hist_ptr, sg_ptr);
    bool backup_whole_realm = false;
    bool produced_new_realm_version = hist.integrate_client_changesets(
        m_work.changesets_from_downstream, m_work.version_info, backup_whole_realm, m_work.integration_result,
        wlogger); // Throws
    bool produced_new_sync_version = !m_work.integration_result.integrated_changesets.empty();
    REALM_ASSERT(!produced_new_sync_version || produced_new_realm_version);
    if (produced_new_realm_version) {
        m_work.produced_new_realm_version = true;
        if (produced_new_sync_version) {
            m_work.produced_new_sync_version = true;
        }
    }
    return produced_new_sync_version;
}

ServerHistory& ServerFile::get_client_file_history(WorkerState& state, std::unique_ptr<ServerHistory>& hist_ptr,
                                                   DBRef& sg_ptr)
{
    if (state.use_file_cache)
        return worker_access().history; // Throws
    const std::string& path = m_worker_file.realm_path;
    hist_ptr = m_server.make_history_for_path();                   // Throws
    DBOptions options = m_worker_file.make_shared_group_options(); // Throws
    sg_ptr = DB::create(*hist_ptr, path, options);                 // Throws
    sg_ptr->claim_sync_agent();                                    // Throws
    return *hist_ptr;                                              // Throws
}


// When worker thread finishes work unit.
void ServerFile::group_postprocess_stage_1()
{
    REALM_ASSERT(m_has_work_in_progress);

    group_finalize_work_stage_1(); // Throws
    group_finalize_work_stage_2(); // Throws
    group_postprocess_stage_2();   // Throws
}


void ServerFile::group_postprocess_stage_2()
{
    REALM_ASSERT(m_has_work_in_progress);
    group_postprocess_stage_3(); // Throws
    // Suicide may have happened at this point
}


// When all files, including the reference file, have been backed up.
void ServerFile::group_postprocess_stage_3()
{
    REALM_ASSERT(m_has_work_in_progress);
    m_has_work_in_progress = false;

    logger.trace("Work unit postprocessing complete"); // Throws
    if (m_has_blocked_work)
        group_unblock_work(); // Throws
}


void ServerFile::finalize_work_stage_1()
{
    if (m_unblocked_changesets_from_downstream_byte_size > 0) {
        // Report the byte size of completed downstream changesets.
        std::size_t byte_size = m_unblocked_changesets_from_downstream_byte_size;
        get_server().dec_byte_size_for_pending_downstream_changesets(byte_size); // Throws
        m_unblocked_changesets_from_downstream_byte_size = 0;
    }

    // Deal with errors (bad changesets) pertaining to downstream clients
    std::size_t num_changesets_removed = 0;
    std::size_t num_bytes_removed = 0;
    for (const auto& entry : m_work.integration_result.excluded_client_files) {
        file_ident_type client_file_ident = entry.first;
        ExtendedIntegrationError error = entry.second;
        ProtocolError error_2 = ProtocolError::other_session_error;
        switch (error) {
            case ExtendedIntegrationError::client_file_expired:
                logger.debug("Changeset integration failed: Client file entry "
                             "expired during session"); // Throws
                error_2 = ProtocolError::client_file_expired;
                break;
            case ExtendedIntegrationError::bad_origin_file_ident:
                error_2 = ProtocolError::bad_origin_file_ident;
                break;
            case ExtendedIntegrationError::bad_changeset:
                error_2 = ProtocolError::bad_changeset;
                break;
        }
        auto i = m_identified_sessions.find(client_file_ident);
        if (i != m_identified_sessions.end()) {
            Session& sess = *i->second;
            SyncConnection& conn = sess.get_connection();
            conn.protocol_error(error_2, &sess); // Throws
        }
        const IntegratableChangesetList& list = m_changesets_from_downstream[client_file_ident];
        std::size_t num_changesets = list.changesets.size();
        std::size_t num_bytes = 0;
        for (const IntegratableChangeset& ic : list.changesets)
            num_bytes += ic.changeset.size();
        logger.info("Excluded %1 changesets of combined byte size %2 for client file %3", num_changesets, num_bytes,
                    client_file_ident); // Throws
        num_changesets_removed += num_changesets;
        num_bytes_removed += num_bytes;
        m_changesets_from_downstream.erase(client_file_ident);
    }

    REALM_ASSERT(num_changesets_removed <= m_num_changesets_from_downstream);
    REALM_ASSERT(num_bytes_removed <= m_blocked_changesets_from_downstream_byte_size);

    if (num_changesets_removed == 0)
        return;

    m_num_changesets_from_downstream -= num_changesets_removed;

    // The byte size of the blocked changesets must be decremented.
    if (num_bytes_removed > 0) {
        m_blocked_changesets_from_downstream_byte_size -= num_bytes_removed;
        get_server().dec_byte_size_for_pending_downstream_changesets(num_bytes_removed); // Throws
    }
}


void ServerFile::finalize_work_stage_2()
{
    // Expose new snapshot to remote peers
    REALM_ASSERT(m_work.produced_new_realm_version || m_work.version_info.realm_version == 0);
    if (m_work.version_info.realm_version > m_version_info.realm_version) {
        REALM_ASSERT(m_work.version_info.sync_version.version >= m_version_info.sync_version.version);
        m_version_info = m_work.version_info;
    }

    bool resume_download_and_upload = m_work.produced_new_sync_version;

