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

sync/engine/sync_scheduler_impl.h

Index: sync/engine/sync_scheduler_impl.h
diff --git a/sync/engine/sync_scheduler_impl.h b/sync/engine/sync_scheduler_impl.h
index e5f3acf61d27aef8463e987beab557e914f059fc..e086c84d68ebe60e21303fc550b3bb91479e220f 100644
--- a/sync/engine/sync_scheduler_impl.h
+++ b/sync/engine/sync_scheduler_impl.h
@@ -313,9 +313,24 @@ class SyncSchedulerImpl : public SyncScheduler {
   // 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_;