RCORE-2063 Fix some client resets potentially failing with AutoClientResetFailed if a new client reset condition occurred before the first one completed

CHANGELOG.md

@@ -6,7 +6,7 @@
 
 ### Fixed
 * <How do the end-user experience this issue? what was the impact?> ([#????](https://github.com/realm/realm-core/issues/????), since v?.?.?)
-* None.
+* Fix some client resets (such as migrating to flexible sync) potentially failing with AutoClientResetFailed if a new client reset condition (such as rolling back a flexible sync migration) occurred before the first one completed. ([PR #7542](https://github.com/realm/realm-core/pull/7542), since v13.11.0)
 
 ### Breaking changes
 * None.

src/realm/sync/client.cpp

@@ -1067,6 +1067,11 @@ SyncClientHookAction SessionImpl::call_debug_hook(SyncClientHookEvent event, con
     return call_debug_hook(data);
 }
 
+SyncClientHookAction SessionImpl::call_debug_hook(SyncClientHookEvent event)
+{
+    return call_debug_hook(event, m_progress, m_last_sent_flx_query_version, DownloadBatchState::SteadyState, 0);
+}
+
 bool SessionImpl::is_steady_state_download_message(DownloadBatchState batch_state, int64_t query_version)
 {
     // Should never be called if session is not active

src/realm/sync/config.hpp

@@ -132,6 +132,7 @@ enum class SyncClientHookEvent {
     SessionActivating,
     SessionSuspended,
     BindMessageSent,
+    ClientResetMergeComplete,
     BootstrapBatchAboutToProcess,
 };
 

src/realm/sync/noinst/client_impl_base.cpp

@@ -1727,8 +1727,7 @@ void Session::activate()
     reset_protocol_state();
     m_state = Active;
 
-    call_debug_hook(SyncClientHookEvent::SessionActivating, m_progress, m_last_sent_flx_query_version,
-                    DownloadBatchState::SteadyState, 0);
+    call_debug_hook(SyncClientHookEvent::SessionActivating);
 
     REALM_ASSERT(!m_suspended);
     m_conn.one_more_active_unsuspended_session(); // Throws
@@ -1946,8 +1945,7 @@ void Session::send_bind_message()
     m_conn.initiate_write_message(out, this); // Throws
 
     m_bind_message_sent = true;
-    call_debug_hook(SyncClientHookEvent::BindMessageSent, m_progress, m_last_sent_flx_query_version,
-                    DownloadBatchState::SteadyState, 0);
+    call_debug_hook(SyncClientHookEvent::BindMessageSent);
 
     // Ready to send the IDENT message if the file identifier pair is already
     // available.
@@ -2273,6 +2271,8 @@ bool Session::client_reset_if_needed()
     bool did_reset =
         client_reset::perform_client_reset(logger, *get_db(), std::move(*client_reset_config), m_client_file_ident,
                                            get_flx_subscription_store(), on_flx_version_complete);
+
+    call_debug_hook(SyncClientHookEvent::ClientResetMergeComplete);
     if (!did_reset) {
         return false;
     }

src/realm/sync/noinst/client_impl_base.hpp

@@ -1180,6 +1180,7 @@ class ClientImpl::Session {
                                          size_t);
     SyncClientHookAction call_debug_hook(SyncClientHookEvent event, const ProtocolErrorInfo&);
     SyncClientHookAction call_debug_hook(const SyncClientHookData& data);
+    SyncClientHookAction call_debug_hook(SyncClientHookEvent event);
 
     bool is_steady_state_download_message(DownloadBatchState batch_state, int64_t query_version);
 

src/realm/sync/noinst/client_reset.cpp

@@ -416,38 +416,49 @@ ClientResyncMode reset_precheck_guard(const TransactionRef& wt_local, ClientResy
 {
     if (auto previous_reset = sync::PendingResetStore::has_pending_reset(wt_local)) {
         logger.info(util::LogCategory::reset, "Found a previous %1", *previous_reset);
-        switch (previous_reset->mode) {
-            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->mode, 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->mode, previous_reset->time, mode));
-                    case ClientResyncMode::RecoverOrDiscard:
-                        mode = ClientResyncMode::DiscardLocal;
-                        logger.info(util::LogCategory::reset,
-                                    "A previous '%1' mode reset from %2 downgrades this mode ('%3') to DiscardLocal",
-                                    previous_reset->mode, previous_reset->time, mode);
-                        sync::PendingResetStore::clear_pending_reset(wt_local);
-                        break;
-                    case ClientResyncMode::DiscardLocal:
-                        sync::PendingResetStore::clear_pending_reset(wt_local);
-                        // 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->mode, previous_reset->time, mode));
+        if (action != previous_reset->action) {
+            // IF a different client reset is being performed, cler the pending client reset and start over.
+            logger.info(util::LogCategory::reset,
+                        "New '%1' client reset of type: '%2' is incompatible - clearing previous reset", action,
+                        mode);
+            sync::PendingResetStore::clear_pending_reset(wt_local);
+        }
+        else {
+            switch (previous_reset->mode) {
+                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->mode, 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->mode, previous_reset->time, mode));
+                        case ClientResyncMode::RecoverOrDiscard:
+                            mode = ClientResyncMode::DiscardLocal;
+                            logger.info(
+                                util::LogCategory::reset,
+                                "A previous '%1' mode reset from %2 downgrades this mode ('%3') to DiscardLocal",
+                                previous_reset->mode, previous_reset->time, mode);
+                            sync::PendingResetStore::clear_pending_reset(wt_local);
+                            break;
+                        case ClientResyncMode::DiscardLocal:
+                            sync::PendingResetStore::clear_pending_reset(wt_local);
+                            // 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->mode, previous_reset->time, mode));
+            }
         }
     }
     if (action == PendingReset::Action::ClientResetNoRecovery) {

