sync: remove buggy freshness condition in SyncSchedulerImpl add defensive DCHECKs.

sync/engine/sync_scheduler_impl.h

 // 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.
 
 #ifndef SYNC_ENGINE_SYNC_SCHEDULER_IMPL_H_
 #define SYNC_ENGINE_SYNC_SCHEDULER_IMPL_H_
 
 #include <string>
 
 #include "base/callback.h"
@@ skipping 295 matching lines @@
 
   // Server-tweakable sessions commit delay.
   base::TimeDelta sessions_commit_delay_;
 
   // Periodic timer for polling.  See AdjustPolling.
   base::RepeatingTimer<SyncSchedulerImpl> poll_timer_;
 
   // The mode of operation.
   Mode mode_;
 
-  // TODO(tim): Bug 26339. This needs to track more than just time I think,
-  // since the nudges could be for different types. Current impl doesn't care.
-  base::TimeTicks last_sync_session_end_time_;
+  // Tracks the time that the last successful nudge or poll completed.  This
+  // is an optimization to avoid performing unnecessary POLLs (if a nudge
+  // completed after the poll was scheduled to start) and unnecessary NUDGEs
+  // (if a POLL [which uses all data types] took place *after* the nudge was

[rlarocque, 2012/08/20 19:02:20]

Can a POLL really replace a NUDGE?  What if the NUDGE had a notification
payload?

+  // supposed to start. Note that this does not result in dropping a NUDGE if
+  // another more recent NUDGE took place, because those NUDGEs would have
+  // been coalesced.  The job will still be scheduled in that case (since we
+  // don't cancel outstanding jobs when coalescing), but this will be detected
+  // as a cancelled NUDGE by the session-equality check in DoSyncSessionJob).
+  // This excludes CONFIGURATION because 1) CONFIGURATION jobs are not subject
+  // to the freshness condition, and 2) a successful CONFIGURATION job should
+  // not trump a new NUDGE / POLL since there is no correlation between data
+  // types.
+  // TODO(tim): If NUDGEs are made to operate only on a subset of enabled types
+  // in the future, this will be insufficient as a NUDGE can't trump a full-
+  // datatype POLL.  On the flip side, if POLLs are one day eliminated, this
+  // optimization is no longer necessary.
+  base::TimeTicks last_nudge_or_poll_end_time_;
 
   // The latest connection code we got while trying to connect.
   HttpResponse::ServerConnectionCode connection_code_;
 
   // Tracks in-flight nudges so we can coalesce.
   scoped_ptr<SyncSessionJob> pending_nudge_;
 
   // Current wait state.  Null if we're not in backoff and not throttled.
   scoped_ptr<WaitInterval> wait_interval_;
 
   scoped_ptr<BackoffDelayProvider> delay_provider_;
 
   // Invoked to run through the sync cycle.
   scoped_ptr<Syncer> syncer_;
 
   sessions::SyncSessionContext *session_context_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncSchedulerImpl);
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_ENGINE_SYNC_SCHEDULER_IMPL_H_