RCORE-2129 Do not send empty upload messages if there are no downloads to ack

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 ### Fixed
 * When a public name is defined on a property, calling `realm::Results::sort()` or `realm::Results::distinct()` with the internal name could throw an error like `Cannot sort on key path 'NAME': property 'PersonObject.NAME' does not exist`. ([realm/realm-js#6779](https://github.com/realm/realm-js/issues/6779), since v12.12.0)
+* Fixed unnecessary server roundtrips when there is no download to acknowledge ([#2129](https://jira.mongodb.org/browse/RCORE-2129), since v14.8.0).
 
 ### Breaking changes
 * None.

src/realm/sync/noinst/client_impl_base.cpp

@@ -2028,17 +2028,21 @@ void Session::send_upload_message()
         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 we need to limit upload up to some version other than the last client version available and there are no
-        // changes to upload, then there is no need to send an empty message.
-        if (m_pending_flx_sub_set) {
-            logger.debug("Empty UPLOAD was skipped (progress_client_version=%1, progress_server_version=%2)",
+        // 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
@@ -2054,11 +2058,12 @@ void Session::send_upload_message()
     version_type progress_server_version = m_upload_progress.last_integrated_server_version;
 
     if (!upload_messages_allowed()) {
-        logger.debug("UPLOAD not allowed: upload progress(progress_client_version=%1, progress_server_version=%2, "
+        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
-        return;
+        // 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, "

test/object-store/sync/flx_sync.cpp

@@ -5023,6 +5023,54 @@ TEST_CASE("flx: nested collections in mixed", "[sync][flx][baas]") {
     CHECK(nested_list.get_any(1) == "foo");
 }
 
+TEST_CASE("flx: no upload during bootstraps", "[sync][flx][bootstrap][baas]") {
+    FLXSyncTestHarness harness("flx_bootstrap_no_upload", {g_large_array_schema, {"queryable_int_field"}});
+    fill_large_array_schema(harness);
+    auto config = harness.make_test_file();
+    enum class TestState { Start, BootstrapInProgress, BootstrapProcessed, BootstrapAck };
+    TestingStateMachine<TestState> state(TestState::Start);
+    config.sync_config->on_sync_client_event_hook = [&](std::weak_ptr<SyncSession>, const SyncClientHookData& data) {
+        if (data.query_version == 0) {
+            return SyncClientHookAction::NoAction;
+        }
+        state.transition_with([&](TestState cur_state) -> std::optional<TestState> {
+            // Check no upload messages are sent during bootstrap.
+            if (data.event == SyncClientHookEvent::BootstrapMessageProcessed) {
+                CHECK((cur_state == TestState::Start || cur_state == TestState::BootstrapInProgress));
+                return TestState::BootstrapInProgress;
+            }
+            else if (data.event == SyncClientHookEvent::DownloadMessageIntegrated &&
+                     data.batch_state == sync::DownloadBatchState::LastInBatch) {
+                CHECK(cur_state == TestState::BootstrapInProgress);
+                return TestState::BootstrapProcessed;
+            }
+            else if (data.event == SyncClientHookEvent::UploadMessageSent) {
+                // Uploads are allowed before a bootstrap starts.
+                if (cur_state == TestState::Start) {
+                    return std::nullopt; // Don't transition
+                }
+                CHECK(cur_state == TestState::BootstrapProcessed);
+                return TestState::BootstrapAck;
+            }
+            return std::nullopt;
+        });
+        return SyncClientHookAction::NoAction;
+    };
+
+    auto realm = Realm::get_shared_realm(config);
+    auto table = realm->read_group().get_table("class_TopLevel");
+    auto new_subs = realm->get_latest_subscription_set().make_mutable_copy();
+    new_subs.insert_or_assign(Query(table));
+    new_subs.commit();
+    state.wait_for(TestState::BootstrapAck);
+
+    // Commiting an empty changeset does not upload a message.
+    realm->begin_transaction();
+    realm->commit_transaction();
+    // Give the sync client the chance to send an upload after mark.
+    wait_for_download(*realm);
+}
+
 } // namespace realm::app
 
 #endif // REALM_ENABLE_AUTH_TESTS