sync: Retry soon when server returns conflict

sync: Retry soon when server returns conflict

When the server gives us a conflict response, we ought to fetch updates
in order to figure out what the server is complaining about, resolve any
conflicts locally, then re-commit.

The current syncer, although not intentionally built to handle this
scenario, does the right thing.  It considers the server's return code
to be indicative of a transient error, so it backs off then retries
later.  The retry sync cycle will fetch updates, resolve conflicts, and
recommit, which is exactly what we want.  Unfortunately, the backoff can
take five minutes.

This commit reduces the time spent unnecessarily backing off.  It's not
the cleanest solution, but its implementation is easy and safe.

BUG=157955
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=169158

[rlarocque, 2012-11-16 20:30:01]

Here's the simpler version of the server conflict handling patch.  Please
review.

[rlarocque, 2012-11-21 21:32:46]

I had forgotten about this.

Ping?

[tim (not reviewing), 2012-11-21 21:37:17]

LGTM with nit

https://codereview.chromium.org/11411041/diff/1/sync/engine/backoff_delay_pro...
File sync/engine/backoff_delay_provider.cc (right):

https://codereview.chromium.org/11411041/diff/1/sync/engine/backoff_delay_pro...
sync/engine/backoff_delay_provider.cc:103: // things.  There's no need to back
off, we can do this immediately.
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.

[commit-bot: I haz the power, 2012-11-21 21:45:05]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rlarocque@chromium.org/11411041/7001

[commit-bot: I haz the power, 2012-11-21 23:27:56]

Change committed as 169158