1817

Committed 04 Nov 2023 12:29AM UTC coverage: 91.695% (+0.04%) from 91.66%

Build # 1817

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Commit Message

Use a single write transaction for DiscardLocal client resets on FLX realms (#7110)

Updating the subscription store in a separate write transaction from the
recovery means that we temporarily commit an invalid state. If the application
crashes between committing the client reset diff and updating the subscription
store, the next launch of the application would try to use the now-invalid
pending subscriptions that should have been discarded.

Run Details

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93.69

/src/realm/sync/noinst/client_reset.cpp

///////////////////////////////////////////////////////////////////////////
//
// Copyright 2021 Realm Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
////////////////////////////////////////////////////////////////////////////

#include <realm/transaction.hpp>
#include <realm/dictionary.hpp>
#include <realm/object_converter.hpp>
#include <realm/table_view.hpp>
#include <realm/set.hpp>

#include <realm/sync/history.hpp>
#include <realm/sync/changeset_parser.hpp>
#include <realm/sync/instruction_applier.hpp>
#include <realm/sync/noinst/client_history_impl.hpp>
#include <realm/sync/noinst/client_reset.hpp>
#include <realm/sync/noinst/client_reset_recovery.hpp>
#include <realm/sync/subscriptions.hpp>

#include <realm/util/compression.hpp>

#include <algorithm>
#include <chrono>
#include <vector>

using namespace realm;
using namespace _impl;
using namespace sync;

namespace realm {

std::ostream& operator<<(std::ostream& os, const ClientResyncMode& mode)
{
    switch (mode) {
        case ClientResyncMode::Manual:
            os << "Manual";
            break;
        case ClientResyncMode::DiscardLocal:
            os << "DiscardLocal";
            break;
        case ClientResyncMode::Recover:
            os << "Recover";
            break;
        case ClientResyncMode::RecoverOrDiscard:
            os << "RecoverOrDiscard";
            break;
    }
    return os;
}

} // namespace realm

namespace realm::_impl::client_reset {

static inline bool should_skip_table(const Transaction& group, TableKey key)
{
    return !group.table_is_public(key);
}

void transfer_group(const Transaction& group_src, Transaction& group_dst, util::Logger& logger,
                    bool allow_schema_additions)
{
    logger.debug("transfer_group, src size = %1, dst size = %2, allow_schema_additions = %3", group_src.size(),
                 group_dst.size(), allow_schema_additions);

    // Turn off the sync history tracking during state transfer since it will be thrown
    // away immediately after anyways. This reduces the memory footprint of a client reset.
    ClientReplication* client_repl = dynamic_cast<ClientReplication*>(group_dst.get_replication());
    REALM_ASSERT_RELEASE(client_repl);
    TempShortCircuitReplication sync_history_guard(*client_repl);

    // Find all tables in dst that should be removed.
    std::set<std::string> tables_to_remove;
    for (auto table_key : group_dst.get_table_keys()) {
        if (should_skip_table(group_dst, table_key))
            continue;
        StringData table_name = group_dst.get_table_name(table_key);
        logger.debug("key = %1, table_name = %2", table_key.value, table_name);
        ConstTableRef table_src = group_src.get_table(table_name);
        if (!table_src) {
            logger.debug("Table '%1' will be removed", table_name);
            tables_to_remove.insert(table_name);
            continue;
        }
        // Check whether the table type is the same.
        TableRef table_dst = group_dst.get_table(table_key);
        auto pk_col_src = table_src->get_primary_key_column();
        auto pk_col_dst = table_dst->get_primary_key_column();
        bool has_pk_src = bool(pk_col_src);
        bool has_pk_dst = bool(pk_col_dst);
        if (has_pk_src != has_pk_dst) {
            throw ClientResetFailed(util::format("Client reset requires a primary key column in %1 table '%2'",
                                                 (has_pk_src ? "dest" : "source"), table_name));
        }
        if (!has_pk_src)
            continue;

