sync: Retry soon when server returns conflict

sync/engine/backoff_delay_provider.cc

Index: sync/engine/backoff_delay_provider.cc
diff --git a/sync/engine/backoff_delay_provider.cc b/sync/engine/backoff_delay_provider.cc
index d030e26f327dceed2e04e35550f599132ce5fab7..f47211b356d236f479bccd78162c06055f3ca04e 100644
--- a/sync/engine/backoff_delay_provider.cc
+++ b/sync/engine/backoff_delay_provider.cc
@@ -97,6 +97,14 @@ TimeDelta BackoffDelayProvider::GetInitialDelay(
     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.

[tim (not reviewing), 2012/11/21 21:37:18]

I agree this solution is safe and easy and we should land it.  Can you also add
a TODO though, and perhaps reference a bug, to say that ideally we would have
dealt with protocol errors like this before dispatching to BackoffDelayProvider,
since they don't actually require backoff.

+  if (state.commit_result == SERVER_RETURN_CONFLICT) {
+    return short_initial_backoff_;
+  }
+
   return default_initial_backoff_;
 }