thomas.goyne_478

Committed 02 Aug 2024 05:19PM UTC coverage: 91.089% (-0.01%) from 91.1%

Build # thomas.goyne_478

Build Type

Pull #7944

Evergreen

Committed by

tgoyne

Commit Message

Only track pending client resets done by the same core version

If the previous attempt at performing a client reset was done with a different
core version then we should retry the client reset as the new version may have
fixed a bug that made the previous attempt fail (or may be a downgrade to a
version before when the bug was introduced). This also simplifies the tracking
as it means that we don't need to be able to read trackers created by different
versions.

This also means that we can freely change the schema of the table, which this
takes advantage of to drop the unused primary key and make the error required,
as we never actually stored null and the code reading it would have crashed if
it encountered a null error.

Pull Request Pull Request #7944: Only track pending client resets done by the same core version

Run Details

102704 of 181534 branches covered (56.58%)

138 of 153 new or added lines in 10 files covered. (90.2%)

85 existing lines in 16 files now uncovered.

216717 of 237917 relevant lines covered (91.09%)

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Source File
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83.34

/src/realm/sync/noinst/client_impl_base.cpp

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

#include <realm/impl/simulated_failure.hpp>
#include <realm/sync/changeset_parser.hpp>
#include <realm/sync/impl/clock.hpp>
#include <realm/sync/network/http.hpp>
#include <realm/sync/network/websocket.hpp>
#include <realm/sync/noinst/client_history_impl.hpp>
#include <realm/sync/noinst/client_reset_operation.hpp>
#include <realm/sync/noinst/sync_schema_migration.hpp>
#include <realm/sync/protocol.hpp>
#include <realm/util/assert.hpp>
#include <realm/util/basic_system_errors.hpp>
#include <realm/util/memory_stream.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/to_string.hpp>
#include <realm/util/uri.hpp>
#include <realm/version.hpp>

#include <system_error>
#include <sstream>

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

using namespace realm;
using namespace _impl;
using namespace realm::util;
using namespace realm::sync;
using namespace realm::sync::websocket;

// clang-format off
using Connection      = ClientImpl::Connection;
using Session         = ClientImpl::Session;
using UploadChangeset = ClientHistory::UploadChangeset;

// These are a work-around for a bug in MSVC. It cannot find in-class types
// mentioned in signature of out-of-line member function definitions.
using ConnectionTerminationReason = ClientImpl::ConnectionTerminationReason;
using OutputBuffer                = ClientImpl::OutputBuffer;
using ReceivedChangesets          = ClientProtocol::ReceivedChangesets;
// clang-format on

void ClientImpl::ReconnectInfo::reset() noexcept
{
    m_backoff_state.reset();
    scheduled_reset = false;
}


void ClientImpl::ReconnectInfo::update(ConnectionTerminationReason new_reason,
                                       std::optional<ResumptionDelayInfo> new_delay_info)
{
    m_backoff_state.update(new_reason, new_delay_info);
}


std::chrono::milliseconds ClientImpl::ReconnectInfo::delay_interval()
{
    if (scheduled_reset) {
        reset();
    }

    if (!m_backoff_state.triggering_error) {
        return std::chrono::milliseconds::zero();
    }

    switch (*m_backoff_state.triggering_error) {
        case ConnectionTerminationReason::closed_voluntarily:
            return std::chrono::milliseconds::zero();
        case ConnectionTerminationReason::server_said_do_not_reconnect:
            return std::chrono::milliseconds::max();
        default:
            if (m_reconnect_mode == ReconnectMode::testing) {
                return std::chrono::milliseconds::max();
            }

            REALM_ASSERT(m_reconnect_mode == ReconnectMode::normal);
            return m_backoff_state.delay_interval();
    }
}


bool ClientImpl::decompose_server_url(const std::string& url, ProtocolEnvelope& protocol, std::string& address,
                                      port_type& port, std::string& path) const
{
    util::Uri uri(url); // Throws
    uri.canonicalize(); // Throws
    std::string userinfo, address_2, port_2;
    bool realm_scheme = (uri.get_scheme() == "realm:" || uri.get_scheme() == "realms:");
    bool ws_scheme = (uri.get_scheme() == "ws:" || uri.get_scheme() == "wss:");
    bool good = ((realm_scheme || ws_scheme) && uri.get_auth(userinfo, address_2, port_2) && userinfo.empty() &&
                 !address_2.empty() && uri.get_query().empty() && uri.get_frag().empty()); // Throws
    if (REALM_UNLIKELY(!good))
        return false;
    ProtocolEnvelope protocol_2;
    port_type port_3;
    if (realm_scheme) {
        if (uri.get_scheme() == "realm:") {
            protocol_2 = ProtocolEnvelope::realm;
            port_3 = (m_enable_default_port_hack ? 80 : 7800);
        }
        else {
            protocol_2 = ProtocolEnvelope::realms;
            port_3 = (m_enable_default_port_hack ? 443 : 7801);
        }
    }
    else {
        REALM_ASSERT(ws_scheme);
        if (uri.get_scheme() == "ws:") {
            protocol_2 = ProtocolEnvelope::ws;
            port_3 = 80;
        }
        else {
            protocol_2 = ProtocolEnvelope::wss;
            port_3 = 443;
        }
    }
    if (!port_2.empty()) {
        std::istringstream in(port_2);    // Throws
        in.imbue(std::locale::classic()); // Throws
        in >> port_3;
        if (REALM_UNLIKELY(!in || !in.eof() || port_3 < 1))
            return false;
    }
    std::string path_2 = uri.get_path(); // Throws (copy)

    protocol = protocol_2;
    address = std::move(address_2);
    port = port_3;
    path = std::move(path_2);
    return true;
}

ClientImpl::ClientImpl(ClientConfig config)
    : logger(std::make_shared<util::CategoryLogger>(util::LogCategory::session, std::move(config.logger)))
    , m_reconnect_mode{config.reconnect_mode}
    , m_connect_timeout{config.connect_timeout}
    , m_connection_linger_time{config.one_connection_per_session ? 0 : config.connection_linger_time}
    , m_ping_keepalive_period{config.ping_keepalive_period}
    , m_pong_keepalive_timeout{config.pong_keepalive_timeout}
    , m_fast_reconnect_limit{config.fast_reconnect_limit}
    , m_reconnect_backoff_info{config.reconnect_backoff_info}
    , m_disable_upload_activation_delay{config.disable_upload_activation_delay}
    , m_dry_run{config.dry_run}
    , m_enable_default_port_hack{config.enable_default_port_hack}
    , m_fix_up_object_ids{config.fix_up_object_ids}
    , m_roundtrip_time_handler{std::move(config.roundtrip_time_handler)}
    , m_socket_provider{std::move(config.socket_provider)}
    , m_client_protocol{} // Throws
    , m_one_connection_per_session{config.one_connection_per_session}
    , m_random{}
{
    // FIXME: Would be better if seeding was up to the application.
    util::seed_prng_nondeterministically(m_random); // Throws

    logger.info("Realm sync client (%1)", REALM_VER_CHUNK); // Throws
    logger.debug("Supported protocol versions: %1-%2", get_oldest_supported_protocol_version(),
                 get_current_protocol_version()); // Throws
    logger.info("Platform: %1", util::get_platform_info());
    const char* build_mode;
#if REALM_DEBUG
    build_mode = "Debug";
#else
    build_mode = "Release";
#endif
    logger.debug("Build mode: %1", build_mode);
    logger.debug("Config param: one_connection_per_session = %1",
                 config.one_connection_per_session); // Throws
    logger.debug("Config param: connect_timeout = %1 ms",
                 config.connect_timeout); // Throws
    logger.debug("Config param: connection_linger_time = %1 ms",
                 config.connection_linger_time); // Throws
    logger.debug("Config param: ping_keepalive_period = %1 ms",
                 config.ping_keepalive_period); // Throws
    logger.debug("Config param: pong_keepalive_timeout = %1 ms",
                 config.pong_keepalive_timeout); // Throws
    logger.debug("Config param: fast_reconnect_limit = %1 ms",
                 config.fast_reconnect_limit); // Throws
    logger.debug("Config param: disable_sync_to_disk = %1",
                 config.disable_sync_to_disk); // Throws
    logger.debug(
        "Config param: reconnect backoff info: max_delay: %1 ms, initial_delay: %2 ms, multiplier: %3, jitter: 1/%4",
        m_reconnect_backoff_info.max_resumption_delay_interval.count(),
        m_reconnect_backoff_info.resumption_delay_interval.count(),
        m_reconnect_backoff_info.resumption_delay_backoff_multiplier, m_reconnect_backoff_info.delay_jitter_divisor);

    if (config.reconnect_mode != ReconnectMode::normal) {
        logger.warn("Testing/debugging feature 'nonnormal reconnect mode' enabled - "
                    "never do this in production!");
    }

    if (config.dry_run) {
        logger.warn("Testing/debugging feature 'dry run' enabled - "
                    "never do this in production!");
    }

    REALM_ASSERT_EX(m_socket_provider, "Must provide socket provider in sync Client config");

    if (m_one_connection_per_session) {
        logger.warn("Testing/debugging feature 'one connection per session' enabled - "
                    "never do this in production");
    }

    if (config.disable_upload_activation_delay) {
        logger.warn("Testing/debugging feature 'disable_upload_activation_delay' enabled - "
                    "never do this in production");
    }

    if (config.disable_sync_to_disk) {
        logger.warn("Testing/debugging feature 'disable_sync_to_disk' enabled - "
                    "never do this in production");
    }

    m_actualize_and_finalize = create_trigger([this](Status status) {
        if (status == ErrorCodes::OperationAborted)
            return;
        else if (!status.is_ok())
            throw Exception(status);
        actualize_and_finalize_session_wrappers(); // Throws
    });
}

void ClientImpl::incr_outstanding_posts()
{
    util::CheckedLockGuard lock(m_drain_mutex);
    ++m_outstanding_posts;
    m_drained = false;
}

void ClientImpl::decr_outstanding_posts()
{
    util::CheckedLockGuard lock(m_drain_mutex);
    REALM_ASSERT(m_outstanding_posts);
    if (--m_outstanding_posts <= 0) {
        // Notify must happen with lock held or another thread could destroy
        // ClientImpl between when we release the lock and when we call notify
        m_drain_cv.notify_all();
    }
}

void ClientImpl::post(SyncSocketProvider::FunctionHandler&& handler)
{
    REALM_ASSERT(m_socket_provider);
    incr_outstanding_posts();
    m_socket_provider->post([handler = std::move(handler), this](Status status) {
        auto decr_guard = util::make_scope_exit([&]() noexcept {
            decr_outstanding_posts();
        });
        handler(status);
    });
}

void ClientImpl::post(util::UniqueFunction<void()>&& handler)
{
    REALM_ASSERT(m_socket_provider);
    incr_outstanding_posts();
    m_socket_provider->post([handler = std::move(handler), this](Status status) {
        auto decr_guard = util::make_scope_exit([&]() noexcept {
            decr_outstanding_posts();
        });
        if (status == ErrorCodes::OperationAborted)
            return;
        if (!status.is_ok())
            throw Exception(status);
        handler();
    });
}


void ClientImpl::drain_connections()
{
    logger.debug("Draining connections during sync client shutdown");
    for (auto& server_slot_pair : m_server_slots) {
        auto& server_slot = server_slot_pair.second;

        if (server_slot.connection) {
            auto& conn = server_slot.connection;
            conn->force_close();
        }
        else {
            for (auto& conn_pair : server_slot.alt_connections) {
                conn_pair.second->force_close();
            }
        }
    }
}


SyncSocketProvider::SyncTimer ClientImpl::create_timer(std::chrono::milliseconds delay,
                                                       SyncSocketProvider::FunctionHandler&& handler)
{
    REALM_ASSERT(m_socket_provider);
    incr_outstanding_posts();
    return m_socket_provider->create_timer(delay, [handler = std::move(handler), this](Status status) {
        auto decr_guard = util::make_scope_exit([&]() noexcept {
            decr_outstanding_posts();
        });
        handler(status);
    });
}


ClientImpl::SyncTrigger ClientImpl::create_trigger(SyncSocketProvider::FunctionHandler&& handler)
{
    REALM_ASSERT(m_socket_provider);
    return std::make_unique<Trigger<ClientImpl>>(this, std::move(handler));
}