    // Deliver allocated file identifiers to requesters
    REALM_ASSERT(m_file_ident_requests.size() >= m_work.file_ident_alloc_slots.size());
    auto begin = m_file_ident_requests.begin();
    auto i = begin;
    for (const FileIdentAllocSlot& slot : m_work.file_ident_alloc_slots) {
        FileIdentRequestInfo& info = i->second;
        REALM_ASSERT(info.proxy_file == slot.proxy_file);
        REALM_ASSERT(info.client_type == slot.client_type);
        if (FileIdentReceiver* receiver = info.receiver) {
            info.receiver = nullptr;
            receiver->receive_file_ident(slot.file_ident); // Throws
        }
        ++i;
    }
    m_file_ident_requests.erase(begin, i);

    // Resume download to downstream clients
    if (resume_download_and_upload) {
        resume_download();
    }
}

// ============================ Worker implementation ============================

Worker::Worker(ServerImpl& server)
    : logger_ptr{std::make_shared<util::PrefixLogger>(util::LogCategory::server, "Worker: ", server.logger_ptr)}
    // Throws
    , logger(*logger_ptr)
    , m_server{server}
    , m_file_access_cache{server.get_config().max_open_files, logger, *this, server.get_config().encryption_key}
{
    util::seed_prng_nondeterministically(m_random); // Throws
}


void Worker::enqueue(ServerFile* file)
{
    util::LockGuard lock{m_mutex};
    m_queue.push_back(file); // Throws
    m_cond.notify_all();
}


std::mt19937_64& Worker::server_history_get_random() noexcept
{
    return m_random;
}


void Worker::run()
{
    for (;;) {
        ServerFile* file = nullptr;
        {
            util::LockGuard lock{m_mutex};
            for (;;) {
                if (REALM_UNLIKELY(m_stop))
                    return;
                if (!m_queue.empty()) {
                    file = m_queue.front();
                    m_queue.pop_front();
                    break;
                }
                m_cond.wait(lock);
            }
        }
        file->worker_process_work_unit(m_state); // Throws
    }
}


void Worker::stop() noexcept
{
    util::LockGuard lock{m_mutex};
    m_stop = true;
    m_cond.notify_all();
}


// ============================ ServerImpl implementation ============================

ServerImpl::ServerImpl(const std::string& root_dir, util::Optional<sync::PKey> pkey, Server::Config config)
    : logger_ptr{std::make_shared<util::CategoryLogger>(util::LogCategory::server, std::move(config.logger))}
    , logger{*logger_ptr}
    , m_config{std::move(config)}
    , m_max_upload_backlog{determine_max_upload_backlog(config)}
    , m_root_dir{root_dir} // Throws
    , m_access_control{std::move(pkey)}
    , m_protocol_version_range{determine_protocol_version_range(config)}                 // Throws
    , m_file_access_cache{m_config.max_open_files, logger, *this, config.encryption_key} // Throws
    , m_worker{*this}                                                                    // Throws
    , m_acceptor{get_service()}
    , m_server_protocol{}       // Throws
    , m_compress_memory_arena{} // Throws
{
    if (m_config.ssl) {
        m_ssl_context = std::make_unique<network::ssl::Context>();                // Throws
        m_ssl_context->use_certificate_chain_file(m_config.ssl_certificate_path); // Throws
        m_ssl_context->use_private_key_file(m_config.ssl_certificate_key_path);   // Throws
    }
}


ServerImpl::~ServerImpl() noexcept
{
    bool server_destroyed_while_still_running = m_running;
    REALM_ASSERT_RELEASE(!server_destroyed_while_still_running);
}


void ServerImpl::start()
{
    logger.info("Realm sync server started (%1)", REALM_VER_CHUNK); // Throws
    logger.info("Supported protocol versions: %1-%2 (%3-%4 configured)",
                ServerImplBase::get_oldest_supported_protocol_version(), get_current_protocol_version(),
                m_protocol_version_range.first,
                m_protocol_version_range.second); // Throws
    logger.info("Platform: %1", util::get_platform_info());
    bool is_debug_build = false;
#if REALM_DEBUG
    is_debug_build = true;
#endif
    {
        const char* lead_text = "Build mode";
        if (is_debug_build) {
            logger.info("%1: Debug", lead_text); // Throws
        }
        else {
            logger.info("%1: Release", lead_text); // Throws
        }
    }
    if (is_debug_build) {
        logger.warn("Build mode is Debug! CAN SEVERELY IMPACT PERFORMANCE - "
                    "NOT RECOMMENDED FOR PRODUCTION"); // Throws
    }
    logger.info("Directory holding persistent state: %1", m_root_dir);        // Throws
    logger.info("Maximum number of open files: %1", m_config.max_open_files); // Throws
    {
        const char* lead_text = "Encryption";
        if (m_config.encryption_key) {
            logger.info("%1: Yes", lead_text); // Throws
        }
        else {
            logger.info("%1: No", lead_text); // Throws
        }
    }
    logger.info("Log level: %1", logger.get_level_threshold()); // Throws
    {
        const char* lead_text = "Disable sync to disk";
        if (m_config.disable_sync_to_disk) {
            logger.info("%1: All files", lead_text); // Throws
        }
        else {
            logger.info("%1: No", lead_text); // Throws
        }
    }
    if (m_config.disable_sync_to_disk) {
        logger.warn("Testing/debugging feature 'disable sync to disk' enabled - "
                    "never do this in production!"); // Throws
    }
    logger.info("Download compaction: %1",
                (m_config.disable_download_compaction ? "No" : "Yes")); // Throws
    logger.info("Download bootstrap caching: %1",
                (m_config.enable_download_bootstrap_cache ? "Yes" : "No"));                // Throws
    logger.info("Max download size: %1 bytes", m_config.max_download_size);                // Throws
    logger.info("Max upload backlog: %1 bytes", m_max_upload_backlog);                     // Throws
    logger.info("HTTP request timeout: %1 ms", m_config.http_request_timeout);             // Throws
    logger.info("HTTP response timeout: %1 ms", m_config.http_response_timeout);           // Throws
    logger.info("Connection reaper timeout: %1 ms", m_config.connection_reaper_timeout);   // Throws
    logger.info("Connection reaper interval: %1 ms", m_config.connection_reaper_interval); // Throws
    logger.info("Connection soft close timeout: %1 ms", m_config.soft_close_timeout);      // Throws
    logger.debug("Authorization header name: %1", m_config.authorization_header_name);     // Throws

