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

sync/engine/backoff_delay_provider.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 "sync/engine/backoff_delay_provider.h"
 
 #include "base/rand_util.h"
 #include "sync/internal_api/public/engine/polling_constants.h"
 #include "sync/internal_api/public/sessions/model_neutral_state.h"
 #include "sync/internal_api/public/util/syncer_error.h"
@@ skipping 79 matching lines @@
   // Note: If we received a MIGRATION_DONE on download updates, then commit
   // should not have taken place.  Moreover, if we receive a MIGRATION_DONE
   // on commit, it means that download updates succeeded.  Therefore, we only
   // need to check if either code is equal to SERVER_RETURN_MIGRATION_DONE,
   // and not if there were any more serious errors requiring the long retry.
   if (state.last_download_updates_result == SERVER_RETURN_MIGRATION_DONE ||
       state.commit_result == SERVER_RETURN_MIGRATION_DONE) {
     return short_initial_backoff_;
   }
 
+  // If a datatype decides the GetUpdates must be retried (e.g. because the
+  // context has been updated since the request), use the short delay.
+  if (state.last_download_updates_result == DATATYPE_TRIGGERED_RETRY)
+    return short_initial_backoff_;
+
   // When the server tells us we have a conflict, then we should download the
   // latest updates so we can see the conflict ourselves, resolve it locally,
   // then try again to commit.  Running another sync cycle will do all these
   // things.  There's no need to back off, we can do this immediately.
   //
   // TODO(sync): We shouldn't need to handle this in BackoffDelayProvider.
   // There should be a way to deal with protocol errors before we get to this
   // point.
-  if (state.commit_result == SERVER_RETURN_CONFLICT) {
+  if (state.commit_result == SERVER_RETURN_CONFLICT)
     return short_initial_backoff_;
-  }
 
   return default_initial_backoff_;
 }
 
 }  // namespace syncer