Connection::~Connection()
{
    if (m_websocket_sentinel) {
        m_websocket_sentinel->destroyed = true;
        m_websocket_sentinel.reset();
    }
}

void Connection::activate()
{
    REALM_ASSERT(m_on_idle);
    m_activated = true;
    if (m_num_active_sessions == 0)
        m_on_idle->trigger();
    // We cannot in general connect immediately, because a prior failure to
    // connect may require a delay before reconnecting (see `m_reconnect_info`).
    initiate_reconnect_wait(); // Throws
}


void Connection::activate_session(std::unique_ptr<Session> sess)
{
    REALM_ASSERT(sess);
    REALM_ASSERT(&sess->m_conn == this);
    REALM_ASSERT(!m_force_closed);
    Session& sess_2 = *sess;
    session_ident_type ident = sess->m_ident;
    auto p = m_sessions.emplace(ident, std::move(sess)); // Throws
    bool was_inserted = p.second;
    REALM_ASSERT(was_inserted);
    // Save the session ident to the historical list of session idents
    m_session_history.insert(ident);
    sess_2.activate(); // Throws
    if (m_state == ConnectionState::connected) {
        bool fast_reconnect = false;
        sess_2.connection_established(fast_reconnect); // Throws
    }
    ++m_num_active_sessions;
}


void Connection::initiate_session_deactivation(Session* sess)
{
    REALM_ASSERT(sess);
    REALM_ASSERT(&sess->m_conn == this);
    REALM_ASSERT(m_num_active_sessions);
    // Since the client may be waiting for m_num_active_sessions to reach 0
    // in stop_and_wait() (on a separate thread), deactivate Session before
    // decrementing the num active sessions value.
    sess->initiate_deactivation(); // Throws
    if (sess->m_state == Session::Deactivated) {
        finish_session_deactivation(sess);
    }
    if (REALM_UNLIKELY(--m_num_active_sessions == 0)) {
        if (m_activated && m_state == ConnectionState::disconnected)
            m_on_idle->trigger();
    }
}


void Connection::cancel_reconnect_delay()
{
    REALM_ASSERT(m_activated);

    if (m_reconnect_delay_in_progress) {
        if (m_nonzero_reconnect_delay)
            logger.detail("Canceling reconnect delay"); // Throws

        // Cancel the in-progress wait operation by destroying the timer
        // object. Destruction is needed in this case, because a new wait
        // operation might have to be initiated before the previous one
        // completes (its completion handler starts to execute), so the new wait
        // operation must be done on a new timer object.
        m_reconnect_disconnect_timer.reset();
        m_reconnect_delay_in_progress = false;
        m_reconnect_info.reset();
        initiate_reconnect_wait(); // Throws
        return;
    }

    // If we are not disconnected, then we need to make sure the next time we get disconnected
    // that we are allowed to re-connect as quickly as possible.
    //
    // Setting m_reconnect_info.scheduled_reset will cause initiate_reconnect_wait to reset the
    // backoff/delay state before calculating the next delay, unless a PONG message is received
    // for the urgent PING message we send below.
    //
    // If we get a PONG message for the urgent PING message sent below, then the connection is
    // healthy and we can calculate the next delay normally.
    if (m_state != ConnectionState::disconnected) {
        m_reconnect_info.scheduled_reset = true;
        m_ping_after_scheduled_reset_of_reconnect_info = false;

        schedule_urgent_ping(); // Throws
        return;
    }
    // Nothing to do in this case. The next reconnect attemp will be made as
    // soon as there are any sessions that are both active and unsuspended.
}

void Connection::finish_session_deactivation(Session* sess)
{
    REALM_ASSERT(sess->m_state == Session::Deactivated);
    auto ident = sess->m_ident;
    m_sessions.erase(ident);
    m_session_history.erase(ident);
}

void Connection::force_close()
{
    if (m_force_closed) {
        return;
    }

    m_force_closed = true;

    if (m_state != ConnectionState::disconnected) {
        voluntary_disconnect();
    }

    REALM_ASSERT_EX(m_state == ConnectionState::disconnected, m_state);
    if (m_reconnect_delay_in_progress || m_disconnect_delay_in_progress) {
        m_reconnect_disconnect_timer.reset();
        m_reconnect_delay_in_progress = false;
        m_disconnect_delay_in_progress = false;
    }

    // We must copy any session pointers we want to close to a vector because force_closing
    // the session may remove it from m_sessions and invalidate the iterator uses to loop
    // through the map. By copying to a separate vector we ensure our iterators remain valid.
    std::vector<Session*> to_close;
    for (auto& session_pair : m_sessions) {
        if (session_pair.second->m_state == Session::State::Active) {
            to_close.push_back(session_pair.second.get());
        }
    }

    for (auto& sess : to_close) {
        sess->force_close();
    }

    logger.debug("Force closed idle connection");
}


void Connection::websocket_connected_handler(const std::string& protocol)
{
    if (!protocol.empty()) {
        std::string_view expected_prefix =
            is_flx_sync_connection() ? get_flx_websocket_protocol_prefix() : get_pbs_websocket_protocol_prefix();
        // FIXME: Use std::string_view::begins_with() in C++20.
        auto prefix_matches = [&](std::string_view other) {
            return protocol.size() >= other.size() && (protocol.substr(0, other.size()) == other);
        };
        if (prefix_matches(expected_prefix)) {
            util::MemoryInputStream in;
            in.set_buffer(protocol.data() + expected_prefix.size(), protocol.data() + protocol.size());
            in.imbue(std::locale::classic());
            in.unsetf(std::ios_base::skipws);
            int value_2 = 0;
            in >> value_2;
            if (in && in.eof() && value_2 >= 0) {
                bool good_version =
                    (value_2 >= get_oldest_supported_protocol_version() && value_2 <= get_current_protocol_version());
                if (good_version) {
                    logger.detail("Negotiated protocol version: %1", value_2);
                    // For now, grab the connection ID from the websocket if it supports it. In the future, the server
                    // will provide the appservices connection ID via a log message.
                    // TODO: Remove once the server starts sending the connection ID
                    receive_appservices_request_id(m_websocket->get_appservices_request_id());
                    m_negotiated_protocol_version = value_2;
                    handle_connection_established(); // Throws
                    return;
                }
            }
        }
        close_due_to_client_side_error({ErrorCodes::SyncProtocolNegotiationFailed,
                                        util::format("Bad protocol info from server: '%1'", protocol)},
                                       IsFatal{true}, ConnectionTerminationReason::bad_headers_in_http_response);
    }
    else {
        close_due_to_client_side_error(
            {ErrorCodes::SyncProtocolNegotiationFailed, "Missing protocol info from server"}, IsFatal{true},
            ConnectionTerminationReason::bad_headers_in_http_response);
    }
}


bool Connection::websocket_binary_message_received(util::Span<const char> data)
{
    if (m_force_closed) {
        logger.debug("Received binary message after connection was force closed");
        return false;
    }

    using sf = SimulatedFailure;
    if (sf::check_trigger(sf::sync_client__read_head)) {
        close_due_to_client_side_error(
            {ErrorCodes::RuntimeError, "Simulated failure during sync client websocket read"}, IsFatal{false},
            ConnectionTerminationReason::read_or_write_error);
        return bool(m_websocket);
    }

    handle_message_received(data);
    return bool(m_websocket);
}


void Connection::websocket_error_handler()
{
    m_websocket_error_received = true;
}

bool Connection::websocket_closed_handler(bool was_clean, WebSocketError error_code, std::string_view msg)
{
    if (m_force_closed) {
        logger.debug("Received websocket close message after connection was force closed");
        return false;
    }
    logger.info("Closing the websocket with error code=%1, message='%2', was_clean=%3", error_code, msg, was_clean);

    switch (error_code) {
        case WebSocketError::websocket_ok:
            break;
        case WebSocketError::websocket_resolve_failed:
            [[fallthrough]];
        case WebSocketError::websocket_connection_failed: {
            SessionErrorInfo error_info(
                {ErrorCodes::SyncConnectFailed, util::format("Failed to connect to sync: %1", msg)}, IsFatal{false});
            // If the connection fails/times out and the server has not been contacted yet, refresh the location
            // to make sure the websocket URL is correct
            if (!m_server_endpoint.is_verified) {
                error_info.server_requests_action = ProtocolErrorInfo::Action::RefreshLocation;
            }
            involuntary_disconnect(std::move(error_info), ConnectionTerminationReason::connect_operation_failed);
            break;
        }
        case WebSocketError::websocket_read_error:
            [[fallthrough]];
        case WebSocketError::websocket_write_error: {
            close_due_to_transient_error({ErrorCodes::ConnectionClosed, msg},
                                         ConnectionTerminationReason::read_or_write_error);
            break;
        }
        case WebSocketError::websocket_going_away:
            [[fallthrough]];
        case WebSocketError::websocket_protocol_error:
            [[fallthrough]];
        case WebSocketError::websocket_unsupported_data:
            [[fallthrough]];
        case WebSocketError::websocket_invalid_payload_data:
            [[fallthrough]];
        case WebSocketError::websocket_policy_violation:
            [[fallthrough]];
        case WebSocketError::websocket_reserved:
            [[fallthrough]];
        case WebSocketError::websocket_no_status_received:
            [[fallthrough]];
        case WebSocketError::websocket_invalid_extension: {
            close_due_to_client_side_error({ErrorCodes::SyncProtocolInvariantFailed, msg}, IsFatal{false},
                                           ConnectionTerminationReason::websocket_protocol_violation); // Throws
            break;
        }
        case WebSocketError::websocket_message_too_big: {
            auto message = util::format(
                "Sync websocket closed because the server received a message that was too large: %1", msg);
            SessionErrorInfo error_info(Status(ErrorCodes::LimitExceeded, std::move(message)), IsFatal{false});
            error_info.server_requests_action = ProtocolErrorInfo::Action::ClientReset;
            involuntary_disconnect(std::move(error_info),
                                   ConnectionTerminationReason::websocket_protocol_violation); // Throws
            break;
        }
        case WebSocketError::websocket_tls_handshake_failed: {
            close_due_to_client_side_error(
                Status(ErrorCodes::TlsHandshakeFailed, util::format("TLS handshake failed: %1", msg)), IsFatal{false},
                ConnectionTerminationReason::ssl_certificate_rejected); // Throws
            break;
        }
        case WebSocketError::websocket_fatal_error: {
            // Error is fatal if the sync_route has already been verified - if the sync_route has not
            // been verified, then use a non-fatal error and try to perform a location update.
            SessionErrorInfo error_info(
                {ErrorCodes::SyncConnectFailed, util::format("Failed to connect to sync: %1", msg)},
                IsFatal{m_server_endpoint.is_verified});
            ConnectionTerminationReason reason = ConnectionTerminationReason::http_response_says_fatal_error;
            // If the connection fails/times out and the server has not been contacted yet, refresh the location
            // to make sure the websocket URL is correct
            if (!m_server_endpoint.is_verified) {
                error_info.server_requests_action = ProtocolErrorInfo::Action::RefreshLocation;
                reason = ConnectionTerminationReason::connect_operation_failed;
            }
            involuntary_disconnect(std::move(error_info), reason);
            break;
        }
        case WebSocketError::websocket_forbidden: {
            SessionErrorInfo error_info({ErrorCodes::AuthError, msg}, IsFatal{true});
            error_info.server_requests_action = ProtocolErrorInfo::Action::LogOutUser;
            involuntary_disconnect(std::move(error_info),
                                   ConnectionTerminationReason::http_response_says_fatal_error);
            break;
        }
        case WebSocketError::websocket_unauthorized: {
            SessionErrorInfo error_info(
                {ErrorCodes::AuthError,
                 util::format("Websocket was closed because of an authentication issue: %1", msg)},
                IsFatal{false});
            error_info.server_requests_action = ProtocolErrorInfo::Action::RefreshUser;
            involuntary_disconnect(std::move(error_info),
                                   ConnectionTerminationReason::http_response_says_nonfatal_error);
            break;
        }
        case WebSocketError::websocket_moved_permanently: {
            SessionErrorInfo error_info({ErrorCodes::ConnectionClosed, msg}, IsFatal{false});
            error_info.server_requests_action = ProtocolErrorInfo::Action::RefreshLocation;
            involuntary_disconnect(std::move(error_info),
                                   ConnectionTerminationReason::http_response_says_nonfatal_error);
            break;
        }
        case WebSocketError::websocket_abnormal_closure: {
            SessionErrorInfo error_info({ErrorCodes::ConnectionClosed, msg}, IsFatal{false});
            error_info.server_requests_action = ProtocolErrorInfo::Action::RefreshUser;
            involuntary_disconnect(std::move(error_info),
                                   ConnectionTerminationReason::http_response_says_nonfatal_error);
            break;
        }
        case WebSocketError::websocket_internal_server_error:
            [[fallthrough]];
        case WebSocketError::websocket_retry_error: {
            involuntary_disconnect(SessionErrorInfo({ErrorCodes::ConnectionClosed, msg}, IsFatal{false}),
                                   ConnectionTerminationReason::http_response_says_nonfatal_error);
            break;
        }
    }

    return bool(m_websocket);
}

// Guarantees that handle_reconnect_wait() is never called from within the
// execution of initiate_reconnect_wait() (no callback reentrance).
void Connection::initiate_reconnect_wait()
{
    REALM_ASSERT(m_activated);
    REALM_ASSERT(!m_reconnect_delay_in_progress);
    REALM_ASSERT(!m_disconnect_delay_in_progress);