    m_realm_names = _impl::find_realm_files(m_root_dir); // Throws

    initiate_connection_reaper_timer(m_config.connection_reaper_interval); // Throws

    listen(); // Throws
}


void ServerImpl::run()
{
    auto ta = util::make_temp_assign(m_running, true);

    {
        auto worker_thread = util::make_thread_exec_guard(m_worker, *this); // Throws
        std::string name;
        if (util::Thread::get_name(name)) {
            name += "-worker";
            worker_thread.start_with_signals_blocked(name); // Throws
        }
        else {
            worker_thread.start_with_signals_blocked(); // Throws
        }

        m_service.run(); // Throws

        worker_thread.stop_and_rethrow(); // Throws
    }

    logger.info("Realm sync server stopped");
}


void ServerImpl::stop() noexcept
{
    util::LockGuard lock{m_mutex};
    if (m_stopped)
        return;
    m_stopped = true;
    m_wait_or_service_stopped_cond.notify_all();
    m_service.stop();
}


void ServerImpl::inc_byte_size_for_pending_downstream_changesets(std::size_t byte_size)
{
    m_pending_changesets_from_downstream_byte_size += byte_size;
    logger.debug("Byte size for pending downstream changesets incremented by "
                 "%1 to reach a total of %2",
                 byte_size,
                 m_pending_changesets_from_downstream_byte_size); // Throws
}


void ServerImpl::dec_byte_size_for_pending_downstream_changesets(std::size_t byte_size)
{
    REALM_ASSERT(byte_size <= m_pending_changesets_from_downstream_byte_size);
    m_pending_changesets_from_downstream_byte_size -= byte_size;
    logger.debug("Byte size for pending downstream changesets decremented by "
                 "%1 to reach a total of %2",
                 byte_size,
                 m_pending_changesets_from_downstream_byte_size); // Throws
}


std::mt19937_64& ServerImpl::server_history_get_random() noexcept
{
    return get_random();
}


void ServerImpl::listen()
{
    network::Resolver resolver{get_service()};
    network::Resolver::Query query(m_config.listen_address, m_config.listen_port,
                                   network::Resolver::Query::passive | network::Resolver::Query::address_configured);
    network::Endpoint::List endpoints = resolver.resolve(query); // Throws

    auto i = endpoints.begin();
    auto end = endpoints.end();
    for (;;) {
        std::error_code ec;
        m_acceptor.open(i->protocol(), ec);
        if (!ec) {
            using SocketBase = network::SocketBase;
            m_acceptor.set_option(SocketBase::reuse_address(m_config.reuse_address), ec);
            if (!ec) {
                m_acceptor.bind(*i, ec);
                if (!ec)
                    break;
            }
            m_acceptor.close();
        }
        if (i + 1 == end) {
            for (auto i2 = endpoints.begin(); i2 != i; ++i2) {
                // FIXME: We don't have the error code for previous attempts, so
                // can't print a nice message.
                logger.error("Failed to bind to %1:%2", i2->address(),
                             i2->port()); // Throws
            }
            logger.error("Failed to bind to %1:%2: %3", i->address(), i->port(),
                         ec.message()); // Throws
            throw std::runtime_error("Could not create a listening socket: All endpoints failed");
        }
    }

    m_acceptor.listen(m_config.listen_backlog);

    network::Endpoint local_endpoint = m_acceptor.local_endpoint();
    const char* ssl_mode = (m_ssl_context ? "TLS" : "non-TLS");
    logger.info("Listening on %1:%2 (max backlog is %3, %4)", local_endpoint.address(), local_endpoint.port(),
                m_config.listen_backlog, ssl_mode); // Throws

    initiate_accept();
}


void ServerImpl::initiate_accept()
{
    auto handler = [this](std::error_code ec) {
        if (ec != util::error::operation_aborted)
            handle_accept(ec);
    };
    bool is_ssl = bool(m_ssl_context);
    m_next_http_conn.reset(new HTTPConnection(*this, ++m_next_conn_id, is_ssl));                            // Throws
    m_acceptor.async_accept(m_next_http_conn->get_socket(), m_next_http_conn_endpoint, std::move(handler)); // Throws
}


void ServerImpl::handle_accept(std::error_code ec)
{
    if (ec) {
        if (ec != util::error::connection_aborted) {
            REALM_ASSERT(ec != util::error::operation_aborted);