        // Now the tables both have primary keys. Check type.
        if (pk_col_src.get_type() != pk_col_dst.get_type()) {
            throw ClientResetFailed(
                util::format("Client reset found incompatible primary key types (%1 vs %2) on '%3'",
                             pk_col_src.get_type(), pk_col_dst.get_type(), table_name));
        }
        // Check collection type, nullability etc. but having an index doesn't matter;
        ColumnAttrMask pk_col_src_attr = pk_col_src.get_attrs();
        ColumnAttrMask pk_col_dst_attr = pk_col_dst.get_attrs();
        pk_col_src_attr.reset(ColumnAttr::col_attr_Indexed);
        pk_col_dst_attr.reset(ColumnAttr::col_attr_Indexed);
        if (pk_col_src_attr != pk_col_dst_attr) {
            throw ClientResetFailed(
                util::format("Client reset found incompatible primary key attributes (%1 vs %2) on '%3'",
                             pk_col_src.value, pk_col_dst.value, table_name));
        }
        // Check name.
        StringData pk_col_name_src = table_src->get_column_name(pk_col_src);
        StringData pk_col_name_dst = table_dst->get_column_name(pk_col_dst);
        if (pk_col_name_src != pk_col_name_dst) {
            throw ClientResetFailed(
                util::format("Client reset requires equal pk column names but '%1' != '%2' on '%3'", pk_col_name_src,
                             pk_col_name_dst, table_name));
        }
        // The table survives.
        logger.debug("Table '%1' will remain", table_name);
    }

    // If there have been any tables marked for removal stop.
    // We consider two possible options for recovery:
    // 1: Remove the tables. But this will generate destructive schema
    //    schema changes that the local Realm cannot advance through.
    //    Since this action will fail down the line anyway, give up now.
    // 2: Keep the tables locally and ignore them. But the local app schema
    //    still has these classes and trying to modify anything in them will
    //    create sync instructions on tables that sync doesn't know about.
    // As an exception in recovery mode, we assume that the corresponding
    // additive schema changes will be part of the recovery upload. If they
    // are present, then the server can choose to allow them (if in dev mode).
    // If they are not present, then the server will emit an error the next time
    // a value is set on the unknown property.
    if (!allow_schema_additions && !tables_to_remove.empty()) {
        std::string names_list;
        for (const std::string& table_name : tables_to_remove) {
            names_list += Group::table_name_to_class_name(table_name);
            names_list += ", ";
        }
        if (names_list.size() > 2) {
            // remove the final ", "
            names_list = names_list.substr(0, names_list.size() - 2);
        }
        throw ClientResetFailed(
            util::format("Client reset cannot recover when classes have been removed: {%1}", names_list));
    }

    // Create new tables in dst if needed.
    for (auto table_key : group_src.get_table_keys()) {
        if (should_skip_table(group_src, table_key))
            continue;
        ConstTableRef table_src = group_src.get_table(table_key);
        StringData table_name = table_src->get_name();
        auto pk_col_src = table_src->get_primary_key_column();
        TableRef table_dst = group_dst.get_table(table_name);
        if (!table_dst) {
            // Create the table.
            if (table_src->is_embedded()) {
                REALM_ASSERT(!pk_col_src);
                group_dst.add_table(table_name, Table::Type::Embedded);
            }
            else {
                REALM_ASSERT(pk_col_src); // a sync table will have a pk
                auto pk_col_src = table_src->get_primary_key_column();
                DataType pk_type = DataType(pk_col_src.get_type());
                StringData pk_col_name = table_src->get_column_name(pk_col_src);
                group_dst.add_table_with_primary_key(table_name, pk_type, pk_col_name, pk_col_src.is_nullable(),
                                                     table_src->get_table_type());
            }
        }
    }