    // If we've been force closed then we don't need/want to reconnect. Just return early here.
    if (m_force_closed) {
        return;
    }

    m_reconnect_delay_in_progress = true;
    auto delay = m_reconnect_info.delay_interval();
    if (delay == std::chrono::milliseconds::max()) {
        logger.detail("Reconnection delayed indefinitely"); // Throws
        // Not actually starting a timer corresponds to an infinite wait
        m_nonzero_reconnect_delay = true;
        return;
    }

    if (delay == std::chrono::milliseconds::zero()) {
        m_nonzero_reconnect_delay = false;
    }
    else {
        logger.detail("Allowing reconnection in %1 milliseconds", delay.count()); // Throws
        m_nonzero_reconnect_delay = true;
    }

    // We create a timer for the reconnect_disconnect timer even if the delay is zero because
    // we need it to be cancelable in case the connection is terminated before the timer
    // callback is run.
    m_reconnect_disconnect_timer = m_client.create_timer(delay, [this](Status status) {
        // If the operation is aborted, the connection object may have been
        // destroyed.
        if (status != ErrorCodes::OperationAborted)
            handle_reconnect_wait(status); // Throws
    });                                    // Throws
}


void Connection::handle_reconnect_wait(Status status)
{
    if (!status.is_ok()) {
        REALM_ASSERT(status != ErrorCodes::OperationAborted);
        throw Exception(status);
    }

    REALM_ASSERT(m_reconnect_delay_in_progress);
    m_reconnect_delay_in_progress = false;

    if (m_num_active_unsuspended_sessions > 0)
        initiate_reconnect(); // Throws
}

struct Connection::WebSocketObserverShim : public sync::WebSocketObserver {
    explicit WebSocketObserverShim(Connection* conn)
        : conn(conn)
        , sentinel(conn->m_websocket_sentinel)
    {
    }

    Connection* conn;
    util::bind_ptr<LifecycleSentinel> sentinel;

    void websocket_connected_handler(const std::string& protocol) override
    {
        if (sentinel->destroyed) {
            return;
        }

        return conn->websocket_connected_handler(protocol);
    }

    void websocket_error_handler() override
    {
        if (sentinel->destroyed) {
            return;
        }

        conn->websocket_error_handler();
    }

    bool websocket_binary_message_received(util::Span<const char> data) override
    {
        if (sentinel->destroyed) {
            return false;
        }

        return conn->websocket_binary_message_received(data);
    }

    bool websocket_closed_handler(bool was_clean, WebSocketError error_code, std::string_view msg) override
    {
        if (sentinel->destroyed) {
            return true;
        }

        return conn->websocket_closed_handler(was_clean, error_code, msg);
    }
};

void Connection::initiate_reconnect()
{
    REALM_ASSERT(m_activated);

    m_state = ConnectionState::connecting;
    report_connection_state_change(ConnectionState::connecting); // Throws
    if (m_websocket_sentinel) {
        m_websocket_sentinel->destroyed = true;
    }
    m_websocket_sentinel = util::make_bind<LifecycleSentinel>();
    m_websocket.reset();

    // Watchdog
    initiate_connect_wait(); // Throws

    std::vector<std::string> sec_websocket_protocol;
    {
        auto protocol_prefix =
            is_flx_sync_connection() ? get_flx_websocket_protocol_prefix() : get_pbs_websocket_protocol_prefix();
        int min = get_oldest_supported_protocol_version();
        int max = get_current_protocol_version();
        REALM_ASSERT_3(min, <=, max);
        // List protocol version in descending order to ensure that the server
        // selects the highest possible version.
        for (int version = max; version >= min; --version) {
            sec_websocket_protocol.push_back(util::format("%1%2", protocol_prefix, version)); // Throws
        }
    }

    logger.info("Connecting to '%1%2:%3%4'", to_string(m_server_endpoint.envelope), m_server_endpoint.address,
                m_server_endpoint.port, m_http_request_path_prefix);

    m_websocket_error_received = false;
    m_websocket =
        m_client.m_socket_provider->connect(std::make_unique<WebSocketObserverShim>(this),
                                            WebSocketEndpoint{
                                                m_server_endpoint.address,
                                                m_server_endpoint.port,
                                                get_http_request_path(),
                                                std::move(sec_websocket_protocol),
                                                is_ssl(m_server_endpoint.envelope),
                                                /// DEPRECATED - The following will be removed in a future release
                                                {m_custom_http_headers.begin(), m_custom_http_headers.end()},
                                                m_verify_servers_ssl_certificate,
                                                m_ssl_trust_certificate_path,
                                                m_ssl_verify_callback,
                                                m_proxy_config,
                                            });
}


void Connection::initiate_connect_wait()
{
    // Deploy a watchdog to enforce an upper bound on the time it can take to
    // fully establish the connection (including SSL and WebSocket
    // handshakes). Without such a watchdog, connect operations could take very
    // long, or even indefinite time.
    milliseconds_type time = m_client.m_connect_timeout;

    m_connect_timer = m_client.create_timer(std::chrono::milliseconds(time), [this](Status status) {
        // If the operation is aborted, the connection object may have been
        // destroyed.
        if (status != ErrorCodes::OperationAborted)
            handle_connect_wait(status); // Throws
    });                                  // Throws
}


void Connection::handle_connect_wait(Status status)
{
    if (!status.is_ok()) {
        REALM_ASSERT(status != ErrorCodes::OperationAborted);
        throw Exception(status);
    }

    REALM_ASSERT_EX(m_state == ConnectionState::connecting, m_state);
    logger.info("Connect timeout"); // Throws
    SessionErrorInfo error_info({ErrorCodes::SyncConnectTimeout, "Sync connection was not fully established in time"},
                                IsFatal{false});
    // If the connection fails/times out and the server has not been contacted yet, refresh the location
    // to make sure the websocket URL is correct
    if (!m_server_endpoint.is_verified) {
        error_info.server_requests_action = ProtocolErrorInfo::Action::RefreshLocation;
    }
    involuntary_disconnect(std::move(error_info), ConnectionTerminationReason::sync_connect_timeout); // Throws
}


void Connection::handle_connection_established()
{
    // Cancel connect timeout watchdog
    m_connect_timer.reset();

    m_state = ConnectionState::connected;
    m_server_endpoint.is_verified = true; // sync route is valid since connection is successful

    milliseconds_type now = monotonic_clock_now();
    m_pong_wait_started_at = now; // Initially, no time was spent waiting for a PONG message
    initiate_ping_delay(now);     // Throws

    bool fast_reconnect = false;
    if (m_disconnect_has_occurred) {
        milliseconds_type time = now - m_disconnect_time;
        if (time <= m_client.m_fast_reconnect_limit)
            fast_reconnect = true;
    }

    for (auto& p : m_sessions) {
        Session& sess = *p.second;
        sess.connection_established(fast_reconnect); // Throws
    }

    report_connection_state_change(ConnectionState::connected); // Throws
}


void Connection::schedule_urgent_ping()
{
    REALM_ASSERT_EX(m_state != ConnectionState::disconnected, m_state);
    if (m_ping_delay_in_progress) {
        m_heartbeat_timer.reset();
        m_ping_delay_in_progress = false;
        m_minimize_next_ping_delay = true;
        milliseconds_type now = monotonic_clock_now();
        initiate_ping_delay(now); // Throws
        return;
    }
    REALM_ASSERT_EX(m_state == ConnectionState::connecting || m_waiting_for_pong, m_state);
    if (!m_send_ping)
        m_minimize_next_ping_delay = true;
}


void Connection::initiate_ping_delay(milliseconds_type now)
{
    REALM_ASSERT(!m_ping_delay_in_progress);
    REALM_ASSERT(!m_waiting_for_pong);
    REALM_ASSERT(!m_send_ping);

    milliseconds_type delay = 0;
    if (!m_minimize_next_ping_delay) {
        delay = m_client.m_ping_keepalive_period;
        // Make a randomized deduction of up to 10%, or up to 100% if this is
        // the first PING message to be sent since the connection was
        // established. The purpose of this randomized deduction is to reduce
        // the risk of many connections sending PING messages simultaneously to
        // the server.
        milliseconds_type max_deduction = (m_ping_sent ? delay / 10 : delay);
        auto distr = std::uniform_int_distribution<milliseconds_type>(0, max_deduction);
        milliseconds_type randomized_deduction = distr(m_client.get_random());
        delay -= randomized_deduction;
        // Deduct the time spent waiting for PONG
        REALM_ASSERT_3(now, >=, m_pong_wait_started_at);
        milliseconds_type spent_time = now - m_pong_wait_started_at;
        if (spent_time < delay) {
            delay -= spent_time;
        }
        else {
            delay = 0;
        }
    }
    else {
        m_minimize_next_ping_delay = false;
    }


    m_ping_delay_in_progress = true;

    m_heartbeat_timer = m_client.create_timer(std::chrono::milliseconds(delay), [this](Status status) {
        if (status == ErrorCodes::OperationAborted)
            return;
        else if (!status.is_ok())
            throw Exception(status);

        handle_ping_delay();                                    // Throws
    });                                                         // Throws
    logger.debug("Will emit a ping in %1 milliseconds", delay); // Throws
}


void Connection::handle_ping_delay()
{
    REALM_ASSERT(m_ping_delay_in_progress);
    m_ping_delay_in_progress = false;
    m_send_ping = true;

    initiate_pong_timeout(); // Throws

    if (m_state == ConnectionState::connected && !m_sending)
        send_next_message(); // Throws
}


void Connection::initiate_pong_timeout()
{
    REALM_ASSERT(!m_ping_delay_in_progress);
    REALM_ASSERT(!m_waiting_for_pong);
    REALM_ASSERT(m_send_ping);

    m_waiting_for_pong = true;
    m_pong_wait_started_at = monotonic_clock_now();

    milliseconds_type time = m_client.m_pong_keepalive_timeout;
    m_heartbeat_timer = m_client.create_timer(std::chrono::milliseconds(time), [this](Status status) {
        if (status == ErrorCodes::OperationAborted)
            return;
        else if (!status.is_ok())
            throw Exception(status);

        handle_pong_timeout(); // Throws
    });                        // Throws
}


void Connection::handle_pong_timeout()
{
    REALM_ASSERT(m_waiting_for_pong);
    logger.debug("Timeout on reception of PONG message"); // Throws
    close_due_to_transient_error({ErrorCodes::ConnectionClosed, "Timed out waiting for PONG response from server"},
                                 ConnectionTerminationReason::pong_timeout);
}


void Connection::initiate_write_message(const OutputBuffer& out, Session* sess)
{
    // Stop sending messages if an websocket error was received.
    if (m_websocket_error_received)
        return;

    m_websocket->async_write_binary(out.as_span(), [this, sentinel = m_websocket_sentinel](Status status) {
        if (sentinel->destroyed) {
            return;
        }
        if (!status.is_ok()) {
            if (status != ErrorCodes::Error::OperationAborted) {
                // Write errors will be handled by the websocket_write_error_handler() callback
                logger.error("Connection: write failed %1: %2", status.code_string(), status.reason());
            }
            return;
        }
        handle_write_message(); // Throws
    });                         // Throws
    m_sending_session = sess;
    m_sending = true;
}


void Connection::handle_write_message()
{
    m_sending_session->message_sent(); // Throws
    if (m_sending_session->m_state == Session::Deactivated) {
        finish_session_deactivation(m_sending_session);
    }
    m_sending_session = nullptr;
    m_sending = false;
    send_next_message(); // Throws
}


void Connection::send_next_message()
{
    REALM_ASSERT_EX(m_state == ConnectionState::connected, m_state);
    REALM_ASSERT(!m_sending_session);
    REALM_ASSERT(!m_sending);
    if (m_send_ping) {
        send_ping(); // Throws
        return;
    }
    while (!m_sessions_enlisted_to_send.empty()) {
        // The state of being connected is not supposed to be able to change
        // across this loop thanks to the "no callback reentrance" guarantee
        // provided by Websocket::async_write_text(), and friends.
        REALM_ASSERT_EX(m_state == ConnectionState::connected, m_state);

        Session& sess = *m_sessions_enlisted_to_send.front();
        m_sessions_enlisted_to_send.pop_front();
        sess.send_message(); // Throws

        if (sess.m_state == Session::Deactivated) {
            finish_session_deactivation(&sess);
        }