            // We close the reserved files to get a few extra file descriptors.
            for (size_t i = 0; i < sizeof(m_reserved_files) / sizeof(m_reserved_files[0]); ++i) {
                m_reserved_files[i].reset();
            }

            // FIXME: There are probably errors that need to be treated
            // specially, and not cause the server to "crash".

            if (ec == make_basic_system_error_code(EMFILE)) {
                logger.error("Failed to accept a connection due to the file descriptor limit, "
                             "consider increasing the limit in your system config"); // Throws
                throw OutOfFilesError(ec);
            }
            else {
                throw std::system_error(ec);
            }
        }
        logger.debug("Skipping aborted connection"); // Throws
    }
    else {
        HTTPConnection& conn = *m_next_http_conn;
        if (m_config.tcp_no_delay)
            conn.get_socket().set_option(network::SocketBase::no_delay(true));  // Throws
        m_http_connections.emplace(conn.get_id(), std::move(m_next_http_conn)); // Throws
        Formatter& formatter = m_misc_buffers.formatter;
        formatter.reset();
        formatter << "[" << m_next_http_conn_endpoint.address() << "]:" << m_next_http_conn_endpoint.port(); // Throws
        std::string remote_endpoint = {formatter.data(), formatter.size()};                                  // Throws
        conn.initiate(std::move(remote_endpoint));                                                           // Throws
    }
    initiate_accept(); // Throws
}


void ServerImpl::remove_http_connection(std::int_fast64_t conn_id) noexcept
{
    m_http_connections.erase(conn_id);
}


void ServerImpl::add_sync_connection(int_fast64_t connection_id, std::unique_ptr<SyncConnection>&& sync_conn)
{
    m_sync_connections.emplace(connection_id, std::move(sync_conn));
}


void ServerImpl::remove_sync_connection(int_fast64_t connection_id)
{
    m_sync_connections.erase(connection_id);
}


void ServerImpl::set_connection_reaper_timeout(milliseconds_type timeout)
{
    get_service().post([this, timeout](Status) {
        m_config.connection_reaper_timeout = timeout;
    });
}


void ServerImpl::close_connections()
{
    get_service().post([this](Status) {
        do_close_connections(); // Throws
    });
}


bool ServerImpl::map_virtual_to_real_path(const std::string& virt_path, std::string& real_path)
{
    return _impl::map_virt_to_real_realm_path(m_root_dir, virt_path, real_path); // Throws
}


void ServerImpl::recognize_external_change(const std::string& virt_path)
{
    std::string virt_path_2 = virt_path; // Throws (copy)
    get_service().post([this, virt_path = std::move(virt_path_2)](Status) {
        do_recognize_external_change(virt_path); // Throws
    });                                          // Throws
}


void ServerImpl::stop_sync_and_wait_for_backup_completion(
    util::UniqueFunction<void(bool did_backup)> completion_handler, milliseconds_type timeout)
{
    logger.info("stop_sync_and_wait_for_backup_completion() called with "
                "timeout = %1",
                timeout); // Throws

    get_service().post([this, completion_handler = std::move(completion_handler), timeout](Status) mutable {
        do_stop_sync_and_wait_for_backup_completion(std::move(completion_handler),
                                                    timeout); // Throws
    });
}


void ServerImpl::initiate_connection_reaper_timer(milliseconds_type timeout)
{
    m_connection_reaper_timer.emplace(get_service());
    m_connection_reaper_timer->async_wait(std::chrono::milliseconds(timeout), [this, timeout](Status status) {
        if (status != ErrorCodes::OperationAborted) {
            reap_connections();                        // Throws
            initiate_connection_reaper_timer(timeout); // Throws
        }
    }); // Throws
}


void ServerImpl::reap_connections()
{
    logger.debug("Discarding dead connections"); // Throws
    SteadyTimePoint now = steady_clock_now();
    {
        auto end = m_http_connections.end();
        auto i = m_http_connections.begin();
        while (i != end) {
            HTTPConnection& conn = *i->second;
            ++i;
            // Suicide
            conn.terminate_if_dead(now); // Throws
        }
    }
    {
        auto end = m_sync_connections.end();
        auto i = m_sync_connections.begin();
        while (i != end) {
            SyncConnection& conn = *i->second;
            ++i;
            // Suicide
            conn.terminate_if_dead(now); // Throws
        }
    }
}


void ServerImpl::do_close_connections()
{
    for (auto& entry : m_sync_connections) {
        SyncConnection& conn = *entry.second;
        conn.initiate_soft_close(); // Throws
    }
}


void ServerImpl::do_recognize_external_change(const std::string& virt_path)
{
    auto i = m_files.find(virt_path);
    if (i == m_files.end())
        return;
    ServerFile& file = *i->second;
    file.recognize_external_change();
}


void ServerImpl::do_stop_sync_and_wait_for_backup_completion(
    util::UniqueFunction<void(bool did_complete)> completion_handler, milliseconds_type timeout)
{
    static_cast<void>(timeout);
    if (m_sync_stopped)
        return;
    do_close_connections(); // Throws
    m_sync_stopped = true;
    bool completion_reached = false;
    completion_handler(completion_reached); // Throws
}


