Fix DiscardLocal client reset failure with unset callbacks

CHANGELOG.md

@@ -18,6 +18,7 @@
 * Calling `Realm::close()` or `Realm::invalidate()` from the async write callbacks could result in crashes (since v11.8.0).
 * Asynchronous writes did not work with queue-confined Realms (since v11.8.0).
 * Releasing all references to a Realm while an asynchronous write was in progress would sometimes result in use-after-frees (since v11.8.0).
+* Fixed a fatal sync error "Automatic recovery failed" during DiscardLocal client reset if the reset notifier callbacks were not set to something. ([#5223](https://github.com/realm/realm-core/issues/5223), since v11.5.0)
 * Throwing exceptions from asynchronous write callbacks would result in crashes or the Realm being in an invalid state (since v11.8.0).
 
 ### Breaking changes

src/realm/object-store/sync/sync_session.cpp

@@ -529,6 +529,46 @@ void SyncSession::create_sync_session()
     do_create_sync_session();
 }
 
+sync::Session::Config::ClientReset make_client_reset_config(SyncConfig& session_config, DBRef&& fresh_copy)
+{
+    sync::Session::Config::ClientReset config;
+    config.discard_local = (session_config.client_resync_mode == ClientResyncMode::DiscardLocal);
+    config.notify_after_client_reset = [&session_config](std::string local_path, VersionID previous_version) {
+        REALM_ASSERT(!local_path.empty());
+        SharedRealm frozen_before, active_after;
+        if (auto local_coordinator = RealmCoordinator::get_existing_coordinator(local_path)) {
+            auto local_config = local_coordinator->get_config();
+            active_after = local_coordinator->get_realm(local_config, util::none);
+            local_config.scheduler = nullptr;
+            frozen_before = local_coordinator->get_realm(local_config, previous_version);
+            REALM_ASSERT(active_after);
+            REALM_ASSERT(!active_after->is_frozen());
+            REALM_ASSERT(frozen_before);
+            REALM_ASSERT(frozen_before->is_frozen());
+        }
+        if (session_config.notify_after_client_reset) {
+            session_config.notify_after_client_reset(frozen_before, active_after);
+        }
+    };
+    config.notify_before_client_reset = [&session_config](std::string local_path) {
+        REALM_ASSERT(!local_path.empty());
+        SharedRealm frozen_local;
+        Realm::Config local_config;
+        if (auto local_coordinator = RealmCoordinator::get_existing_coordinator(local_path)) {
+            local_config = local_coordinator->get_config();
+            local_config.scheduler = nullptr;
+            frozen_local = local_coordinator->get_realm(local_config, VersionID());
+            REALM_ASSERT(frozen_local);
+            REALM_ASSERT(frozen_local->is_frozen());
+        }
+        if (session_config.notify_before_client_reset) {
+            session_config.notify_before_client_reset(frozen_local);
+        }
+    };
+    config.fresh_copy = std::move(fresh_copy);
+    return config;
+}
+
 void SyncSession::do_create_sync_session()
 {
     sync::Session::Config session_config;
@@ -558,38 +598,7 @@ void SyncSession::do_create_sync_session()
     session_config.custom_http_headers = m_config.custom_http_headers;
 
     if (m_force_client_reset) {
-        sync::Session::Config::ClientReset config;
-        config.discard_local = (m_config.client_resync_mode == ClientResyncMode::DiscardLocal);
-        config.notify_after_client_reset = [this](std::string local_path, VersionID previous_version) {
-            REALM_ASSERT(!local_path.empty());
-            SharedRealm frozen_before, active_after;
-            if (auto local_coordinator = RealmCoordinator::get_existing_coordinator(local_path)) {
-                auto local_config = local_coordinator->get_config();
-                active_after = local_coordinator->get_realm(local_config, util::none);
-                local_config.scheduler = nullptr;
-                frozen_before = local_coordinator->get_realm(local_config, previous_version);
-                REALM_ASSERT(active_after);
-                REALM_ASSERT(!active_after->is_frozen());
-                REALM_ASSERT(frozen_before);
-                REALM_ASSERT(frozen_before->is_frozen());
-            }
-            m_config.notify_after_client_reset(frozen_before, active_after);
-        };
-        config.notify_before_client_reset = [this](std::string local_path) {
-            REALM_ASSERT(!local_path.empty());
-            SharedRealm frozen_local;
-            Realm::Config local_config;
-            if (auto local_coordinator = RealmCoordinator::get_existing_coordinator(local_path)) {
-                local_config = local_coordinator->get_config();
-                local_config.scheduler = nullptr;
-                frozen_local = local_coordinator->get_realm(local_config, VersionID());
-                REALM_ASSERT(frozen_local);
-                REALM_ASSERT(frozen_local->is_frozen());
-            }
-            m_config.notify_before_client_reset(frozen_local);
-        };
-        config.fresh_copy = std::move(m_client_reset_fresh_copy);
-        session_config.client_reset_config = std::move(config);
+        session_config.client_reset_config = make_client_reset_config(m_config, std::move(m_client_reset_fresh_copy));
         m_force_client_reset = false;
     }
 

test/object-store/sync/client_reset.cpp

@@ -341,6 +341,14 @@ TEST_CASE("sync: client reset", "[client reset]") {
             }
         }
 
+        SECTION("can be reset without notifiers") {
+            local_config.sync_config->notify_before_client_reset = nullptr;
+            local_config.sync_config->notify_after_client_reset = nullptr;
+            make_reset(local_config, remote_config)->run();
+            REQUIRE(before_callback_invoctions == 0);
+            REQUIRE(after_callback_invocations == 0);
+        }
+
         SECTION("an interrupted reset can recover on the next session") {
             struct SessionInterruption : public std::runtime_error {
                 using std::runtime_error::runtime_error;