        // An enlisted session may choose to not send a message. In that case,
        // we should pass the opportunity to the next enlisted session.
        if (m_sending)
            break;
    }
}


void Connection::send_ping()
{
    REALM_ASSERT(!m_ping_delay_in_progress);
    REALM_ASSERT(m_waiting_for_pong);
    REALM_ASSERT(m_send_ping);

    m_send_ping = false;
    if (m_reconnect_info.scheduled_reset)
        m_ping_after_scheduled_reset_of_reconnect_info = true;

    m_last_ping_sent_at = monotonic_clock_now();
    logger.debug("Sending: PING(timestamp=%1, rtt=%2)", m_last_ping_sent_at,
                 m_previous_ping_rtt); // Throws

    ClientProtocol& protocol = get_client_protocol();
    OutputBuffer& out = get_output_buffer();
    protocol.make_ping(out, m_last_ping_sent_at, m_previous_ping_rtt); // Throws
    initiate_write_ping(out);                                          // Throws
    m_ping_sent = true;
}


void Connection::initiate_write_ping(const OutputBuffer& out)
{
    m_websocket->async_write_binary(out.as_span(), [this, sentinel = m_websocket_sentinel](Status status) {
        if (sentinel->destroyed) {
            return;
        }
        if (!status.is_ok()) {
            if (status != ErrorCodes::Error::OperationAborted) {
                // Write errors will be handled by the websocket_write_error_handler() callback
                logger.error("Connection: send ping failed %1: %2", status.code_string(), status.reason());
            }
            return;
        }
        handle_write_ping(); // Throws
    });                      // Throws
    m_sending = true;
}


void Connection::handle_write_ping()
{
    REALM_ASSERT(m_sending);
    REALM_ASSERT(!m_sending_session);
    m_sending = false;
    send_next_message(); // Throws
}


void Connection::handle_message_received(util::Span<const char> data)
{
    // parse_message_received() parses the message and calls the proper handler
    // on the Connection object (this).
    get_client_protocol().parse_message_received<Connection>(*this, std::string_view(data.data(), data.size()));
}


void Connection::initiate_disconnect_wait()
{
    REALM_ASSERT(!m_reconnect_delay_in_progress);

    if (m_disconnect_delay_in_progress) {
        m_reconnect_disconnect_timer.reset();
        m_disconnect_delay_in_progress = false;
    }

    milliseconds_type time = m_client.m_connection_linger_time;

    m_reconnect_disconnect_timer = m_client.create_timer(std::chrono::milliseconds(time), [this](Status status) {
        // If the operation is aborted, the connection object may have been
        // destroyed.
        if (status != ErrorCodes::OperationAborted)
            handle_disconnect_wait(status); // Throws
    });                                     // Throws
    m_disconnect_delay_in_progress = true;
}


void Connection::handle_disconnect_wait(Status status)
{
    if (!status.is_ok()) {
        REALM_ASSERT(status != ErrorCodes::OperationAborted);
        throw Exception(status);
    }

    m_disconnect_delay_in_progress = false;

    REALM_ASSERT_EX(m_state != ConnectionState::disconnected, m_state);
    if (m_num_active_unsuspended_sessions == 0) {
        if (m_client.m_connection_linger_time > 0)
            logger.detail("Linger time expired"); // Throws
        voluntary_disconnect();                   // Throws
        logger.info("Disconnected");              // Throws
    }
}


void Connection::close_due_to_protocol_error(Status status)
{
    SessionErrorInfo error_info(std::move(status), IsFatal{true});
    error_info.server_requests_action = ProtocolErrorInfo::Action::ProtocolViolation;
    involuntary_disconnect(std::move(error_info),
                           ConnectionTerminationReason::sync_protocol_violation); // Throws
}


void Connection::close_due_to_client_side_error(Status status, IsFatal is_fatal, ConnectionTerminationReason reason)
{
    logger.info("Connection closed due to error: %1", status); // Throws

    involuntary_disconnect(SessionErrorInfo{std::move(status), is_fatal}, reason); // Throw
}


void Connection::close_due_to_transient_error(Status status, ConnectionTerminationReason reason)
{
    logger.info("Connection closed due to transient error: %1", status); // Throws
    SessionErrorInfo error_info{std::move(status), IsFatal{false}};
    error_info.server_requests_action = ProtocolErrorInfo::Action::Transient;

    involuntary_disconnect(std::move(error_info), reason); // Throw
}


// Close connection due to error discovered on the server-side, and then
// reported to the client by way of a connection-level ERROR message.
void Connection::close_due_to_server_side_error(ProtocolError error_code, const ProtocolErrorInfo& info)
{
    logger.info("Connection closed due to error reported by server: %1 (%2)", info.message,
                int(error_code)); // Throws

    const auto reason = info.is_fatal ? ConnectionTerminationReason::server_said_do_not_reconnect
                                      : ConnectionTerminationReason::server_said_try_again_later;
    involuntary_disconnect(SessionErrorInfo{info, protocol_error_to_status(error_code, info.message)},
                           reason); // Throws
}


void Connection::disconnect(const SessionErrorInfo& info)
{
    // Cancel connect timeout watchdog
    m_connect_timer.reset();

    if (m_state == ConnectionState::connected) {
        m_disconnect_time = monotonic_clock_now();
        m_disconnect_has_occurred = true;

        // Sessions that are in the Deactivating state at this time can be
        // immediately discarded, in part because they are no longer enlisted to
        // send. Such sessions will be taken to the Deactivated state by
        // Session::connection_lost(), and then they will be removed from
        // `m_sessions`.
        auto i = m_sessions.begin(), end = m_sessions.end();
        while (i != end) {
            // Prevent invalidation of the main iterator when erasing elements
            auto j = i++;
            Session& sess = *j->second;
            sess.connection_lost(); // Throws
            if (sess.m_state == Session::Unactivated || sess.m_state == Session::Deactivated)
                m_sessions.erase(j);
        }
    }

    change_state_to_disconnected();

    m_ping_delay_in_progress = false;
    m_waiting_for_pong = false;
    m_send_ping = false;
    m_minimize_next_ping_delay = false;
    m_ping_after_scheduled_reset_of_reconnect_info = false;
    m_ping_sent = false;
    m_heartbeat_timer.reset();
    m_previous_ping_rtt = 0;

    m_websocket_sentinel->destroyed = true;
    m_websocket_sentinel.reset();
    m_websocket.reset();
    m_input_body_buffer.reset();
    m_sending_session = nullptr;
    m_sessions_enlisted_to_send.clear();
    m_sending = false;

    if (!m_appservices_coid.empty()) {
        m_appservices_coid.clear();
        logger.base_logger = make_logger(m_ident, std::nullopt, get_client().logger.base_logger);
        for (auto& [ident, sess] : m_sessions) {
            sess->logger.base_logger = Session::make_logger(ident, logger.base_logger);
        }
    }

    report_connection_state_change(ConnectionState::disconnected, info); // Throws
    initiate_reconnect_wait();                                           // Throws
}

bool Connection::is_flx_sync_connection() const noexcept
{
    return m_server_endpoint.server_mode != SyncServerMode::PBS;
}

void Connection::receive_pong(milliseconds_type timestamp)
{
    logger.debug("Received: PONG(timestamp=%1)", timestamp);

    bool legal_at_this_time = (m_waiting_for_pong && !m_send_ping);
    if (REALM_UNLIKELY(!legal_at_this_time)) {
        close_due_to_protocol_error(
            {ErrorCodes::SyncProtocolInvariantFailed, "Received PONG message when it was not valid"}); // Throws
        return;
    }

    if (REALM_UNLIKELY(timestamp != m_last_ping_sent_at)) {
        close_due_to_protocol_error(
            {ErrorCodes::SyncProtocolInvariantFailed,
             util::format("Received PONG message with an invalid timestamp (expected %1, received %2)",
                          m_last_ping_sent_at, timestamp)}); // Throws
        return;
    }

    milliseconds_type now = monotonic_clock_now();
    milliseconds_type round_trip_time = now - timestamp;
    logger.debug("Round trip time was %1 milliseconds", round_trip_time);
    m_previous_ping_rtt = round_trip_time;

    // If this PONG message is a response to a PING mesage that was sent after
    // the last invocation of cancel_reconnect_delay(), then the connection is
    // still good, and we do not have to skip the next reconnect delay.
    if (m_ping_after_scheduled_reset_of_reconnect_info) {
        REALM_ASSERT(m_reconnect_info.scheduled_reset);
        m_ping_after_scheduled_reset_of_reconnect_info = false;
        m_reconnect_info.scheduled_reset = false;
    }

    m_heartbeat_timer.reset();
    m_waiting_for_pong = false;

    initiate_ping_delay(now); // Throws

    if (m_client.m_roundtrip_time_handler)
        m_client.m_roundtrip_time_handler(m_previous_ping_rtt); // Throws
}

Session* Connection::find_and_validate_session(session_ident_type session_ident, std::string_view message) noexcept
{
    if (session_ident == 0) {
        return nullptr;
    }

    auto* sess = get_session(session_ident);
    if (REALM_LIKELY(sess)) {
        return sess;
    }
    // Check the history to see if the message received was for a previous session
    if (auto it = m_session_history.find(session_ident); it == m_session_history.end()) {
        logger.error("Bad session identifier in %1 message, session_ident = %2", message, session_ident);
        close_due_to_protocol_error(
            {ErrorCodes::SyncProtocolInvariantFailed,
             util::format("Received message %1 for session iden %2 when that session never existed", message,
                          session_ident)});
    }
    else {
        logger.error("Received %1 message for closed session, session_ident = %2", message,
                     session_ident); // Throws
    }
    return nullptr;
}

void Connection::receive_error_message(const ProtocolErrorInfo& info, session_ident_type session_ident)
{
    Session* sess = nullptr;
    if (session_ident != 0) {
        sess = find_and_validate_session(session_ident, "ERROR");
        if (REALM_UNLIKELY(!sess)) {
            return;
        }
        if (auto status = sess->receive_error_message(info); !status.is_ok()) {
            close_due_to_protocol_error(std::move(status)); // Throws
            return;
        }

        if (sess->m_state == Session::Deactivated) {
            finish_session_deactivation(sess);
        }
        return;
    }

    logger.info("Received: ERROR \"%1\" (error_code=%2, is_fatal=%3, session_ident=%4, error_action=%5)",
                info.message, info.raw_error_code, info.is_fatal, session_ident,
                info.server_requests_action); // Throws

    bool known_error_code = bool(get_protocol_error_message(info.raw_error_code));
    if (REALM_LIKELY(known_error_code)) {
        ProtocolError error_code = ProtocolError(info.raw_error_code);
        if (REALM_LIKELY(!is_session_level_error(error_code))) {
            close_due_to_server_side_error(error_code, info); // Throws
            return;
        }
        close_due_to_protocol_error(
            {ErrorCodes::SyncProtocolInvariantFailed,
             util::format("Received ERROR message with a non-connection-level error code %1 without a session ident",
                          info.raw_error_code)});
    }
    else {
        close_due_to_protocol_error(
            {ErrorCodes::SyncProtocolInvariantFailed,
             util::format("Received ERROR message with unknown error code %1", info.raw_error_code)});
    }
}


void Connection::receive_query_error_message(int raw_error_code, std::string_view message, int64_t query_version,
                                             session_ident_type session_ident)
{
    if (session_ident == 0) {
        return close_due_to_protocol_error(
            {ErrorCodes::SyncProtocolInvariantFailed, "Received query error message for session ident 0"});
    }

    if (!is_flx_sync_connection()) {
        return close_due_to_protocol_error({ErrorCodes::SyncProtocolInvariantFailed,
                                            "Received a FLX query error message on a non-FLX sync connection"});
    }

    if (Session* sess = find_and_validate_session(session_ident, "QUERY_ERROR")) {
        sess->receive_query_error_message(raw_error_code, message, query_version);
    }
}


void Connection::receive_ident_message(session_ident_type session_ident, SaltedFileIdent client_file_ident)
{
    Session* sess = find_and_validate_session(session_ident, "IDENT");
    if (REALM_UNLIKELY(!sess)) {
        return;
    }

    if (auto status = sess->receive_ident_message(client_file_ident); !status.is_ok())
        close_due_to_protocol_error(std::move(status)); // Throws
}

void Connection::receive_download_message(session_ident_type session_ident, const DownloadMessage& message)
{
    Session* sess = find_and_validate_session(session_ident, "DOWNLOAD");
    if (REALM_UNLIKELY(!sess)) {
        return;
    }

    if (auto status = sess->receive_download_message(message); !status.is_ok()) {
        close_due_to_protocol_error(std::move(status));
    }
}

void Connection::receive_mark_message(session_ident_type session_ident, request_ident_type request_ident)
{
    Session* sess = find_and_validate_session(session_ident, "MARK");
    if (REALM_UNLIKELY(!sess)) {
        return;
    }

    if (auto status = sess->receive_mark_message(request_ident); !status.is_ok())
        close_due_to_protocol_error(std::move(status)); // Throws
}