    // Now the class tables are identical.
    size_t num_tables;
    {
        size_t num_tables_src = 0;
        for (auto table_key : group_src.get_table_keys()) {
            if (!should_skip_table(group_src, table_key))
                ++num_tables_src;
        }
        size_t num_tables_dst = 0;
        for (auto table_key : group_dst.get_table_keys()) {
            if (!should_skip_table(group_dst, table_key))
                ++num_tables_dst;
        }
        REALM_ASSERT_EX(allow_schema_additions || num_tables_src == num_tables_dst, num_tables_src, num_tables_dst);
        num_tables = num_tables_src;
    }
    logger.debug("The number of tables is %1", num_tables);

    // Remove columns in dst if they are absent in src.
    for (auto table_key : group_src.get_table_keys()) {
        if (should_skip_table(group_src, table_key))
            continue;
        ConstTableRef table_src = group_src.get_table(table_key);
        StringData table_name = table_src->get_name();
        TableRef table_dst = group_dst.get_table(table_name);
        REALM_ASSERT(table_dst);
        std::vector<std::string> columns_to_remove;
        for (ColKey col_key : table_dst->get_column_keys()) {
            StringData col_name = table_dst->get_column_name(col_key);
            ColKey col_key_src = table_src->get_column_key(col_name);
            if (!col_key_src) {
                columns_to_remove.push_back(col_name);
                continue;
            }
        }
        if (!allow_schema_additions && !columns_to_remove.empty()) {
            std::string columns_list;
            for (const std::string& col_name : columns_to_remove) {
                columns_list += col_name;
                columns_list += ", ";
            }
            throw ClientResetFailed(
                util::format("Client reset cannot recover when columns have been removed from '%1': {%2}", table_name,
                             columns_list));
        }
    }

    // Add columns in dst if present in src and absent in dst.
    for (auto table_key : group_src.get_table_keys()) {
        if (should_skip_table(group_src, table_key))
            continue;
        ConstTableRef table_src = group_src.get_table(table_key);
        StringData table_name = table_src->get_name();
        TableRef table_dst = group_dst.get_table(table_name);
        REALM_ASSERT(table_dst);
        for (ColKey col_key : table_src->get_column_keys()) {
            StringData col_name = table_src->get_column_name(col_key);
            ColKey col_key_dst = table_dst->get_column_key(col_name);
            if (!col_key_dst) {
                DataType col_type = table_src->get_column_type(col_key);
                bool nullable = col_key.is_nullable();
                auto search_index_type = table_src->search_index_type(col_key);
                logger.trace("Create column, table = %1, column name = %2, "
                             " type = %3, nullable = %4, search_index = %5",
                             table_name, col_name, col_key.get_type(), nullable, search_index_type);
                ColKey col_key_dst;
                if (Table::is_link_type(col_key.get_type())) {
                    ConstTableRef target_src = table_src->get_link_target(col_key);
                    TableRef target_dst = group_dst.get_table(target_src->get_name());
                    if (col_key.is_list()) {
                        col_key_dst = table_dst->add_column_list(*target_dst, col_name);
                    }
                    else if (col_key.is_set()) {
                        col_key_dst = table_dst->add_column_set(*target_dst, col_name);
                    }
                    else if (col_key.is_dictionary()) {
                        DataType key_type = table_src->get_dictionary_key_type(col_key);
                        col_key_dst = table_dst->add_column_dictionary(*target_dst, col_name, key_type);
                    }
                    else {
                        REALM_ASSERT(!col_key.is_collection());
                        col_key_dst = table_dst->add_column(*target_dst, col_name);
                    }
                }
                else if (col_key.is_list()) {
                    col_key_dst = table_dst->add_column_list(col_type, col_name, nullable);
                }
                else if (col_key.is_set()) {
                    col_key_dst = table_dst->add_column_set(col_type, col_name, nullable);
                }
                else if (col_key.is_dictionary()) {
                    DataType key_type = table_src->get_dictionary_key_type(col_key);
                    col_key_dst = table_dst->add_column_dictionary(col_type, col_name, nullable, key_type);
                }
                else {
                    REALM_ASSERT(!col_key.is_collection());
                    col_key_dst = table_dst->add_column(col_type, col_name, nullable);
                }

