[Sync] Support commit only types.

components/sync/engine_impl/syncer.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/sync/engine_impl/syncer.h"
 
 #include <memory>
 
 #include "base/auto_reset.h"
 #include "base/logging.h"
@@ skipping 96 matching lines @@
 bool Syncer::PostClearServerData(SyncCycle* cycle) {
   DCHECK(cycle);
   ClearServerData clear_server_data(cycle->context()->account_name());
   return clear_server_data.SendRequest(cycle) == SYNCER_OK;
 }
 
 bool Syncer::DownloadAndApplyUpdates(ModelTypeSet* request_types,
                                      SyncCycle* cycle,
                                      const GetUpdatesDelegate& delegate,
                                      bool create_mobile_bookmarks_folder) {
+  ModelTypeSet add_for_commit = Intersection(*request_types, CommitOnlyTypes());

[Patrick Noland, 2017/06/01 22:58:48]

This name made it harder for me to follow what you're doing here. Maybe
requested_commit_only_types would be easier to follow? And a comment explaining
why you remove these types for download and then add them back for the GU
request?

[skym, 2017/06/01 23:15:21]

Done.

+  ModelTypeSet download_types = Difference(*request_types, add_for_commit);
   GetUpdatesProcessor get_updates_processor(
       cycle->context()->model_type_registry()->update_handler_map(), delegate);
   SyncerError download_result = UNSET;
   do {
     download_result = get_updates_processor.DownloadUpdates(
-        request_types, cycle, create_mobile_bookmarks_folder);
+        &download_types, cycle, create_mobile_bookmarks_folder);
   } while (download_result == SERVER_MORE_TO_DOWNLOAD);
 
+  // It is our responsibility to propagate the removal of types that occurred in
+  // GetUpdatesProcessor::DownloadUpdates().
+  *request_types = Union(download_types, add_for_commit);
+
   // Exit without applying if we're shutting down or an error was detected.
   if (download_result != SYNCER_OK || ExitRequested())
     return false;
 
   {
     TRACE_EVENT0("sync", "ApplyUpdates");
 
     // Control type updates always get applied first.
     ApplyControlDataUpdates(cycle->context()->directory());
 
     // Apply updates to the other types. May or may not involve cross-thread
     // traffic, depending on the underlying update handlers and the GU type's
     // delegate.
-    get_updates_processor.ApplyUpdates(*request_types,
+    get_updates_processor.ApplyUpdates(download_types,
                                        cycle->mutable_status_controller());
 
     cycle->context()->set_hierarchy_conflict_detected(
         cycle->status_controller().num_hierarchy_conflicts() > 0);
     cycle->SendEventNotification(SyncCycleEvent::STATUS_CHANGED);
   }
 
   return !ExitRequested();
 }
 
@@ skipping 45 matching lines @@
   bool success =
       !HasSyncerError(cycle->status_controller().model_neutral_state());
   if (success && source == sync_pb::GetUpdatesCallerInfo::PERIODIC) {
     cycle->mutable_status_controller()->UpdatePollTime();
   }
 
   return success;
 }
 
 }  // namespace syncer