// ============================ SyncConnection implementation ============================

SyncConnection::~SyncConnection() noexcept
{
    m_sessions_enlisted_to_send.clear();
    m_sessions.clear();
}


void SyncConnection::initiate()
{
    m_last_activity_at = steady_clock_now();
    logger.debug("Sync Connection initiated");
    m_websocket.initiate_server_websocket_after_handshake();
    send_log_message(util::Logger::Level::info, "Client connection established with server", 0,
                     m_appservices_request_id);
}


template <class... Params>
void SyncConnection::terminate(Logger::Level log_level, const char* log_message, Params... log_params)
{
    terminate_sessions();                              // Throws
    logger.log(log_level, log_message, log_params...); // Throws
    m_websocket.stop();
    m_ssl_stream.reset();
    m_socket.reset();
    // Suicide
    m_server.remove_sync_connection(m_id);
}


void SyncConnection::terminate_if_dead(SteadyTimePoint now)
{
    milliseconds_type time = steady_duration(m_last_activity_at, now);
    const Server::Config& config = m_server.get_config();
    if (m_is_closing) {
        if (time >= config.soft_close_timeout) {
            // Suicide
            terminate(Logger::Level::detail,
                      "Sync connection closed (timeout during soft close)"); // Throws
        }
    }
    else {
        if (time >= config.connection_reaper_timeout) {
            // Suicide
            terminate(Logger::Level::detail,
                      "Sync connection closed (no heartbeat)"); // Throws
        }
    }
}


void SyncConnection::enlist_to_send(Session* sess) noexcept
{
    REALM_ASSERT(m_send_trigger);
    REALM_ASSERT(!m_is_closing);
    REALM_ASSERT(!sess->is_enlisted_to_send());
    m_sessions_enlisted_to_send.push_back(sess);
    m_send_trigger->trigger();
}


void SyncConnection::handle_protocol_error(Status status)
{
    logger.error("%1", status);
    switch (status.code()) {
        case ErrorCodes::SyncProtocolInvariantFailed:
            protocol_error(ProtocolError::bad_syntax); // Throws
            break;
        case ErrorCodes::LimitExceeded:
            protocol_error(ProtocolError::limits_exceeded); // Throws
            break;
        default:
            protocol_error(ProtocolError::other_error);
            break;
    }
}

void SyncConnection::receive_bind_message(session_ident_type session_ident, std::string path,
                                          std::string signed_user_token, bool need_client_file_ident,
                                          bool is_subserver)
{
    auto p = m_sessions.emplace(session_ident, nullptr); // Throws
    bool was_inserted = p.second;
    if (REALM_UNLIKELY(!was_inserted)) {
        logger.error("Overlapping reuse of session identifier %1 in BIND message",
                     session_ident);                           // Throws
        protocol_error(ProtocolError::reuse_of_session_ident); // Throws
        return;
    }
    try {
        p.first->second.reset(new Session(*this, session_ident)); // Throws
    }
    catch (...) {
        m_sessions.erase(p.first);
        throw;
    }

    Session& sess = *p.first->second;
    sess.initiate(); // Throws
    ProtocolError error;
    bool success =
        sess.receive_bind_message(std::move(path), std::move(signed_user_token), need_client_file_ident, is_subserver,
                                  error); // Throws
    if (REALM_UNLIKELY(!success))         // Throws
        protocol_error(error, &sess);     // Throws
}


void SyncConnection::receive_ident_message(session_ident_type session_ident, file_ident_type client_file_ident,
                                           salt_type client_file_ident_salt, version_type scan_server_version,
                                           version_type scan_client_version, version_type latest_server_version,
                                           salt_type latest_server_version_salt)
{
    auto i = m_sessions.find(session_ident);
    if (REALM_UNLIKELY(i == m_sessions.end())) {
        bad_session_ident("IDENT", session_ident); // Throws
        return;
    }
    Session& sess = *i->second;
    if (REALM_UNLIKELY(sess.unbind_message_received())) {
        message_after_unbind("IDENT", session_ident); // Throws
        return;
    }
    if (REALM_UNLIKELY(sess.error_occurred())) {
        // Protocol state is SendError or WaitForUnbindErr. In these states, all
        // messages, other than UNBIND, must be ignored.
        return;
    }
    if (REALM_UNLIKELY(sess.must_send_ident_message())) {
        logger.error("Received IDENT message before IDENT message was sent"); // Throws
        protocol_error(ProtocolError::bad_message_order);                     // Throws
        return;
    }
    if (REALM_UNLIKELY(sess.ident_message_received())) {
        logger.error("Received second IDENT message for session"); // Throws
        protocol_error(ProtocolError::bad_message_order);          // Throws
        return;
    }