void Connection::receive_unbound_message(session_ident_type session_ident)
{
    Session* sess = find_and_validate_session(session_ident, "UNBOUND");
    if (REALM_UNLIKELY(!sess)) {
        return;
    }

    if (auto status = sess->receive_unbound_message(); !status.is_ok()) {
        close_due_to_protocol_error(std::move(status)); // Throws
        return;
    }

    if (sess->m_state == Session::Deactivated) {
        finish_session_deactivation(sess);
    }
}


void Connection::receive_test_command_response(session_ident_type session_ident, request_ident_type request_ident,
                                               std::string_view body)
{
    Session* sess = find_and_validate_session(session_ident, "TEST_COMMAND");
    if (REALM_UNLIKELY(!sess)) {
        return;
    }

    if (auto status = sess->receive_test_command_response(request_ident, body); !status.is_ok()) {
        close_due_to_protocol_error(std::move(status));
    }
}


void Connection::receive_server_log_message(session_ident_type session_ident, util::Logger::Level level,
                                            std::string_view message)
{
    if (session_ident != 0) {
        if (auto sess = get_session(session_ident)) {
            sess->logger.log(LogCategory::session, level, "Server log: %1", message);
            return;
        }

        logger.log(util::LogCategory::session, level, "Server log for unknown session %1: %2", session_ident,
                   message);
        return;
    }

    logger.log(level, "Server log: %1", message);
}


void Connection::receive_appservices_request_id(std::string_view coid)
{
    if (coid.empty() || !m_appservices_coid.empty()) {
        return;
    }
    m_appservices_coid = coid;
    logger.log(util::LogCategory::session, util::LogCategory::Level::info,
               "Connected to app services with request id: \"%1\". Further log entries for this connection will be "
               "prefixed with \"Connection[%2:%1]\" instead of \"Connection[%2]\"",
               m_appservices_coid, m_ident);
    logger.base_logger = make_logger(m_ident, m_appservices_coid, get_client().logger.base_logger);

    for (auto& [ident, sess] : m_sessions) {
        sess->logger.base_logger = Session::make_logger(ident, logger.base_logger);
    }
}


void Connection::handle_protocol_error(Status status)
{
    close_due_to_protocol_error(std::move(status));
}


// Sessions are guaranteed to be granted the opportunity to send a message in
// the order that they enlist. Note that this is important to ensure
// nonoverlapping communication with the server for consecutive sessions
// associated with the same Realm file.
//
// CAUTION: The specified session may get destroyed before this function
// returns, but only if its Session::send_message() puts it into the Deactivated
// state.
void Connection::enlist_to_send(Session* sess)
{
    REALM_ASSERT_EX(m_state == ConnectionState::connected, m_state);
    m_sessions_enlisted_to_send.push_back(sess); // Throws
    if (!m_sending)
        send_next_message(); // Throws
}


std::string Connection::get_active_appservices_connection_id()
{
    return m_appservices_coid;
}

void Session::cancel_resumption_delay()
{
    REALM_ASSERT_EX(m_state == Active, m_state);

    if (!m_suspended)
        return;

    m_suspended = false;

    logger.debug("Resumed"); // Throws

    if (unbind_process_complete())
        initiate_rebind(); // Throws

    try {
        process_pending_flx_bootstrap(); // throws
    }
    catch (const IntegrationException& error) {
        on_integration_failure(error);
    }
    catch (...) {
        on_integration_failure(IntegrationException(exception_to_status()));
    }

    m_conn.one_more_active_unsuspended_session(); // Throws
    if (m_try_again_activation_timer) {
        m_try_again_activation_timer.reset();
    }

    on_resumed(); // Throws
}


void Session::gather_pending_compensating_writes(util::Span<Changeset> changesets,
                                                 std::vector<ProtocolErrorInfo>* out)
{
    if (m_pending_compensating_write_errors.empty() || changesets.empty()) {
        return;
    }

#ifdef REALM_DEBUG
    REALM_ASSERT_DEBUG(
        std::is_sorted(m_pending_compensating_write_errors.begin(), m_pending_compensating_write_errors.end(),
                       [](const ProtocolErrorInfo& lhs, const ProtocolErrorInfo& rhs) {
                           REALM_ASSERT_DEBUG(lhs.compensating_write_server_version.has_value());
                           REALM_ASSERT_DEBUG(rhs.compensating_write_server_version.has_value());
                           return *lhs.compensating_write_server_version < *rhs.compensating_write_server_version;
                       }));
#endif

    while (!m_pending_compensating_write_errors.empty() &&
           *m_pending_compensating_write_errors.front().compensating_write_server_version <=
               changesets.back().version) {
        auto& cur_error = m_pending_compensating_write_errors.front();
        REALM_ASSERT_3(*cur_error.compensating_write_server_version, >=, changesets.front().version);
        out->push_back(std::move(cur_error));
        m_pending_compensating_write_errors.pop_front();
    }
}


void Session::integrate_changesets(const SyncProgress& progress, std::uint_fast64_t downloadable_bytes,
                                   const ReceivedChangesets& received_changesets, VersionInfo& version_info,
                                   DownloadBatchState download_batch_state)
{
    auto& history = get_history();
    if (received_changesets.empty()) {
        if (download_batch_state == DownloadBatchState::MoreToCome) {
            throw IntegrationException(ErrorCodes::SyncProtocolInvariantFailed,
                                       "received empty download message that was not the last in batch",
                                       ProtocolError::bad_progress);
        }
        history.set_sync_progress(progress, downloadable_bytes, version_info); // Throws
        return;
    }

    std::vector<ProtocolErrorInfo> pending_compensating_write_errors;
    auto transact = get_db()->start_read();
    history.integrate_server_changesets(
        progress, downloadable_bytes, received_changesets, version_info, download_batch_state, logger, transact,
        [&](const Transaction&, util::Span<Changeset> changesets) {
            gather_pending_compensating_writes(changesets, &pending_compensating_write_errors);
        }); // Throws
    if (received_changesets.size() == 1) {
        logger.debug("1 remote changeset integrated, producing client version %1",
                     version_info.sync_version.version); // Throws
    }
    else {
        logger.debug("%2 remote changesets integrated, producing client version %1",
                     version_info.sync_version.version, received_changesets.size()); // Throws
    }

    for (const auto& pending_error : pending_compensating_write_errors) {
        logger.info("Reporting compensating write for client version %1 in server version %2: %3",
                    pending_error.compensating_write_rejected_client_version,
                    *pending_error.compensating_write_server_version, pending_error.message);
        try {
            on_connection_state_changed(
                m_conn.get_state(),
                SessionErrorInfo{pending_error,
                                 protocol_error_to_status(static_cast<ProtocolError>(pending_error.raw_error_code),
                                                          pending_error.message)});
        }
        catch (...) {
            logger.error("Exception thrown while reporting compensating write: %1", exception_to_status());
        }
    }
}


void Session::on_integration_failure(const IntegrationException& error)
{
    REALM_ASSERT_EX(m_state == Active, m_state);
    REALM_ASSERT(!m_client_error && !m_error_to_send);
    logger.error("Failed to integrate downloaded changesets: %1", error.to_status());

    m_client_error = util::make_optional<IntegrationException>(error);
    m_error_to_send = true;
    SessionErrorInfo error_info{error.to_status(), IsFatal{false}};
    error_info.server_requests_action = ProtocolErrorInfo::Action::Warning;
    // Surface the error to the user otherwise is lost.
    on_connection_state_changed(m_conn.get_state(), std::move(error_info));

    // Since the deactivation process has not been initiated, the UNBIND
    // message cannot have been sent unless an ERROR message was received.
    REALM_ASSERT(m_suspended || m_error_message_received || !m_unbind_message_sent);
    if (m_ident_message_sent && !m_error_message_received && !m_suspended) {
        ensure_enlisted_to_send(); // Throws
    }
}

void Session::on_changesets_integrated(version_type client_version, const SyncProgress& progress)
{
    REALM_ASSERT_EX(m_state == Active, m_state);
    REALM_ASSERT_3(progress.download.server_version, >=, m_download_progress.server_version);

    m_download_progress = progress.download;
    m_progress = progress;

    if (progress.upload.client_version > m_upload_progress.client_version)
        m_upload_progress = progress.upload;

    do_recognize_sync_version(client_version); // Allows upload process to resume

    check_for_download_completion(); // Throws

    // If the client migrated from PBS to FLX, create subscriptions when new tables are received from server.
    if (auto migration_store = get_migration_store(); migration_store && m_is_flx_sync_session) {
        auto& flx_subscription_store = *get_flx_subscription_store();
        get_migration_store()->create_subscriptions(flx_subscription_store);
    }

    // Since the deactivation process has not been initiated, the UNBIND
    // message cannot have been sent unless an ERROR message was received.
    REALM_ASSERT(m_suspended || m_error_message_received || !m_unbind_message_sent);
    if (m_ident_message_sent && !m_error_message_received && !m_suspended) {
        ensure_enlisted_to_send(); // Throws
    }
}


Session::~Session()
{
    //    REALM_ASSERT_EX(m_state == Unactivated || m_state == Deactivated, m_state);
}


std::shared_ptr<util::Logger> Session::make_logger(session_ident_type ident,
                                                   std::shared_ptr<util::Logger> base_logger)
{
    auto prefix = util::format("Session[%1]: ", ident);
    return std::make_shared<util::PrefixLogger>(util::LogCategory::session, std::move(prefix),
                                                std::move(base_logger));
}

void Session::activate()
{
    REALM_ASSERT_EX(m_state == Unactivated, m_state);

    logger.debug("Activating"); // Throws

    if (REALM_LIKELY(!get_client().is_dry_run())) {
        bool file_exists = util::File::exists(get_realm_path());

        logger.info("client_reset_config = %1, Realm exists = %2, upload messages allowed = %3",
                    get_client_reset_config().has_value(), file_exists, upload_messages_allowed() ? "yes" : "no");
        get_history().get_status(m_last_version_available, m_client_file_ident, m_progress); // Throws
    }
    logger.debug("client_file_ident = %1, client_file_ident_salt = %2", m_client_file_ident.ident,
                 m_client_file_ident.salt); // Throws
    m_upload_progress = m_progress.upload;
    m_download_progress = m_progress.download;
    REALM_ASSERT_3(m_last_version_available, >=, m_progress.upload.client_version);
    init_progress_handler();

    logger.debug("last_version_available = %1", m_last_version_available);                     // Throws
    logger.debug("progress_download_server_version = %1", m_progress.download.server_version); // Throws
    logger.debug("progress_download_client_version = %1",
                 m_progress.download.last_integrated_client_version);                                      // Throws
    logger.debug("progress_upload_server_version = %1", m_progress.upload.last_integrated_server_version); // Throws
    logger.debug("progress_upload_client_version = %1", m_progress.upload.client_version);                 // Throws

    reset_protocol_state();
    m_state = Active;

    call_debug_hook(SyncClientHookEvent::SessionActivating);

    REALM_ASSERT(!m_suspended);
    m_conn.one_more_active_unsuspended_session(); // Throws

    try {
        process_pending_flx_bootstrap(); // throws
    }
    catch (const IntegrationException& error) {
        on_integration_failure(error);
    }
    catch (...) {
        on_integration_failure(IntegrationException(exception_to_status()));
    }

    // Checks if there is a pending client reset
    handle_pending_client_reset_acknowledgement();
}


// The caller (Connection) must discard the session if the session has become
// deactivated upon return.
void Session::initiate_deactivation()
{
    REALM_ASSERT_EX(m_state == Active, m_state);

    logger.debug("Initiating deactivation"); // Throws

    m_state = Deactivating;

    if (!m_suspended)
        m_conn.one_less_active_unsuspended_session(); // Throws

    if (m_enlisted_to_send) {
        REALM_ASSERT(!unbind_process_complete());
        return;
    }

    // Deactivate immediately if the BIND message has not yet been sent and the
    // session is not enlisted to send, or if the unbinding process has already
    // completed.
    if (!m_bind_message_sent || unbind_process_complete()) {
        complete_deactivation(); // Throws
        // Life cycle state is now Deactivated
        return;
    }

    // Ready to send the UNBIND message, if it has not already been sent
    if (!m_unbind_message_sent) {
        enlist_to_send(); // Throws
        return;
    }
}


void Session::complete_deactivation()
{
    REALM_ASSERT_EX(m_state == Deactivating, m_state);
    m_state = Deactivated;

    logger.debug("Deactivation completed"); // Throws
}


// Called by the associated Connection object when this session is granted an
// opportunity to send a message.
//
// The caller (Connection) must discard the session if the session has become
// deactivated upon return.
void Session::send_message()
{
    REALM_ASSERT_EX(m_state == Active || m_state == Deactivating, m_state);
    REALM_ASSERT(m_enlisted_to_send);
    m_enlisted_to_send = false;
    if (m_state == Deactivating || m_error_message_received || m_suspended) {
        // Deactivation has been initiated. If the UNBIND message has not been
        // sent yet, there is no point in sending it. Instead, we can let the
        // deactivation process complete.
        if (!m_bind_message_sent) {
            return complete_deactivation(); // Throws
            // Life cycle state is now Deactivated
        }