                if (search_index_type != IndexType::None)
                    table_dst->add_search_index(col_key_dst, search_index_type);
            }
            else {
                // column preexists in dest, make sure the types match
                if (col_key.get_type() != col_key_dst.get_type()) {
                    throw ClientResetFailed(util::format(
                        "Incompatable column type change detected during client reset for '%1.%2' (%3 vs %4)",
                        table_name, col_name, col_key.get_type(), col_key_dst.get_type()));
                }
                ColumnAttrMask src_col_attrs = col_key.get_attrs();
                ColumnAttrMask dst_col_attrs = col_key_dst.get_attrs();
                src_col_attrs.reset(ColumnAttr::col_attr_Indexed);
                dst_col_attrs.reset(ColumnAttr::col_attr_Indexed);
                // make sure the attributes such as collection type, nullability etc. match
                // but index equality doesn't matter here.
                if (src_col_attrs != dst_col_attrs) {
                    throw ClientResetFailed(util::format(
                        "Incompatable column attribute change detected during client reset for '%1.%2' (%3 vs %4)",
                        table_name, col_name, col_key.value, col_key_dst.value));
                }
            }
        }
    }

    // Now the schemas are identical.

    // Remove objects in dst that are absent in src.
    for (auto table_key : group_src.get_table_keys()) {
        if (should_skip_table(group_src, table_key))
            continue;
        auto table_src = group_src.get_table(table_key);
        // There are no primary keys in embedded tables but this is ok, because
        // embedded objects are tied to the lifetime of top level objects.
        if (table_src->is_embedded())
            continue;
        StringData table_name = table_src->get_name();
        logger.debug("Removing objects in '%1'", table_name);
        auto table_dst = group_dst.get_table(table_name);

        auto pk_col = table_dst->get_primary_key_column();
        REALM_ASSERT_DEBUG(pk_col); // sync realms always have a pk
        std::vector<std::pair<Mixed, ObjKey>> objects_to_remove;
        for (auto obj : *table_dst) {
            auto pk = obj.get_any(pk_col);
            if (!table_src->find_primary_key(pk)) {
                objects_to_remove.emplace_back(pk, obj.get_key());
            }
        }
        for (auto& pair : objects_to_remove) {
            logger.debug("  removing '%1'", pair.first);
            table_dst->remove_object(pair.second);
        }
    }

    // We must re-create any missing objects that are absent in dst before trying to copy
    // their properties because creating them may re-create any dangling links which would
    // otherwise cause inconsistencies when re-creating lists of links.
    for (auto table_key : group_src.get_table_keys()) {
        ConstTableRef table_src = group_src.get_table(table_key);
        auto table_name = table_src->get_name();
        if (should_skip_table(group_src, table_key) || table_src->is_embedded())
            continue;
        TableRef table_dst = group_dst.get_table(table_name);
        auto pk_col = table_src->get_primary_key_column();
        REALM_ASSERT(pk_col);
        logger.debug("Creating missing objects for table '%1', number of rows = %2, "
                     "primary_key_col = %3, primary_key_type = %4",
                     table_name, table_src->size(), pk_col.get_index().val, pk_col.get_type());
        for (const Obj& src : *table_src) {
            bool created = false;
            table_dst->create_object_with_primary_key(src.get_primary_key(), &created);
            if (created) {
                logger.debug("   created %1", src.get_primary_key());
            }
        }
    }