test/object-store/sync/flx_migration.cpp

@@ -320,6 +320,8 @@ TEST_CASE("Test client migration and rollback", "[sync][flx][flx migration][baas
 
 TEST_CASE("Test client migration and rollback with recovery", "[sync][flx][flx migration][baas]") {
     auto logger_ptr = util::Logger::get_default_logger();
+    enum TestState { idle, wait_for_merge, merge_complete, rollback_complete };
+    TestingStateMachine<TestState> test_state(TestState::idle);
 
     const std::string partition = "migration-test";
     const Schema mig_schema{
@@ -331,6 +333,20 @@ TEST_CASE("Test client migration and rollback with recovery", "[sync][flx][flx m
     SyncTestFile config(session.app()->current_user(), partition, server_app_config.schema);
     config.sync_config->client_resync_mode = ClientResyncMode::Recover;
     config.schema_version = 0;
+    config.sync_config->on_sync_client_event_hook = [&](std::weak_ptr<SyncSession>, const SyncClientHookData& data) {
+        test_state.transition_with([data](TestState cur_state) -> std::optional<TestState> {
+            if (data.event == SyncClientHookEvent::ClientResetMergeComplete &&
+                cur_state == TestState::wait_for_merge) {
+                return TestState::merge_complete;
+            }
+            return std::nullopt;
+        });
+        if (test_state.get() == TestState::merge_complete) {
+            // Wait for the FLX->PBS rollback to complete before continuing
+            test_state.wait_for(TestState::rollback_complete, std::chrono::seconds(25));
+        }
+        return SyncClientHookAction::NoAction;
+    };
 
     // Fill some objects
     auto objects = fill_test_data(config); // 5 objects starting at 1 with no partition value set
@@ -437,15 +453,16 @@ TEST_CASE("Test client migration and rollback with recovery", "[sync][flx][flx m
         REALM_ASSERT(result.get_value() == sync::SubscriptionSet::State::Superseded);
     }
 
+    test_state.transition_to(TestState::wait_for_merge);
+
     //  Migrate back to FLX - and keep the realm session open
     trigger_server_migration(session.app_session(), MigrateToFLX, logger_ptr);
 
-    // wait for the subscription store to initialize after downloading
-    timed_wait_for(
-        [&outer_realm]() {
-            return outer_realm->sync_session() && outer_realm->sync_session()->get_flx_subscription_store();
-        },
-        std::chrono::seconds(180));
+    // Cancel any connect waits (since sync session is still active) and try to connect now
+    outer_realm->sync_session()->handle_reconnect();
+
+    // wait for the fresh realm to download and merge with the current local realm
+    test_state.wait_for(TestState::merge_complete, std::chrono::seconds(180));
 
     // Verify data has been sync'ed and there is only 1 subscription for the Object table
     {
@@ -460,9 +477,18 @@ TEST_CASE("Test client migration and rollback with recovery", "[sync][flx][flx m
         REQUIRE(active_subs.find("flx_migrated_Object"));
     }
 
-    // Roll back to PBS once again - and keep the realm session open
+    // Roll back to PBS once again before the client reset is complete and keep the realm session open
+    // NOTE: the realm session is blocked in the hook callback until the rollback is complete
     trigger_server_migration(session.app_session(), RollbackToPBS, logger_ptr);
 
+    // Release the realm session; will reconnect and perform the rollback to PBS client reset
+    test_state.transition_to(TestState::rollback_complete);
+
+    // Cancel any connect waits (since sync session is still active) and try to connect now
+    outer_realm->sync_session()->handle_reconnect();
+
+    // During the rollback client reset, the previous migrate to flx client reset operation is still
+    // tracked, but will be removed since the new rollback server requests action is incompatible.
     REQUIRE(!wait_for_upload(*outer_realm));
     REQUIRE(!wait_for_download(*outer_realm));
 

test/object-store/util/test_utils.hpp

@@ -68,10 +68,10 @@ class TestingStateMachine {
         m_cv.notify_one();
     }
 
-    void wait_for(E target)
+    bool wait_for(E target, std::chrono::milliseconds period = std::chrono::seconds(15))
     {
         std::unique_lock lock{m_mutex};
-        m_cv.wait(lock, [&] {
+        return m_cv.wait_for(lock, period, [&] {
             return m_cur_state == target;
         });
     }