sync: Add pre-commit update avoidance mode + flag

sync/sessions/sync_session_context.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.
 
 // SyncSessionContext encapsulates the contextual information and engine
 // components specific to a SyncSession. A context is accessible via
 // a SyncSession so that session SyncerCommands and parts of the engine have
 // a convenient way to access other parts. In this way it can be thought of as
 // the surrounding environment for the SyncSession. The components of this
 // environment are either valid or not valid for the entire context lifetime,
@@ skipping 37 matching lines @@
 class SYNC_EXPORT_PRIVATE SyncSessionContext {
  public:
   SyncSessionContext(ServerConnectionManager* connection_manager,
                      syncable::Directory* directory,
                      const std::vector<ModelSafeWorker*>& workers,
                      ExtensionsActivityMonitor* extensions_activity_monitor,
                      const std::vector<SyncEngineEventListener*>& listeners,
                      DebugInfoGetter* debug_info_getter,
                      TrafficRecorder* traffic_recorder,
                      bool keystore_encryption_enabled,
+                     bool always_fetch_updates_before_commit,
                      const std::string& invalidator_client_id);
 
   ~SyncSessionContext();
 
   ServerConnectionManager* connection_manager() {
     return connection_manager_;
   }
   syncable::Directory* directory() {
     return directory_;
   }
@@ skipping 54 matching lines @@
   }
 
   const sync_pb::ClientStatus& client_status() const {
     return client_status_;
   }
 
   const std::string& invalidator_client_id() const {
     return invalidator_client_id_;
   }
 
+  const bool should_fetch_updates_before_commit() const {
+    return !(pre_commit_update_avoidance_experiment_enabled_ ||
+             force_enable_pre_commit_update_avoidance_);
+  }
+
+  void set_pre_commit_update_avoidance_experiment_enabled(bool value) {
+    pre_commit_update_avoidance_experiment_enabled_ = value;
+  }
+
  private:
   // Rather than force clients to set and null-out various context members, we
   // extend our encapsulation boundary to scoped helpers that take care of this
   // once they are allocated. See definitions of these below.
   friend class TestScopedSessionEventListener;
 
   ObserverList<SyncEngineEventListener> listeners_;
 
   ServerConnectionManager* const connection_manager_;
   syncable::Directory* const directory_;
@@ skipping 32 matching lines @@
   // we should attempt performing keystore encryption related work, false if
   // the experiment is not enabled.
   bool keystore_encryption_enabled_;
 
   // This is a copy of the identifier the that the invalidations client used to
   // register itself with the invalidations server during startup.  We need to
   // provide this to the sync server when we make changes to enable it to
   // prevent us from receiving notifications of changes we make ourselves.
   const std::string invalidator_client_id_;
 
+  // Flag to enable or disable the no pre-commit GetUpdates experiment.  When
+  // this flag is set to false, the syncer has the option of not performing at
+  // GetUpdates request when there is nothing to fetch.
+  bool pre_commit_update_avoidance_experiment_enabled_;

[Nicolas Zea, 2013/07/16 00:24:32]

Do we plan for this to stick around (i.e. is the eventual state for whether we
do pre-commit GU going to remain server controlled)? I wonder if we shouldn't
call it an experiment then.

[rlarocque, 2013/07/16 01:08:27]

Good point.  I was hoping to remove this flag and make this behaviour the
default eventually, but it could be a long time before we get to that point.  I
could go either way on the name.  

Raz, any thoughts?

+
+  // If true, indicates that we've been passed a command-line flag to force
+  // enable the pre-commit update avoidance experiment described above.
+  bool force_enable_pre_commit_update_avoidance_;
+
   DISALLOW_COPY_AND_ASSIGN(SyncSessionContext);
 };
 
 }  // namespace sessions
 }  // namespace syncer
 
 #endif  // SYNC_SESSIONS_SYNC_SESSION_CONTEXT_H_