[sync] Prevent sessions flare on startup

components/sync/model/syncable_service.h

 // Copyright 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.
 
 #ifndef COMPONENTS_SYNC_MODEL_SYNCABLE_SERVICE_H_
 #define COMPONENTS_SYNC_MODEL_SYNCABLE_SERVICE_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 12 matching lines @@
 class AttachmentService;
 class SyncErrorFactory;
 
 // TODO(zea): remove SupportsWeakPtr in favor of having all SyncableService
 // implementers provide a way of getting a weak pointer to themselves.
 // See crbug.com/100114.
 class SyncableService : public SyncChangeProcessor,
                         public base::SupportsWeakPtr<SyncableService> {
  public:
   // A StartSyncFlare is useful when your SyncableService has a need for sync
-  // to start ASAP, typically because a local change event has occurred but
-  // MergeDataAndStartSyncing hasn't been called yet, meaning you don't have a
-  // SyncChangeProcessor. The sync subsystem will respond soon after invoking
-  // Run() on your flare by calling MergeDataAndStartSyncing. The ModelType
-  // parameter is included so that the recieving end can track usage and timing
-  // statistics, make optimizations or tradeoffs by type, etc.
+  // to start ASAP. This is typically for one of three reasons:
+  // 1) Because a local change event has occurred but MergeDataAndStartSyncing
+  // hasn't been called yet, meaning you don't have a SyncChangeProcessor. The
+  // sync subsystem will respond soon after invoking Run() on your flare by
+  // calling MergeDataAndStartSyncing.
+  // 2) You want remote data to be visible immediately; for example if the
+  // history page is open, you want remote sessions data to be available there.
+  // 3) You want to signal to sync that it's safe to start now that the
+  // browser's IO-intensive startup process is over. The ModelType parameter is
+  // included so that the recieving end can track usage and timing statistics,
+  // make pptimizations or tradeoffs by type, etc.
   using StartSyncFlare = base::Callback<void(ModelType)>;
 
   // Informs the service to begin syncing the specified synced datatype |type|.
   // The service should then merge |initial_sync_data| into it's local data,
   // calling |sync_processor|'s ProcessSyncChanges as necessary to reconcile the
   // two. After this, the SyncableService's local data should match the server
   // data, and the service should be ready to receive and process any further
   // SyncChange's as they occur.
   // Returns: a SyncMergeResult whose error field reflects whether an error
   //          was encountered while merging the two models. The merge result
@@ skipping 37 matching lines @@
   virtual void SetAttachmentService(
       std::unique_ptr<AttachmentService> attachment_service);
 
  protected:
   ~SyncableService() override;
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_MODEL_SYNCABLE_SERVICE_H_