    ProtocolError error = {};
    bool success = sess.receive_ident_message(client_file_ident, client_file_ident_salt, scan_server_version,
                                              scan_client_version, latest_server_version, latest_server_version_salt,
                                              error); // Throws
    if (REALM_UNLIKELY(!success))                     // Throws
        protocol_error(error, &sess);                 // Throws
}

void SyncConnection::receive_upload_message(session_ident_type session_ident, version_type progress_client_version,
                                            version_type progress_server_version, version_type locked_server_version,
                                            const UploadChangesets& upload_changesets)
{
    auto i = m_sessions.find(session_ident);
    if (REALM_UNLIKELY(i == m_sessions.end())) {
        bad_session_ident("UPLOAD", session_ident); // Throws
        return;
    }
    Session& sess = *i->second;
    if (REALM_UNLIKELY(sess.unbind_message_received())) {
        message_after_unbind("UPLOAD", session_ident); // Throws
        return;
    }
    if (REALM_UNLIKELY(sess.error_occurred())) {
        // Protocol state is SendError or WaitForUnbindErr. In these states, all
        // messages, other than UNBIND, must be ignored.
        return;
    }
    if (REALM_UNLIKELY(!sess.ident_message_received())) {
        message_before_ident("UPLOAD", session_ident); // Throws
        return;
    }

    ProtocolError error = {};
    bool success = sess.receive_upload_message(progress_client_version, progress_server_version,
                                               locked_server_version, upload_changesets, error); // Throws
    if (REALM_UNLIKELY(!success))                                                                // Throws
        protocol_error(error, &sess);                                                            // Throws
}


void SyncConnection::receive_mark_message(session_ident_type session_ident, request_ident_type request_ident)
{
    auto i = m_sessions.find(session_ident);
    if (REALM_UNLIKELY(i == m_sessions.end())) {
        bad_session_ident("MARK", session_ident);
        return;
    }
    Session& sess = *i->second;
    if (REALM_UNLIKELY(sess.unbind_message_received())) {
        message_after_unbind("MARK", session_ident); // Throws
        return;
    }
    if (REALM_UNLIKELY(sess.error_occurred())) {
        // Protocol state is SendError or WaitForUnbindErr. In these states, all
        // messages, other than UNBIND, must be ignored.
        return;
    }
    if (REALM_UNLIKELY(!sess.ident_message_received())) {
        message_before_ident("MARK", session_ident); // Throws
        return;
    }

    ProtocolError error;
    bool success = sess.receive_mark_message(request_ident, error); // Throws
    if (REALM_UNLIKELY(!success))                                   // Throws
        protocol_error(error, &sess);                               // Throws
}


void SyncConnection::receive_unbind_message(session_ident_type session_ident)
{
    auto i = m_sessions.find(session_ident); // Throws
    if (REALM_UNLIKELY(i == m_sessions.end())) {
        bad_session_ident("UNBIND", session_ident); // Throws
        return;
    }
    Session& sess = *i->second;
    if (REALM_UNLIKELY(sess.unbind_message_received())) {
        message_after_unbind("UNBIND", session_ident); // Throws
        return;
    }

    sess.receive_unbind_message(); // Throws
    // NOTE: The session might have gotten destroyed at this time!
}


void SyncConnection::receive_ping(milliseconds_type timestamp, milliseconds_type rtt)
{
    logger.debug("Received: PING(timestamp=%1, rtt=%2)", timestamp, rtt); // Throws
    m_send_pong = true;
    m_last_ping_timestamp = timestamp;
    if (!m_is_sending)
        send_next_message();
}


void SyncConnection::receive_error_message(session_ident_type session_ident, int error_code,
                                           std::string_view error_body)
{
    logger.debug("Received: ERROR(error_code=%1, message_size=%2, session_ident=%3)", error_code, error_body.size(),
                 session_ident); // Throws
    auto i = m_sessions.find(session_ident);
    if (REALM_UNLIKELY(i == m_sessions.end())) {
        bad_session_ident("ERROR", session_ident);
        return;
    }
    Session& sess = *i->second;
    if (REALM_UNLIKELY(sess.unbind_message_received())) {
        message_after_unbind("ERROR", session_ident); // Throws
        return;
    }

    sess.receive_error_message(session_ident, error_code, error_body); // Throws
}

void SyncConnection::send_log_message(util::Logger::Level level, const std::string&& message,
                                      session_ident_type sess_ident, std::optional<std::string> co_id)
{
    if (get_client_protocol_version() < SyncConnection::SERVER_LOG_PROTOCOL_VERSION) {
        return logger.log(level, message.c_str());
    }

    LogMessage log_msg{sess_ident, level, std::move(message), std::move(co_id)};
    {
        std::lock_guard lock(m_log_mutex);
        m_log_messages.push(std::move(log_msg));
    }
    m_send_trigger->trigger();
}


void SyncConnection::bad_session_ident(const char* message_type, session_ident_type session_ident)
{
    logger.error("Bad session identifier in %1 message, session_ident = %2", message_type,
                 session_ident);                      // Throws
    protocol_error(ProtocolError::bad_session_ident); // Throws
}


void SyncConnection::message_after_unbind(const char* message_type, session_ident_type session_ident)
{
    logger.error("Received %1 message after UNBIND message, session_ident = %2", message_type,
                 session_ident);                      // Throws
    protocol_error(ProtocolError::bad_message_order); // Throws
}


void SyncConnection::message_before_ident(const char* message_type, session_ident_type session_ident)
{
    logger.error("Received %1 message before IDENT message, session_ident = %2", message_type,
                 session_ident);                      // Throws
    protocol_error(ProtocolError::bad_message_order); // Throws
}


void SyncConnection::handle_message_received(const char* data, size_t size)
{
    // parse_message_received() parses the message and calls the
    // proper handler on the SyncConnection object (this).
    get_server_protocol().parse_message_received<SyncConnection>(*this, std::string_view(data, size));
    return;
}