        // Session life cycle state is Deactivating or the unbinding process has
        // been initiated by a session specific ERROR message
        if (!m_unbind_message_sent)
            send_unbind_message(); // Throws
        return;
    }

    // Session life cycle state is Active and the unbinding process has
    // not been initiated
    REALM_ASSERT(!m_unbind_message_sent);

    if (!m_bind_message_sent)
        return send_bind_message(); // Throws

    // Pending test commands can be sent any time after the BIND message is sent
    const auto has_pending_test_command = std::any_of(m_pending_test_commands.begin(), m_pending_test_commands.end(),
                                                      [](const PendingTestCommand& command) {
                                                          return command.pending;
                                                      });
    if (has_pending_test_command) {
        return send_test_command_message();
    }

    if (!m_ident_message_sent) {
        if (have_client_file_ident())
            send_ident_message(); // Throws
        return;
    }

    if (m_error_to_send)
        return send_json_error_message(); // Throws

    // Stop sending upload, mark and query messages when the client detects an error.
    if (m_client_error) {
        return;
    }

    if (m_target_download_mark > m_last_download_mark_sent)
        return send_mark_message(); // Throws

    auto is_upload_allowed = [&]() -> bool {
        if (!m_is_flx_sync_session) {
            return true;
        }

        auto migration_store = get_migration_store();
        if (!migration_store) {
            return true;
        }

        auto sentinel_query_version = migration_store->get_sentinel_subscription_set_version();
        if (!sentinel_query_version) {
            return true;
        }

        // Do not allow upload if the last query sent is the sentinel one used by the migration store.
        return m_last_sent_flx_query_version != *sentinel_query_version;
    };

    if (!is_upload_allowed()) {
        return;
    }

    auto check_pending_flx_version = [&]() -> bool {
        if (!m_is_flx_sync_session) {
            return false;
        }

        if (m_delay_uploads) {
            return false;
        }

        m_pending_flx_sub_set = get_flx_subscription_store()->get_next_pending_version(m_last_sent_flx_query_version);

        if (!m_pending_flx_sub_set) {
            return false;
        }

        // Send QUERY messages when the upload progress client version reaches the snapshot version
        // of a pending subscription
        return m_upload_progress.client_version >= m_pending_flx_sub_set->snapshot_version;
    };

    if (check_pending_flx_version()) {
        return send_query_change_message(); // throws
    }

    if (!m_delay_uploads && (m_last_version_available > m_upload_progress.client_version)) {
        return send_upload_message(); // Throws
    }
}


void Session::send_bind_message()
{
    REALM_ASSERT_EX(m_state == Active, m_state);

    session_ident_type session_ident = m_ident;
    // Request an ident if we don't already have one and there isn't a pending client reset diff
    // The file ident can be 0 when a client reset is being performed if a brand new local realm
    // has been opened (or using Async open) and a FLX/PBS migration occurs when first connecting
    // to the server.
    bool need_client_file_ident = !have_client_file_ident() && !get_client_reset_config();
    const bool is_subserver = false;

    ClientProtocol& protocol = m_conn.get_client_protocol();
    int protocol_version = m_conn.get_negotiated_protocol_version();
    OutputBuffer& out = m_conn.get_output_buffer();
    // Discard the token since it's ignored by the server.
    std::string empty_access_token;
    if (m_is_flx_sync_session) {
        nlohmann::json bind_json_data;
        if (auto migrated_partition = get_migration_store()->get_migrated_partition()) {
            bind_json_data["migratedPartition"] = *migrated_partition;
        }
        bind_json_data["sessionReason"] = static_cast<uint64_t>(get_session_reason());
        auto schema_version = get_schema_version();
        // Send 0 if schema is not versioned.
        bind_json_data["schemaVersion"] = schema_version != uint64_t(-1) ? schema_version : 0;
        if (logger.would_log(util::Logger::Level::debug)) {
            std::string json_data_dump;
            if (!bind_json_data.empty()) {
                json_data_dump = bind_json_data.dump();
            }
            logger.debug(
                "Sending: BIND(session_ident=%1, need_client_file_ident=%2, is_subserver=%3, json_data=\"%4\")",
                session_ident, need_client_file_ident, is_subserver, json_data_dump);
        }
        protocol.make_flx_bind_message(protocol_version, out, session_ident, bind_json_data, empty_access_token,
                                       need_client_file_ident, is_subserver); // Throws
    }
    else {
        std::string server_path = get_virt_path();
        logger.debug("Sending: BIND(session_ident=%1, need_client_file_ident=%2, is_subserver=%3, server_path=%4)",
                     session_ident, need_client_file_ident, is_subserver, server_path);
        protocol.make_pbs_bind_message(protocol_version, out, session_ident, server_path, empty_access_token,
                                       need_client_file_ident, is_subserver); // Throws
    }
    m_conn.initiate_write_message(out, this); // Throws

    m_bind_message_sent = true;
    call_debug_hook(SyncClientHookEvent::BindMessageSent);

    // If there is a pending client reset diff, process that when the BIND message has
    // been sent successfully and wait before sending the IDENT message. Otherwise,
    // ready to send the IDENT message if the file identifier pair is already available.
    if (!need_client_file_ident)
        enlist_to_send(); // Throws
}


void Session::send_ident_message()
{
    REALM_ASSERT_EX(m_state == Active, m_state);
    REALM_ASSERT(m_bind_message_sent);
    REALM_ASSERT(!m_unbind_message_sent);
    REALM_ASSERT(have_client_file_ident());

    ClientProtocol& protocol = m_conn.get_client_protocol();
    OutputBuffer& out = m_conn.get_output_buffer();
    session_ident_type session_ident = m_ident;

    if (m_is_flx_sync_session) {
        const auto active_query_set = get_flx_subscription_store()->get_active();
        const auto active_query_body = active_query_set.to_ext_json();
        logger.debug("Sending: 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, query_version=%7, query_size=%8, query=\"%9\")",
                     m_client_file_ident.ident, m_client_file_ident.salt, m_progress.download.server_version,
                     m_progress.download.last_integrated_client_version, m_progress.latest_server_version.version,
                     m_progress.latest_server_version.salt, active_query_set.version(), active_query_body.size(),
                     active_query_body); // Throws
        protocol.make_flx_ident_message(out, session_ident, m_client_file_ident, m_progress,
                                        active_query_set.version(), active_query_body); // Throws
        m_last_sent_flx_query_version = active_query_set.version();
    }
    else {
        logger.debug("Sending: 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)",
                     m_client_file_ident.ident, m_client_file_ident.salt, m_progress.download.server_version,
                     m_progress.download.last_integrated_client_version, m_progress.latest_server_version.version,
                     m_progress.latest_server_version.salt);                                  // Throws
        protocol.make_pbs_ident_message(out, session_ident, m_client_file_ident, m_progress); // Throws
    }
    m_conn.initiate_write_message(out, this); // Throws

    m_ident_message_sent = true;
    call_debug_hook(SyncClientHookEvent::IdentMessageSent);

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

void Session::send_query_change_message()
{
    REALM_ASSERT_EX(m_state == Active, m_state);
    REALM_ASSERT(m_ident_message_sent);
    REALM_ASSERT(!m_unbind_message_sent);
    REALM_ASSERT(m_pending_flx_sub_set);
    REALM_ASSERT_3(m_pending_flx_sub_set->query_version, >, m_last_sent_flx_query_version);

    if (REALM_UNLIKELY(get_client().is_dry_run())) {
        return;
    }

    auto sub_store = get_flx_subscription_store();
    auto latest_sub_set = sub_store->get_by_version(m_pending_flx_sub_set->query_version);
    auto latest_queries = latest_sub_set.to_ext_json();
    logger.debug("Sending: QUERY(query_version=%1, query_size=%2, query=\"%3\", snapshot_version=%4)",
                 latest_sub_set.version(), latest_queries.size(), latest_queries, latest_sub_set.snapshot_version());

    OutputBuffer& out = m_conn.get_output_buffer();
    session_ident_type session_ident = get_ident();
    ClientProtocol& protocol = m_conn.get_client_protocol();
    protocol.make_query_change_message(out, session_ident, latest_sub_set.version(), latest_queries);
    m_conn.initiate_write_message(out, this);

    m_last_sent_flx_query_version = latest_sub_set.version();

    request_download_completion_notification();
}

void Session::send_upload_message()
{
    REALM_ASSERT_EX(m_state == Active, m_state);
    REALM_ASSERT(m_ident_message_sent);
    REALM_ASSERT(!m_unbind_message_sent);

    if (REALM_UNLIKELY(get_client().is_dry_run()))
        return;

    version_type target_upload_version = m_last_version_available;
    if (m_pending_flx_sub_set) {
        REALM_ASSERT(m_is_flx_sync_session);
        target_upload_version = m_pending_flx_sub_set->snapshot_version;
    }

    bool server_version_to_ack =
        m_upload_progress.last_integrated_server_version < m_download_progress.server_version;

    std::vector<UploadChangeset> uploadable_changesets;
    version_type locked_server_version = 0;
    get_history().find_uploadable_changesets(m_upload_progress, target_upload_version, uploadable_changesets,
                                             locked_server_version); // Throws

    if (uploadable_changesets.empty()) {
        // Nothing more to upload right now if:
        //  1. We need to limit upload up to some version other than the last client version
        //     available and there are no changes to upload
        //  2. There are no changes to upload and no server version(s) to acknowledge
        if (m_pending_flx_sub_set || !server_version_to_ack) {
            logger.trace("Empty UPLOAD was skipped (progress_client_version=%1, progress_server_version=%2)",
                         m_upload_progress.client_version, m_upload_progress.last_integrated_server_version);
            // Other messages may be waiting to be sent
            return enlist_to_send(); // Throws
        }
    }

    if (m_pending_flx_sub_set && target_upload_version < m_last_version_available) {
        logger.trace("Limiting UPLOAD message up to version %1 to send QUERY version %2",
                     m_pending_flx_sub_set->snapshot_version, m_pending_flx_sub_set->query_version);
    }

    version_type progress_client_version = m_upload_progress.client_version;
    version_type progress_server_version = m_upload_progress.last_integrated_server_version;

    if (!upload_messages_allowed()) {
        logger.trace("UPLOAD not allowed (progress_client_version=%1, progress_server_version=%2, "
                     "locked_server_version=%3, num_changesets=%4)",
                     progress_client_version, progress_server_version, locked_server_version,
                     uploadable_changesets.size()); // Throws
        // Other messages may be waiting to be sent
        return enlist_to_send(); // Throws
    }

    logger.debug("Sending: 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,
                 uploadable_changesets.size()); // Throws

    ClientProtocol& protocol = m_conn.get_client_protocol();
    ClientProtocol::UploadMessageBuilder upload_message_builder = protocol.make_upload_message_builder(); // Throws

    for (const UploadChangeset& uc : uploadable_changesets) {
        logger.debug(util::LogCategory::changeset,
                     "Fetching changeset for upload (client_version=%1, server_version=%2, "
                     "changeset_size=%3, origin_timestamp=%4, origin_file_ident=%5)",
                     uc.progress.client_version, uc.progress.last_integrated_server_version, uc.changeset.size(),
                     uc.origin_timestamp, uc.origin_file_ident); // Throws
        if (logger.would_log(util::Logger::Level::trace)) {
            BinaryData changeset_data = uc.changeset.get_first_chunk();
            if (changeset_data.size() < 1024) {
                logger.trace(util::LogCategory::changeset, "Changeset: %1",
                             _impl::clamped_hex_dump(changeset_data)); // Throws
            }
            else {
                logger.trace(util::LogCategory::changeset, "Changeset(comp): %1 %2", changeset_data.size(),
                             protocol.compressed_hex_dump(changeset_data));
            }

#if REALM_DEBUG
            ChunkedBinaryInputStream in{changeset_data};
            Changeset log;
            try {
                parse_changeset(in, log);
                std::stringstream ss;
                log.print(ss);
                logger.trace(util::LogCategory::changeset, "Changeset (parsed):\n%1", ss.str());
            }
            catch (const BadChangesetError& err) {
                logger.error(util::LogCategory::changeset, "Unable to parse changeset: %1", err.what());
            }
#endif
        }