    converters::EmbeddedObjectConverter embedded_tracker;
    // Now src and dst have identical schemas and no extraneous objects from dst.
    // There may be missing object from src and the values of existing objects may
    // still differ. Diff all the values and create missing objects on the fly.
    for (auto table_key : group_src.get_table_keys()) {
        if (should_skip_table(group_src, table_key))
            continue;
        ConstTableRef table_src = group_src.get_table(table_key);
        // Embedded objects don't have a primary key, so they are handled
        // as a special case when they are encountered as a link value.
        if (table_src->is_embedded())
            continue;
        StringData table_name = table_src->get_name();
        TableRef table_dst = group_dst.get_table(table_name);
        REALM_ASSERT_EX(allow_schema_additions || table_src->get_column_count() == table_dst->get_column_count(),
                        allow_schema_additions, table_src->get_column_count(), table_dst->get_column_count());
        auto pk_col = table_src->get_primary_key_column();
        REALM_ASSERT(pk_col);
        logger.debug("Updating values for table '%1', number of rows = %2, "
                     "number of columns = %3, primary_key_col = %4, "
                     "primary_key_type = %5",
                     table_name, table_src->size(), table_src->get_column_count(), pk_col.get_index().val,
                     pk_col.get_type());

        converters::InterRealmObjectConverter converter(table_src, table_dst, &embedded_tracker);

        for (const Obj& src : *table_src) {
            auto src_pk = src.get_primary_key();
            // create the object - it should have been created above.
            auto dst = table_dst->get_object_with_primary_key(src_pk);
            REALM_ASSERT(dst);

            bool updated = false;
            converter.copy(src, dst, &updated);
            if (updated) {
                logger.debug("  updating %1", src_pk);
            }
        }
        embedded_tracker.process_pending();
    }
}

// A table without a "class_" prefix will not generate sync instructions.
constexpr static std::string_view s_meta_reset_table_name("client_reset_metadata");
constexpr static std::string_view s_pk_col_name("id");
constexpr static std::string_view s_version_column_name("version");
constexpr static std::string_view s_timestamp_col_name("event_time");
constexpr static std::string_view s_reset_type_col_name("type_of_reset");
constexpr int64_t metadata_version = 1;

void remove_pending_client_resets(Transaction& wt)
{
    if (auto table = wt.get_table(s_meta_reset_table_name); table && !table->is_empty()) {
        table->clear();
    }
}

util::Optional<PendingReset> has_pending_reset(const Transaction& rt)
{
    ConstTableRef table = rt.get_table(s_meta_reset_table_name);
    if (!table || table->size() == 0) {
        return util::none;
    }
    ColKey timestamp_col = table->get_column_key(s_timestamp_col_name);
    ColKey type_col = table->get_column_key(s_reset_type_col_name);
    ColKey version_col = table->get_column_key(s_version_column_name);
    REALM_ASSERT(timestamp_col);
    REALM_ASSERT(type_col);
    REALM_ASSERT(version_col);
    if (table->size() > 1) {
        // this may happen if a future version of this code changes the format and expectations around reset metadata.
        throw ClientResetFailed(
            util::format("Previous client resets detected (%1) but only one is expected.", table->size()));
    }
    Obj first = *table->begin();
    REALM_ASSERT(first);
    PendingReset pending;
    int64_t version = first.get<int64_t>(version_col);
    pending.time = first.get<Timestamp>(timestamp_col);
    if (version > metadata_version) {
        throw ClientResetFailed(util::format("Unsupported client reset metadata version: %1 vs %2, from %3", version,
                                             metadata_version, pending.time));
    }
    int64_t type = first.get<int64_t>(type_col);
    if (type == 0) {
        pending.type = ClientResyncMode::DiscardLocal;
    }
    else if (type == 1) {
        pending.type = ClientResyncMode::Recover;
    }
    else {
        throw ClientResetFailed(
            util::format("Unsupported client reset metadata type: %1 from %2", type, pending.time));
    }
    return pending;
}