void SyncConnection::handle_ping_received(const char* data, size_t size)
{
    // parse_message_received() parses the message and calls the
    // proper handler on the SyncConnection object (this).
    get_server_protocol().parse_ping_received<SyncConnection>(*this, std::string_view(data, size));
    return;
}


void SyncConnection::send_next_message()
{
    REALM_ASSERT(!m_is_sending);
    REALM_ASSERT(!m_sending_pong);
    if (m_send_pong) {
        send_pong(m_last_ping_timestamp);
        if (m_sending_pong)
            return;
    }
    for (;;) {
        Session* sess = m_sessions_enlisted_to_send.pop_front();
        if (!sess) {
            // No sessions were enlisted to send
            if (REALM_LIKELY(!m_is_closing))
                break; // Check to see if there are any log messages to go out
            // Send a connection level ERROR
            REALM_ASSERT(!is_session_level_error(m_error_code));
            initiate_write_error(m_error_code, m_error_session_ident); // Throws
            return;
        }
        sess->send_message(); // Throws
        // NOTE: The session might have gotten destroyed at this time!

        // At this point, `m_is_sending` is true if, and only if the session
        // chose to send a message. If it chose to not send a message, we must
        // loop back and give the next session in `m_sessions_enlisted_to_send`
        // a chance.
        if (m_is_sending)
            return;
    }
    {
        std::lock_guard lock(m_log_mutex);
        if (!m_log_messages.empty()) {
            send_log_message(m_log_messages.front());
            m_log_messages.pop();
        }
    }
    // Otherwise, nothing to do
}


void SyncConnection::initiate_write_output_buffer()
{
    auto handler = [this](std::error_code ec, size_t) {
        if (!ec) {
            handle_write_output_buffer();
        }
    };

    m_websocket.async_write_binary(m_output_buffer.data(), m_output_buffer.size(),
                                   std::move(handler)); // Throws
    m_is_sending = true;
}


void SyncConnection::initiate_pong_output_buffer()
{
    auto handler = [this](std::error_code ec, size_t) {
        if (!ec) {
            handle_pong_output_buffer();
        }
    };

    REALM_ASSERT(!m_is_sending);
    REALM_ASSERT(!m_sending_pong);
    m_websocket.async_write_binary(m_output_buffer.data(), m_output_buffer.size(),
                                   std::move(handler)); // Throws

    m_is_sending = true;
    m_sending_pong = true;
}


void SyncConnection::send_pong(milliseconds_type timestamp)
{
    REALM_ASSERT(m_send_pong);
    REALM_ASSERT(!m_sending_pong);
    m_send_pong = false;
    logger.debug("Sending: PONG(timestamp=%1)", timestamp); // Throws

    OutputBuffer& out = get_output_buffer();
    get_server_protocol().make_pong(out, timestamp); // Throws

    initiate_pong_output_buffer(); // Throws
}

void SyncConnection::send_log_message(const LogMessage& log_msg)
{
    OutputBuffer& out = get_output_buffer();
    get_server_protocol().make_log_message(out, log_msg.level, log_msg.message, log_msg.sess_ident,
                                           log_msg.co_id); // Throws

    initiate_write_output_buffer(); // Throws
}


void SyncConnection::handle_write_output_buffer()
{
    release_output_buffer();
    m_is_sending = false;
    send_next_message(); // Throws
}


void SyncConnection::handle_pong_output_buffer()
{
    release_output_buffer();
    REALM_ASSERT(m_is_sending);
    REALM_ASSERT(m_sending_pong);
    m_is_sending = false;
    m_sending_pong = false;
    send_next_message(); // Throws
}


void SyncConnection::initiate_write_error(ProtocolError error_code, session_ident_type session_ident)
{
    const char* message = get_protocol_error_message(int(error_code));
    std::size_t message_size = std::strlen(message);
    bool try_again = determine_try_again(error_code);

    logger.detail("Sending: ERROR(error_code=%1, message_size=%2, try_again=%3, session_ident=%4)", int(error_code),
                  message_size, try_again, session_ident); // Throws

    OutputBuffer& out = get_output_buffer();
    int protocol_version = get_client_protocol_version();
    get_server_protocol().make_error_message(protocol_version, out, error_code, message, message_size, try_again,
                                             session_ident); // Throws

    auto handler = [this](std::error_code ec, size_t) {
        handle_write_error(ec); // Throws
    };
    m_websocket.async_write_binary(out.data(), out.size(), std::move(handler));
    m_is_sending = true;
}


void SyncConnection::handle_write_error(std::error_code ec)
{
    m_is_sending = false;
    REALM_ASSERT(m_is_closing);
    if (!m_ssl_stream) {
        m_socket->shutdown(network::Socket::shutdown_send, ec);
        if (ec && ec != make_basic_system_error_code(ENOTCONN))
            throw std::system_error(ec);
    }
}