        {
            upload_message_builder.add_changeset(uc.progress.client_version,
                                                 uc.progress.last_integrated_server_version, uc.origin_timestamp,
                                                 uc.origin_file_ident,
                                                 uc.changeset); // Throws
        }
    }

    int protocol_version = m_conn.get_negotiated_protocol_version();
    OutputBuffer& out = m_conn.get_output_buffer();
    session_ident_type session_ident = get_ident();
    upload_message_builder.make_upload_message(protocol_version, out, session_ident, progress_client_version,
                                               progress_server_version,
                                               locked_server_version); // Throws
    m_conn.initiate_write_message(out, this);                          // Throws

    call_debug_hook(SyncClientHookEvent::UploadMessageSent);

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


void Session::send_mark_message()
{
    REALM_ASSERT_EX(m_state == Active, m_state);
    REALM_ASSERT(m_ident_message_sent);
    REALM_ASSERT(!m_unbind_message_sent);
    REALM_ASSERT_3(m_target_download_mark, >, m_last_download_mark_sent);

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

    ClientProtocol& protocol = m_conn.get_client_protocol();
    OutputBuffer& out = m_conn.get_output_buffer();
    session_ident_type session_ident = get_ident();
    protocol.make_mark_message(out, session_ident, request_ident); // Throws
    m_conn.initiate_write_message(out, this);                      // Throws

    m_last_download_mark_sent = request_ident;

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


void Session::send_unbind_message()
{
    REALM_ASSERT_EX(m_state == Deactivating || m_error_message_received || m_suspended, m_state);
    REALM_ASSERT(m_bind_message_sent);
    REALM_ASSERT(!m_unbind_message_sent);

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

    ClientProtocol& protocol = m_conn.get_client_protocol();
    OutputBuffer& out = m_conn.get_output_buffer();
    session_ident_type session_ident = get_ident();
    protocol.make_unbind_message(out, session_ident); // Throws
    m_conn.initiate_write_message(out, this);         // Throws

    m_unbind_message_sent = true;
}


void Session::send_json_error_message()
{
    REALM_ASSERT_EX(m_state == Active, m_state);
    REALM_ASSERT(m_ident_message_sent);
    REALM_ASSERT(!m_unbind_message_sent);
    REALM_ASSERT(m_error_to_send);
    REALM_ASSERT(m_client_error);

    ClientProtocol& protocol = m_conn.get_client_protocol();
    OutputBuffer& out = m_conn.get_output_buffer();
    session_ident_type session_ident = get_ident();
    auto protocol_error = m_client_error->error_for_server;

    auto message = util::format("%1", m_client_error->to_status());
    logger.info("Sending: ERROR \"%1\" (error_code=%2, session_ident=%3)", message, static_cast<int>(protocol_error),
                session_ident); // Throws

    nlohmann::json error_body_json;
    error_body_json["message"] = std::move(message);
    protocol.make_json_error_message(out, session_ident, static_cast<int>(protocol_error),
                                     error_body_json.dump()); // Throws
    m_conn.initiate_write_message(out, this);                 // Throws

    m_error_to_send = false;
    enlist_to_send(); // Throws
}


void Session::send_test_command_message()
{
    REALM_ASSERT_EX(m_state == Active, m_state);

    auto it = std::find_if(m_pending_test_commands.begin(), m_pending_test_commands.end(),
                           [](const PendingTestCommand& command) {
                               return command.pending;
                           });
    REALM_ASSERT(it != m_pending_test_commands.end());

    ClientProtocol& protocol = m_conn.get_client_protocol();
    OutputBuffer& out = m_conn.get_output_buffer();
    auto session_ident = get_ident();

    logger.info("Sending: TEST_COMMAND \"%1\" (session_ident=%2, request_ident=%3)", it->body, session_ident, it->id);
    protocol.make_test_command_message(out, session_ident, it->id, it->body);

    m_conn.initiate_write_message(out, this); // Throws;
    it->pending = false;

    enlist_to_send();
}

bool Session::client_reset_if_needed()
{
    // Even if we end up not actually performing a client reset, consume the
    // config to ensure that the resources it holds are released
    auto client_reset_config = std::exchange(get_client_reset_config(), std::nullopt);
    if (!client_reset_config) {
        return false;
    }

    // Save a copy of the status and action in case an error/exception occurs
    Status cr_status = client_reset_config->error;
    ProtocolErrorInfo::Action cr_action = client_reset_config->action;

    try {
        // The file ident from the fresh realm will be copied over to the local realm
        bool did_reset = client_reset::perform_client_reset(logger, *get_db(), std::move(*client_reset_config),
                                                            get_flx_subscription_store());

        call_debug_hook(SyncClientHookEvent::ClientResetMergeComplete);
        if (!did_reset) {
            return false;
        }
    }
    catch (const std::exception& e) {
        auto err_msg = util::format("A fatal error occurred during '%1' client reset diff for %2: '%3'", cr_action,
                                    cr_status, e.what());
        logger.error(err_msg.c_str());
        SessionErrorInfo err_info(Status{ErrorCodes::AutoClientResetFailed, err_msg}, IsFatal{true});
        suspend(err_info);
        return false;
    }

    // The fresh Realm has been used to reset the state
    logger.debug("Client reset is completed, path = %1", get_realm_path()); // Throws

    // Update the version, file ident and progress info after the client reset diff is done
    get_history().get_status(m_last_version_available, m_client_file_ident, m_progress); // Throws
    // Print the version/progress information before performing the asserts
    logger.debug("client_file_ident = %1, client_file_ident_salt = %2", m_client_file_ident.ident,
                 m_client_file_ident.salt);                                // Throws
    logger.debug("last_version_available = %1", m_last_version_available); // Throws
    logger.debug("upload_progress_client_version = %1, upload_progress_server_version = %2",
                 m_progress.upload.client_version,
                 m_progress.upload.last_integrated_server_version); // Throws
    logger.debug("download_progress_client_version = %1, download_progress_server_version = %2",
                 m_progress.download.last_integrated_client_version,
                 m_progress.download.server_version); // Throws

    REALM_ASSERT_EX(m_progress.download.last_integrated_client_version == 0,
                    m_progress.download.last_integrated_client_version);
    REALM_ASSERT_EX(m_progress.upload.client_version == 0, m_progress.upload.client_version);

    m_upload_progress = m_progress.upload;
    m_download_progress = m_progress.download;
    init_progress_handler();
    // In recovery mode, there may be new changesets to upload and nothing left to download.
    // In FLX DiscardLocal mode, there may be new commits due to subscription handling.
    // For both, we want to allow uploads again without needing external changes to download first.
    m_delay_uploads = false;

    // Checks if there is a pending client reset
    handle_pending_client_reset_acknowledgement();

    // If a migration or rollback is in progress, mark it complete when client reset is completed.
    if (auto migration_store = get_migration_store()) {
        migration_store->complete_migration_or_rollback();
    }

    return true;
}

Status Session::receive_ident_message(SaltedFileIdent client_file_ident)
{
    logger.debug("Received: IDENT(client_file_ident=%1, client_file_ident_salt=%2)", client_file_ident.ident,
                 client_file_ident.salt); // Throws

    // Ignore the message if the deactivation process has been initiated,
    // because in that case, the associated Realm and SessionWrapper must
    // not be accessed any longer.
    if (m_state != Active)
        return Status::OK(); // Success

    bool legal_at_this_time = (m_bind_message_sent && !have_client_file_ident() && !m_error_message_received &&
                               !m_unbound_message_received);
    if (REALM_UNLIKELY(!legal_at_this_time)) {
        return {ErrorCodes::SyncProtocolInvariantFailed, "Received IDENT message when it was not legal"};
    }
    if (REALM_UNLIKELY(client_file_ident.ident < 1)) {
        return {ErrorCodes::SyncProtocolInvariantFailed, "Bad client file identifier in IDENT message"};
    }
    if (REALM_UNLIKELY(client_file_ident.salt == 0)) {
        return {ErrorCodes::SyncProtocolInvariantFailed, "Bad client file identifier salt in IDENT message"};
    }

    m_client_file_ident = client_file_ident;

    if (REALM_UNLIKELY(get_client().is_dry_run())) {
        // Ready to send the IDENT message
        ensure_enlisted_to_send(); // Throws
        return Status::OK();       // Success
    }

    get_history().set_client_file_ident(client_file_ident,
                                        m_fix_up_object_ids); // Throws
    m_progress.download.last_integrated_client_version = 0;
    m_progress.upload.client_version = 0;

    // Ready to send the IDENT message
    ensure_enlisted_to_send(); // Throws
    return Status::OK();       // Success
}

Status Session::receive_download_message(const DownloadMessage& message)
{
    // Ignore the message if the deactivation process has been initiated,
    // because in that case, the associated Realm and SessionWrapper must
    // not be accessed any longer.
    if (m_state != Active)
        return Status::OK();

    bool is_flx = m_conn.is_flx_sync_connection();
    int64_t query_version = is_flx ? *message.query_version : 0;

    if (!is_flx || query_version > 0)
        enable_progress_notifications();

    auto&& progress = message.progress;
    if (is_flx) {
        logger.debug("Received: DOWNLOAD(download_server_version=%1, download_client_version=%2, "
                     "latest_server_version=%3, latest_server_version_salt=%4, "
                     "upload_client_version=%5, upload_server_version=%6, progress_estimate=%7, "
                     "batch_state=%8, query_version=%9, num_changesets=%10, ...)",
                     progress.download.server_version, progress.download.last_integrated_client_version,
                     progress.latest_server_version.version, progress.latest_server_version.salt,
                     progress.upload.client_version, progress.upload.last_integrated_server_version,
                     message.downloadable.as_estimate(), message.batch_state, query_version,
                     message.changesets.size()); // Throws
    }
    else {
        logger.debug("Received: DOWNLOAD(download_server_version=%1, download_client_version=%2, "
                     "latest_server_version=%3, latest_server_version_salt=%4, "
                     "upload_client_version=%5, upload_server_version=%6, "
                     "downloadable_bytes=%7, num_changesets=%8, ...)",
                     progress.download.server_version, progress.download.last_integrated_client_version,
                     progress.latest_server_version.version, progress.latest_server_version.salt,
                     progress.upload.client_version, progress.upload.last_integrated_server_version,
                     message.downloadable.as_bytes(), message.changesets.size()); // Throws
    }

    // Ignore download messages when the client detects an error. This is to prevent transforming the same bad
    // changeset over and over again.
    if (m_client_error) {
        logger.debug("Ignoring download message because the client detected an integration error");
        return Status::OK();
    }

    bool legal_at_this_time = (m_ident_message_sent && !m_error_message_received && !m_unbound_message_received);
    if (REALM_UNLIKELY(!legal_at_this_time)) {
        return {ErrorCodes::SyncProtocolInvariantFailed, "Received DOWNLOAD message when it was not legal"};
    }
    if (auto status = check_received_sync_progress(progress); REALM_UNLIKELY(!status.is_ok())) {
        logger.error("Bad sync progress received (%1)", status);
        return status;
    }

    version_type server_version = m_progress.download.server_version;
    version_type last_integrated_client_version = m_progress.download.last_integrated_client_version;
    for (const RemoteChangeset& changeset : message.changesets) {
        // Check that per-changeset server version is strictly increasing, except in FLX sync where the server
        // version must be increasing, but can stay the same during bootstraps.
        bool good_server_version = m_is_flx_sync_session ? (changeset.remote_version >= server_version)
                                                         : (changeset.remote_version > server_version);
        // Each server version cannot be greater than the one in the header of the download message.
        good_server_version = good_server_version && (changeset.remote_version <= progress.download.server_version);
        if (!good_server_version) {
            return {ErrorCodes::SyncProtocolInvariantFailed,
                    util::format("Bad server version in changeset header (DOWNLOAD) (%1, %2, %3)",
                                 changeset.remote_version, server_version, progress.download.server_version)};
        }
        server_version = changeset.remote_version;

        // Check that per-changeset last integrated client version is "weakly"
        // increasing.
        bool good_client_version =
            (changeset.last_integrated_local_version >= last_integrated_client_version &&
             changeset.last_integrated_local_version <= progress.download.last_integrated_client_version);
        if (!good_client_version) {
            return {ErrorCodes::SyncProtocolInvariantFailed,
                    util::format("Bad last integrated client version in changeset header (DOWNLOAD) "
                                 "(%1, %2, %3)",
                                 changeset.last_integrated_local_version, last_integrated_client_version,
                                 progress.download.last_integrated_client_version)};
        }
        last_integrated_client_version = changeset.last_integrated_local_version;
        // Server shouldn't send our own changes, and zero is not a valid client
        // file identifier.
        bool good_file_ident =
            (changeset.origin_file_ident > 0 && changeset.origin_file_ident != m_client_file_ident.ident);
        if (!good_file_ident) {
            return {ErrorCodes::SyncProtocolInvariantFailed,
                    util::format("Bad origin file identifier in changeset header (DOWNLOAD)",
                                 changeset.origin_file_ident)};
        }
    }

    auto hook_action = call_debug_hook(SyncClientHookEvent::DownloadMessageReceived, progress, query_version,
                                       message.batch_state, message.changesets.size());
    if (hook_action == SyncClientHookAction::EarlyReturn) {
        return Status::OK();
    }
    REALM_ASSERT_EX(hook_action == SyncClientHookAction::NoAction, hook_action);

    if (process_flx_bootstrap_message(message)) {
        clear_resumption_delay_state();
        return Status::OK();
    }

    initiate_integrate_changesets(message.downloadable.as_bytes(), message.batch_state, progress,
                                  message.changesets); // Throws

    hook_action = call_debug_hook(SyncClientHookEvent::DownloadMessageIntegrated, progress, query_version,
                                  message.batch_state, message.changesets.size());
    if (hook_action == SyncClientHookAction::EarlyReturn) {
        return Status::OK();
    }
    REALM_ASSERT_EX(hook_action == SyncClientHookAction::NoAction, hook_action);