void track_reset(Transaction& wt, ClientResyncMode mode)
{
    REALM_ASSERT(mode != ClientResyncMode::Manual);
    TableRef table = wt.get_table(s_meta_reset_table_name);
    ColKey version_col, timestamp_col, type_col;
    if (!table) {
        table = wt.add_table_with_primary_key(s_meta_reset_table_name, type_ObjectId, s_pk_col_name);
        REALM_ASSERT(table);
        version_col = table->add_column(type_Int, s_version_column_name);
        timestamp_col = table->add_column(type_Timestamp, s_timestamp_col_name);
        type_col = table->add_column(type_Int, s_reset_type_col_name);
    }
    else {
        version_col = table->get_column_key(s_version_column_name);
        timestamp_col = table->get_column_key(s_timestamp_col_name);
        type_col = table->get_column_key(s_reset_type_col_name);
    }
    REALM_ASSERT(version_col);
    REALM_ASSERT(timestamp_col);
    REALM_ASSERT(type_col);
    int64_t mode_val = 0; // Discard
    if (mode == ClientResyncMode::Recover || mode == ClientResyncMode::RecoverOrDiscard) {
        mode_val = 1; // Recover
    }

    if (table->size() > 1) {
        // this may happen if a future version of this code changes the format and expectations around reset metadata.
        throw ClientResetFailed(
            util::format("Previous client resets detected (%1) but only one is expected.", table->size()));
    }
    table->create_object_with_primary_key(ObjectId::gen(),
                                          {{version_col, metadata_version},
                                           {timestamp_col, Timestamp(std::chrono::system_clock::now())},
                                           {type_col, mode_val}});
}

static ClientResyncMode reset_precheck_guard(Transaction& wt, ClientResyncMode mode, bool recovery_is_allowed,
                                             util::Logger& logger)
{
    if (auto previous_reset = has_pending_reset(wt)) {
        logger.info("A previous reset was detected of type: '%1' at: %2", previous_reset->type, previous_reset->time);
        switch (previous_reset->type) {
            case ClientResyncMode::Manual:
                REALM_UNREACHABLE();
            case ClientResyncMode::DiscardLocal:
                throw ClientResetFailed(util::format("A previous '%1' mode reset from %2 did not succeed, "
                                                     "giving up on '%3' mode to prevent a cycle",
                                                     previous_reset->type, previous_reset->time, mode));
            case ClientResyncMode::Recover:
                switch (mode) {
                    case ClientResyncMode::Recover:
                        throw ClientResetFailed(util::format("A previous '%1' mode reset from %2 did not succeed, "
                                                             "giving up on '%3' mode to prevent a cycle",
                                                             previous_reset->type, previous_reset->time, mode));
                    case ClientResyncMode::RecoverOrDiscard:
                        mode = ClientResyncMode::DiscardLocal;
                        logger.info("A previous '%1' mode reset from %2 downgrades this mode ('%3') to DiscardLocal",
                                    previous_reset->type, previous_reset->time, mode);
                        remove_pending_client_resets(wt);
                        break;
                    case ClientResyncMode::DiscardLocal:
                        remove_pending_client_resets(wt);
                        // previous mode Recover and this mode is Discard, this is not a cycle yet
                        break;
                    case ClientResyncMode::Manual:
                        REALM_UNREACHABLE();
                }
                break;
            case ClientResyncMode::RecoverOrDiscard:
                throw ClientResetFailed(util::format("Unexpected previous '%1' mode reset from %2 did not "
                                                     "succeed, giving up on '%3' mode to prevent a cycle",
                                                     previous_reset->type, previous_reset->time, mode));
        }
    }
    if (!recovery_is_allowed) {
        if (mode == ClientResyncMode::Recover) {
            throw ClientResetFailed(
                "Client reset mode is set to 'Recover' but the server does not allow recovery for this client");
        }
        else if (mode == ClientResyncMode::RecoverOrDiscard) {
            logger.info("Client reset in 'RecoverOrDiscard' is choosing 'DiscardLocal' because the server does not "
                        "permit recovery for this client");
            mode = ClientResyncMode::DiscardLocal;
        }
    }
    track_reset(wt, mode);
    return mode;
}