// For connection level errors, `sess` is ignored. For session level errors, a
// session must be specified.
//
// If a session is specified, that session object will have been detached from
// the ServerFile object (and possibly destroyed) upon return from
// protocol_error().
//
// If a session is specified for a protocol level error, that session object
// will have been destroyed upon return from protocol_error(). For session level
// errors, the specified session will have been destroyed upon return from
// protocol_error() if, and only if the negotiated protocol version is less than
// 23.
void SyncConnection::protocol_error(ProtocolError error_code, Session* sess)
{
    REALM_ASSERT(!m_is_closing);
    bool session_level = is_session_level_error(error_code);
    REALM_ASSERT(!session_level || sess);
    REALM_ASSERT(!sess || m_sessions.count(sess->get_session_ident()) == 1);
    if (logger.would_log(util::Logger::Level::debug)) {
        const char* message = get_protocol_error_message(int(error_code));
        Logger& logger_2 = (session_level ? sess->logger : logger);
        logger_2.debug("Protocol error: %1 (error_code=%2)", message, int(error_code)); // Throws
    }
    session_ident_type session_ident = (session_level ? sess->get_session_ident() : 0);
    if (session_level) {
        sess->initiate_deactivation(error_code); // Throws
        return;
    }
    do_initiate_soft_close(error_code, session_ident); // Throws
}


void SyncConnection::do_initiate_soft_close(ProtocolError error_code, session_ident_type session_ident)
{
    REALM_ASSERT(get_protocol_error_message(int(error_code)));

    // With recent versions of the protocol (when the version is greater than,
    // or equal to 23), this function will only be called for connection level
    // errors, never for session specific errors. However, for the purpose of
    // emulating earlier protocol versions, this function might be called for
    // session specific errors too.
    REALM_ASSERT(is_session_level_error(error_code) == (session_ident != 0));
    REALM_ASSERT(!is_session_level_error(error_code));

    REALM_ASSERT(m_send_trigger);
    REALM_ASSERT(!m_is_closing);
    m_is_closing = true;

    m_error_code = error_code;
    m_error_session_ident = session_ident;

    // Don't waste time and effort sending any other messages
    m_send_pong = false;
    m_sessions_enlisted_to_send.clear();

    m_receiving_session = nullptr;

    terminate_sessions(); // Throws

    m_send_trigger->trigger();
}


void SyncConnection::close_due_to_close_by_client(std::error_code ec)
{
    auto log_level = (ec == util::MiscExtErrors::end_of_input ? Logger::Level::detail : Logger::Level::info);
    // Suicide
    terminate(log_level, "Sync connection closed by client: %1", ec.message()); // Throws
}


void SyncConnection::close_due_to_error(std::error_code ec)
{
    // Suicide
    terminate(Logger::Level::error, "Sync connection closed due to error: %1",
              ec.message()); // Throws
}


void SyncConnection::terminate_sessions()
{
    for (auto& entry : m_sessions) {
        Session& sess = *entry.second;
        sess.terminate(); // Throws
    }
    m_sessions_enlisted_to_send.clear();
    m_sessions.clear();
}


void SyncConnection::initiate_soft_close()
{
    if (!m_is_closing) {
        session_ident_type session_ident = 0;                                    // Not session specific
        do_initiate_soft_close(ProtocolError::connection_closed, session_ident); // Throws
    }
}


void SyncConnection::discard_session(session_ident_type session_ident) noexcept
{
    m_sessions.erase(session_ident);
}

} // anonymous namespace


// ============================ sync::Server implementation ============================

class Server::Implementation : public ServerImpl {
public:
    Implementation(const std::string& root_dir, util::Optional<PKey> pkey, Server::Config config)
        : ServerImpl{root_dir, std::move(pkey), std::move(config)} // Throws
    {
    }
    virtual ~Implementation() {}
};


Server::Server(const std::string& root_dir, util::Optional<sync::PKey> pkey, Config config)
    : m_impl{new Implementation{root_dir, std::move(pkey), std::move(config)}} // Throws
{
}


Server::Server(Server&& serv) noexcept
    : m_impl{std::move(serv.m_impl)}
{
}


Server::~Server() noexcept {}


void Server::start()
{
    m_impl->start(); // Throws
}


void Server::start(const std::string& listen_address, const std::string& listen_port, bool reuse_address)
{
    m_impl->start(listen_address, listen_port, reuse_address); // Throws
}


network::Endpoint Server::listen_endpoint() const
{
    return m_impl->listen_endpoint(); // Throws
}


void Server::run()
{
    m_impl->run(); // Throws
}


void Server::stop() noexcept
{
    m_impl->stop();
}


uint_fast64_t Server::errors_seen() const noexcept
{
    return m_impl->errors_seen;
}


void Server::stop_sync_and_wait_for_backup_completion(util::UniqueFunction<void(bool did_backup)> completion_handler,
                                                      milliseconds_type timeout)
{
    m_impl->stop_sync_and_wait_for_backup_completion(std::move(completion_handler), timeout); // Throws
}


void Server::set_connection_reaper_timeout(milliseconds_type timeout)
{
    m_impl->set_connection_reaper_timeout(timeout);
}


void Server::close_connections()
{
    m_impl->close_connections();
}


bool Server::map_virtual_to_real_path(const std::string& virt_path, std::string& real_path)
{
    return m_impl->map_virtual_to_real_path(virt_path, real_path); // Throws
}


void Server::recognize_external_change(const std::string& virt_path)
{
    m_impl->recognize_external_change(virt_path); // Throws
}


void Server::get_workunit_timers(milliseconds_type& parallel_section, milliseconds_type& sequential_section)
{
    m_impl->get_workunit_timers(parallel_section, sequential_section);
}

realm / realm-core / 2293

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