    // When we receive a DOWNLOAD message successfully, we can clear the backoff timer value used to reconnect
    // after a retryable session error.
    clear_resumption_delay_state();
    return Status::OK();
}

Status Session::receive_mark_message(request_ident_type request_ident)
{
    logger.debug("Received: MARK(request_ident=%1)", request_ident); // Throws

    // Ignore the message if the deactivation process has been initiated,
    // because in that case, the associated Realm and SessionWrapper must
    // not be accessed any longer.
    if (m_state != Active)
        return Status::OK(); // Success

    bool legal_at_this_time = (m_ident_message_sent && !m_error_message_received && !m_unbound_message_received);
    if (REALM_UNLIKELY(!legal_at_this_time)) {
        return {ErrorCodes::SyncProtocolInvariantFailed, "Received MARK message when it was not legal"};
    }
    bool good_request_ident =
        (request_ident <= m_last_download_mark_sent && request_ident > m_last_download_mark_received);
    if (REALM_UNLIKELY(!good_request_ident)) {
        return {
            ErrorCodes::SyncProtocolInvariantFailed,
            util::format(
                "Received MARK message with invalid request identifer (last mark sent: %1 last mark received: %2)",
                m_last_download_mark_sent, m_last_download_mark_received)};
    }

    m_server_version_at_last_download_mark = m_progress.download.server_version;
    m_last_download_mark_received = request_ident;
    check_for_download_completion(); // Throws

    return Status::OK(); // Success
}


// The caller (Connection) must discard the session if the session has become
// deactivated upon return.
Status Session::receive_unbound_message()
{
    logger.debug("Received: UNBOUND");

    bool legal_at_this_time = (m_unbind_message_sent && !m_error_message_received && !m_unbound_message_received);
    if (REALM_UNLIKELY(!legal_at_this_time)) {
        return {ErrorCodes::SyncProtocolInvariantFailed, "Received UNBOUND message when it was not legal"};
    }

    // The fact that the UNBIND message has been sent, but an ERROR message has
    // not been received, implies that the deactivation process must have been
    // initiated, so this session must be in the Deactivating state or the session
    // has been suspended because of a client side error.
    REALM_ASSERT_EX(m_state == Deactivating || m_suspended, m_state);

    m_unbound_message_received = true;

    // Detect completion of the unbinding process
    if (m_unbind_message_send_complete && m_state == Deactivating) {
        // The deactivation process completes when the unbinding process
        // completes.
        complete_deactivation(); // Throws
        // Life cycle state is now Deactivated
    }

    return Status::OK(); // Success
}


void Session::receive_query_error_message(int error_code, std::string_view message, int64_t query_version)
{
    logger.info("Received QUERY_ERROR \"%1\" (error_code=%2, query_version=%3)", message, error_code, query_version);
    on_flx_sync_error(query_version, message); // throws
}

// The caller (Connection) must discard the session if the session has become
// deactivated upon return.
Status Session::receive_error_message(const ProtocolErrorInfo& info)
{
    logger.info("Received: ERROR \"%1\" (error_code=%2, is_fatal=%3, error_action=%4)", info.message,
                info.raw_error_code, info.is_fatal, info.server_requests_action); // Throws

    bool legal_at_this_time = (m_bind_message_sent && !m_error_message_received && !m_unbound_message_received);
    if (REALM_UNLIKELY(!legal_at_this_time)) {
        return {ErrorCodes::SyncProtocolInvariantFailed, "Received ERROR message when it was not legal"};
    }

    auto protocol_error = static_cast<ProtocolError>(info.raw_error_code);
    auto status = protocol_error_to_status(protocol_error, info.message);
    if (status != ErrorCodes::UnknownError && REALM_UNLIKELY(!is_session_level_error(protocol_error))) {
        return {ErrorCodes::SyncProtocolInvariantFailed,
                util::format("Received ERROR message for session with non-session-level error code %1",
                             info.raw_error_code)};
    }

    // Can't process debug hook actions once the Session is undergoing deactivation, since
    // the SessionWrapper may not be available
    if (m_state == Active) {
        auto debug_action = call_debug_hook(SyncClientHookEvent::ErrorMessageReceived, &info);
        if (debug_action == SyncClientHookAction::EarlyReturn) {
            return Status::OK();
        }
    }

    // For compensating write errors, we need to defer raising them to the SDK until after the server version
    // containing the compensating write has appeared in a download message.
    if (status == ErrorCodes::SyncCompensatingWrite) {
        // If the client is not active, the compensating writes will not be processed now, but will be
        // sent again the next time the client connects
        if (m_state == Active) {
            REALM_ASSERT(info.compensating_write_server_version.has_value());
            m_pending_compensating_write_errors.push_back(info);
        }
        return Status::OK();
    }

    if (protocol_error == ProtocolError::schema_version_changed) {
        // Enable upload immediately if the session is still active.
        if (m_state == Active) {
            auto wt = get_db()->start_write();
            _impl::sync_schema_migration::track_sync_schema_migration(*wt, *info.previous_schema_version);
            wt->commit();
            // Notify SyncSession a schema migration is required.
            on_connection_state_changed(m_conn.get_state(), SessionErrorInfo{info});
        }
        // Keep the session active to upload any unsynced changes.
        return Status::OK();
    }

    m_error_message_received = true;
    suspend(SessionErrorInfo{info, std::move(status)});
    return Status::OK();
}

void Session::suspend(const SessionErrorInfo& info)
{
    REALM_ASSERT(!m_suspended);
    REALM_ASSERT_EX(m_state == Active || m_state == Deactivating, m_state);
    logger.debug("Suspended"); // Throws

    m_suspended = true;

    // Detect completion of the unbinding process
    if (m_unbind_message_send_complete && m_error_message_received) {
        // The fact that the UNBIND message has been sent, but we are not being suspended because
        // we received an ERROR message implies that the deactivation process must
        // have been initiated, so this session must be in the Deactivating state.
        REALM_ASSERT_EX(m_state == Deactivating, m_state);

        // The deactivation process completes when the unbinding process
        // completes.
        complete_deactivation(); // Throws
        // Life cycle state is now Deactivated
    }

    // Notify the application of the suspension of the session if the session is
    // still in the Active state
    if (m_state == Active) {
        call_debug_hook(SyncClientHookEvent::SessionSuspended, &info);
        m_conn.one_less_active_unsuspended_session(); // Throws
        on_suspended(info);                           // Throws
    }

    if (!info.is_fatal) {
        begin_resumption_delay(info);
    }

    // Ready to send the UNBIND message, if it has not been sent already
    if (!m_unbind_message_sent)
        ensure_enlisted_to_send(); // Throws
}

Status Session::receive_test_command_response(request_ident_type ident, std::string_view body)
{
    logger.info("Received: TEST_COMMAND \"%1\" (session_ident=%2, request_ident=%3)", body, m_ident, ident);
    auto it = std::find_if(m_pending_test_commands.begin(), m_pending_test_commands.end(),
                           [&](const PendingTestCommand& command) {
                               return command.id == ident;
                           });
    if (it == m_pending_test_commands.end()) {
        return {ErrorCodes::SyncProtocolInvariantFailed,
                util::format("Received test command response for a non-existent ident %1", ident)};
    }

    it->promise.emplace_value(std::string{body});
    m_pending_test_commands.erase(it);

    return Status::OK();
}

void Session::begin_resumption_delay(const ProtocolErrorInfo& error_info)
{
    REALM_ASSERT(!m_try_again_activation_timer);

    m_try_again_delay_info.update(static_cast<sync::ProtocolError>(error_info.raw_error_code),
                                  error_info.resumption_delay_interval);
    auto try_again_interval = m_try_again_delay_info.delay_interval();
    if (ProtocolError(error_info.raw_error_code) == ProtocolError::session_closed) {
        // FIXME With compensating writes the server sends this error after completing a bootstrap. Doing the
        // normal backoff behavior would result in waiting up to 5 minutes in between each query change which is
        // not acceptable latency. So for this error code alone, we hard-code a 1 second retry interval.
        try_again_interval = std::chrono::milliseconds{1000};
    }
    logger.debug("Will attempt to resume session after %1 milliseconds", try_again_interval.count());
    m_try_again_activation_timer = get_client().create_timer(try_again_interval, [this](Status status) {
        if (status == ErrorCodes::OperationAborted)
            return;
        else if (!status.is_ok())
            throw Exception(status);

        m_try_again_activation_timer.reset();
        cancel_resumption_delay();
    });
}

void Session::clear_resumption_delay_state()
{
    if (m_try_again_activation_timer) {
        logger.debug("Clearing resumption delay state after successful download");
        m_try_again_delay_info.reset();
    }
}

Status Session::check_received_sync_progress(const SyncProgress& progress) noexcept
{
    const SyncProgress& a = m_progress;
    const SyncProgress& b = progress;
    std::string message;
    if (b.latest_server_version.version < a.latest_server_version.version) {
        message = util::format("Latest server version in download messages must be weakly increasing throughout a "
                               "session (current: %1, received: %2)",
                               a.latest_server_version.version, b.latest_server_version.version);
    }
    if (b.upload.client_version < a.upload.client_version) {
        message = util::format("Last integrated client version in download messages must be weakly increasing "
                               "throughout a session (current: %1, received: %2)",
                               a.upload.client_version, b.upload.client_version);
    }
    if (b.upload.client_version > m_last_version_available) {
        message = util::format("Last integrated client version on server cannot be greater than the latest client "
                               "version in existence (current: %1, received: %2)",
                               m_last_version_available, b.upload.client_version);
    }
    if (b.download.server_version < a.download.server_version) {
        message =
            util::format("Download cursor must be weakly increasing throughout a session (current: %1, received: %2)",
                         a.download.server_version, b.download.server_version);
    }
    if (b.download.server_version > b.latest_server_version.version) {
        message = util::format(
            "Download cursor cannot be greater than the latest server version in existence (cursor: %1, latest: %2)",
            b.download.server_version, b.latest_server_version.version);
    }
    if (b.download.last_integrated_client_version < a.download.last_integrated_client_version) {
        message = util::format(
            "Last integrated client version on the server at the position in the server's history of the download "
            "cursor must be weakly increasing throughout a session (current: %1, received: %2)",
            a.download.last_integrated_client_version, b.download.last_integrated_client_version);
    }
    if (b.download.last_integrated_client_version > b.upload.client_version) {
        message = util::format("Last integrated client version on the server in the position at the server's history "
                               "of the download cursor cannot be greater than the latest client version integrated "
                               "on the server (download: %1, upload: %2)",
                               b.download.last_integrated_client_version, b.upload.client_version);
    }
    if (b.download.server_version < b.upload.last_integrated_server_version) {
        message = util::format(
            "The server version of the download cursor cannot be less than the server version integrated in the "
            "latest client version acknowledged by the server (download: %1, upload: %2)",
            b.download.server_version, b.upload.last_integrated_server_version);
    }

    if (message.empty()) {
        return Status::OK();
    }
    return {ErrorCodes::SyncProtocolInvariantFailed, std::move(message)};
}


void Session::check_for_download_completion()
{
    REALM_ASSERT_3(m_target_download_mark, >=, m_last_download_mark_received);
    REALM_ASSERT_3(m_last_download_mark_received, >=, m_last_triggering_download_mark);
    if (m_last_download_mark_received == m_last_triggering_download_mark)
        return;
    if (m_last_download_mark_received < m_target_download_mark)
        return;
    if (m_download_progress.server_version < m_server_version_at_last_download_mark)
        return;
    m_last_triggering_download_mark = m_target_download_mark;
    if (REALM_UNLIKELY(m_delay_uploads)) {
        // Activate the upload process now, and enable immediate reactivation
        // after a subsequent fast reconnect.
        m_delay_uploads = false;
        ensure_enlisted_to_send(); // Throws
    }
    on_download_completion(); // Throws
}

realm / realm-core / thomas.goyne_478

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