LocalVersionIDs perform_client_reset_diff(DB& db_local, DB& db_remote, sync::SaltedFileIdent client_file_ident,
                                          util::Logger& logger, ClientResyncMode mode, bool recovery_is_allowed,
                                          bool* did_recover_out, sync::SubscriptionStore* sub_store,
                                          util::UniqueFunction<void(int64_t)> on_flx_version_complete)
{
    auto wt_local = db_local.start_write();
    auto actual_mode = reset_precheck_guard(*wt_local, mode, recovery_is_allowed, logger);
    bool recover_local_changes =
        actual_mode == ClientResyncMode::Recover || actual_mode == ClientResyncMode::RecoverOrDiscard;

    logger.info("Client reset: path_local = %1, "
                "client_file_ident = (ident: %2, salt: %3), "
                "remote_path = %4, requested_mode = %5, recovery_is_allowed = %6, "
                "actual_mode = %7, will_recover = %8",
                db_local.get_path(), client_file_ident.ident, client_file_ident.salt, db_remote.get_path(), mode,
                recovery_is_allowed, actual_mode, recover_local_changes);

    auto& repl_local = dynamic_cast<ClientReplication&>(*db_local.get_replication());
    auto& history_local = repl_local.get_history();
    history_local.ensure_updated(wt_local->get_version());
    SaltedFileIdent orig_file_ident = history_local.get_client_file_ident(*wt_local);
    VersionID old_version_local = wt_local->get_version_of_current_transaction();

    auto& repl_remote = dynamic_cast<ClientReplication&>(*db_remote.get_replication());
    auto& history_remote = repl_remote.get_history();

    sync::SaltedVersion fresh_server_version = {0, 0};
    {
        SyncProgress remote_progress;
        sync::version_type remote_version_unused;
        SaltedFileIdent remote_ident_unused;
        history_remote.get_status(remote_version_unused, remote_ident_unused, remote_progress);
        fresh_server_version = remote_progress.latest_server_version;
    }

    if (!recover_local_changes) {
        auto rt_remote = db_remote.start_read();
        // transform the local Realm such that all public tables become identical to the remote Realm
        transfer_group(*rt_remote, *wt_local, logger, false);

        // now that the state of the fresh and local Realms are identical,
        // reset the local sync history and steal the fresh Realm's ident
        history_local.set_client_reset_adjustments(wt_local->get_version(), client_file_ident, fresh_server_version,
                                                   BinaryData());

        int64_t subscription_version = 0;
        if (sub_store) {
            subscription_version = sub_store->set_active_as_latest(*wt_local);
        }

        wt_local->commit_and_continue_as_read();
        if (did_recover_out) {
            *did_recover_out = false;
        }
        if (on_flx_version_complete) {
            on_flx_version_complete(subscription_version);
        }

        VersionID new_version_local = wt_local->get_version_of_current_transaction();
        logger.info("perform_client_reset_diff is done: old_version = (version: %1, index: %2), "
                    "new_version = (version: %3, index: %4)",
                    old_version_local.version, old_version_local.index, new_version_local.version,
                    new_version_local.index);
        return LocalVersionIDs{old_version_local, new_version_local};
    }

    auto remake_active_subscription = [&]() {
        if (!sub_store) {
            return;
        }
        auto subs = sub_store->get_active();
        int64_t before_version = subs.version();
        auto mut_subs = subs.make_mutable_copy();
        mut_subs.update_state(sync::SubscriptionSet::State::Complete);
        auto sub = std::move(mut_subs).commit();
        if (on_flx_version_complete) {
            on_flx_version_complete(sub.version());
        }
        logger.info("Recreated the active subscription set in the complete state (%1 -> %2)", before_version,
                    sub.version());
    };

    auto frozen_pre_local_state = db_local.start_frozen();
    auto local_changes = history_local.get_local_changes(wt_local->get_version());
    logger.info("Local changesets to recover: %1", local_changes.size());

    auto wt_remote = db_remote.start_write();

