[Sync] Add support for retrying a getupdates due to a context change

sync/engine/directory_update_handler.cc

 // Copyright 2014 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 "sync/engine/directory_update_handler.h"
 
 #include "sync/engine/conflict_resolver.h"
 #include "sync/engine/process_updates_util.h"
 #include "sync/engine/update_applicator.h"
 #include "sync/sessions/status_controller.h"
@@ skipping 19 matching lines @@
     sync_pb::DataTypeProgressMarker* progress_marker) const {
   dir_->GetDownloadProgress(type_, progress_marker);
 }
 
 void DirectoryUpdateHandler::GetDataTypeContext(
     sync_pb::DataTypeContext* context) const {
   syncable::ModelNeutralWriteTransaction trans(FROM_HERE, SYNCER, dir_);
   dir_->GetDataTypeContext(&trans, type_, context);
 }
 
-void DirectoryUpdateHandler::ProcessGetUpdatesResponse(
+bool DirectoryUpdateHandler::ProcessGetUpdatesResponse(
     const sync_pb::DataTypeProgressMarker& progress_marker,
     const sync_pb::DataTypeContext& mutated_context,
     const SyncEntityList& applicable_updates,
     sessions::StatusController* status) {
   syncable::ModelNeutralWriteTransaction trans(FROM_HERE, SYNCER, dir_);
-  UpdateSyncEntities(&trans, applicable_updates, status);
-
-  if (IsValidProgressMarker(progress_marker)) {
-    ExpireEntriesIfNeeded(&trans, progress_marker);
-    UpdateProgressMarker(progress_marker);
-  }
-
   if (mutated_context.has_context()) {
     sync_pb::DataTypeContext local_context;
     dir_->GetDataTypeContext(&trans, type_, &local_context);
 
     // Only update the local context if it is still relevant. If the local
     // version is higher, it means a local change happened while the mutation
     // was in flight, and the local context takes priority.
     if (mutated_context.version() >= local_context.version() &&
         local_context.context() != mutated_context.context()) {
       dir_->SetDataTypeContext(&trans, type_, mutated_context);
       // TODO(zea): trigger the datatype's UpdateDataTypeContext method.
+    } else if (mutated_context.version() < local_context.version()) {
+      // A GetUpdates using the old context was in progress when the context was
+      // set. Fail this get updates cycle, to force a retry.
+      DVLOG(1) << "GU Context conflict detected, forcing GU retry.";
+      return false;
     }
   }
+
+  UpdateSyncEntities(&trans, applicable_updates, status);
+
+  if (IsValidProgressMarker(progress_marker)) {
+    ExpireEntriesIfNeeded(&trans, progress_marker);
+    UpdateProgressMarker(progress_marker);
+  }
+  return true;
 }
 
 void DirectoryUpdateHandler::ApplyUpdates(sessions::StatusController* status) {
   if (!IsApplyUpdatesRequired()) {
     return;
   }
 
   // This will invoke handlers that belong to the model and its thread, so we
   // switch to the appropriate thread before we start this work.
   WorkCallback c = base::Bind(
@@ skipping 145 matching lines @@
               new_gc_directive.version_watermark())) {
     ExpireEntriesByVersion(dir_, trans, type_,
                            new_gc_directive.version_watermark());
   }
 
   cached_gc_directive_.reset(
       new sync_pb::GarbageCollectionDirective(new_gc_directive));
 }
 
 }  // namespace syncer