    BinaryData recovered_changeset;

    // FLX with recovery has to be done in multiple commits, which is significantly different than other modes
    if (sub_store) {
        // In FLX recovery, save a copy of the pending subscriptions for later. This
        // needs to be done before they are wiped out by remake_active_subscription()
        std::vector<SubscriptionSet> pending_subscriptions = sub_store->get_pending_subscriptions();
        // transform the local Realm such that all public tables become identical to the remote Realm
        transfer_group(*wt_remote, *wt_local, logger, recover_local_changes);
        // now that the state of the fresh and local Realms are identical,
        // reset the local sync history.
        // Note that we do not set the new file ident yet! This is done in the last commit.
        history_local.set_client_reset_adjustments(wt_local->get_version(), orig_file_ident, fresh_server_version,
                                                   recovered_changeset);
        // The local Realm is committed. There are no changes to the remote Realm.
        wt_remote->rollback_and_continue_as_read();
        wt_local->commit_and_continue_as_read();
        // Make a copy of the active subscription set and mark it as
        // complete. This will cause all other subscription sets to become superceded.
        remake_active_subscription();
        // Apply local changes interleaved with pending subscriptions in separate commits
        // as needed. This has the consequence that there may be extra notifications along
        // the way to the final state, but since separate commits are necessary, this is
        // unavoidable.
        wt_local = db_local.start_write();
        RecoverLocalChangesetsHandler handler{*wt_local, *frozen_pre_local_state, logger, db_local.get_replication()};
        handler.process_changesets(local_changes, std::move(pending_subscriptions)); // throws on error
        // The new file ident is set as part of the final commit. This is to ensure that if
        // there are any exceptions during recovery, or the process is killed for some
        // reason, the client reset cycle detection will catch this and we will not attempt
        // to recover again. If we had set the ident in the first commit, a Realm which was
        // partially recovered, but interrupted may continue sync the next time it is
        // opened with only partially recovered state while having lost the history of any
        // offline modifications.
        history_local.set_client_file_ident_in_wt(wt_local->get_version(), client_file_ident);
        wt_local->commit_and_continue_as_read();
    }
    else {
        // In PBS recovery, the strategy is to apply all local changes to the remote realm first,
        // and then transfer the modified state all at once to the local Realm. This creates a
        // nice side effect for notifications because only the minimal state change is made.
        RecoverLocalChangesetsHandler handler{*wt_remote, *frozen_pre_local_state, logger,
                                              db_remote.get_replication()};
        handler.process_changesets(local_changes, {}); // throws on error
        ChangesetEncoder& encoder = repl_remote.get_instruction_encoder();
        const sync::ChangesetEncoder::Buffer& buffer = encoder.buffer();
        recovered_changeset = {buffer.data(), buffer.size()};

        // transform the local Realm such that all public tables become identical to the remote Realm
        transfer_group(*wt_remote, *wt_local, logger, recover_local_changes);

        // now that the state of the fresh and local Realms are identical,
        // reset the local sync history and steal the fresh Realm's ident
        history_local.set_client_reset_adjustments(wt_local->get_version(), client_file_ident, fresh_server_version,
                                                   recovered_changeset);

        // Finally, the local Realm is committed. The changes to the remote Realm are discarded.
        wt_remote->rollback_and_continue_as_read();
        wt_local->commit_and_continue_as_read();
    }

    if (did_recover_out) {
        *did_recover_out = true;
    }
    VersionID new_version_local = wt_local->get_version_of_current_transaction();
    logger.info("perform_client_reset_diff is done, old_version.version = %1, "
                "old_version.index = %2, new_version.version = %3, "
                "new_version.index = %4",
                old_version_local.version, old_version_local.index, new_version_local.version,
                new_version_local.index);

    return LocalVersionIDs{old_version_local, new_version_local};
}

} // namespace realm::_impl::client_reset

realm / realm-core / 1817

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