Add code for new invalidation local ack system

Add code for new invalidation local ack system

Commits classes that are part of the infrastructure of the new local
acknowledgement system for invalidations, which will be based on
WeakHandles to AckHandlers.

BUG=233437
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=231983

[rlarocque, 2013-10-24 21:09:29]

Please review.

Here's the next chunk of invalidations code.  As before, these new classes are
never instantiated in a real Chrome instance.

https://codereview.chromium.org/40303005/diff/1/sync/notifier/dropped_invalid...
File sync/notifier/dropped_invalidation_tracker.h (right):

https://codereview.chromium.org/40303005/diff/1/sync/notifier/dropped_invalid...
sync/notifier/dropped_invalidation_tracker.h:30: class SYNC_EXPORT
DroppedInvalidationTracker {
The DroppedInvalidationTracker is left over from an earlier version of this
system where drop tracking was a bit more complicated.  Now it's not so
complicated, but a bit strange.  

Consecutive drops are coalesced so that we only need to keep track of the most
recent one.  In order to recover from a drop, we need to acknowledge the
ack_handle of the most recently dropped invalidation.

The DroppedInvalidationTracker hides this system from the InvalidationHandlers. 
It helps to keep the concept of ack handles completely hidden from them, too.  

Overall, I think the DroppedInvalidationTracker's existence makes sense, but I'm
sort of on the fence about it.  I'd like to hear your opinion.

[tim (not reviewing), 2013-10-29 17:21:57]

Looks mostly good, just a few nits.

https://codereview.chromium.org/40303005/diff/1/sync/internal_api/public/base...
File sync/internal_api/public/base/invalidation.h (right):

https://codereview.chromium.org/40303005/diff/1/sync/internal_api/public/base...
sync/internal_api/public/base/invalidation.h:62: void
Drop(DroppedInvalidationTracker* tracker) const;
Comment param.

https://codereview.chromium.org/40303005/diff/1/sync/notifier/dropped_invalid...
File sync/notifier/dropped_invalidation_tracker.h (right):

https://codereview.chromium.org/40303005/diff/1/sync/notifier/dropped_invalid...
sync/notifier/dropped_invalidation_tracker.h:32:
DroppedInvalidationTracker(const invalidation::ObjectId& id);
make constructor explicit.

https://codereview.chromium.org/40303005/diff/1/sync/notifier/dropped_invalid...
sync/notifier/dropped_invalidation_tracker.h:38: void
RecordDropEvent(WeakHandle<AckHandler> handler, AckHandle handle);
Comment on threading and where the best-effort contract is, given presence of
WeakHandle.

[rlarocque, 2013-10-30 00:49:25]

https://codereview.chromium.org/40303005/diff/1/sync/internal_api/public/base...
File sync/internal_api/public/base/invalidation.h (right):

https://codereview.chromium.org/40303005/diff/1/sync/internal_api/public/base...
sync/internal_api/public/base/invalidation.h:62: void
Drop(DroppedInvalidationTracker* tracker) const;
On 2013/10/29 17:21:57, timsteele wrote:
> Comment param.

Done.

https://codereview.chromium.org/40303005/diff/1/sync/notifier/dropped_invalid...
File sync/notifier/dropped_invalidation_tracker.h (right):

https://codereview.chromium.org/40303005/diff/1/sync/notifier/dropped_invalid...
sync/notifier/dropped_invalidation_tracker.h:32:
DroppedInvalidationTracker(const invalidation::ObjectId& id);
On 2013/10/29 17:21:57, timsteele wrote:
> make constructor explicit.

Done.

https://codereview.chromium.org/40303005/diff/1/sync/notifier/dropped_invalid...
sync/notifier/dropped_invalidation_tracker.h:38: void
RecordDropEvent(WeakHandle<AckHandler> handler, AckHandle handle);
On 2013/10/29 17:21:57, timsteele wrote:
> Comment on threading and where the best-effort contract is, given presence of
> WeakHandle.

Added comment to the member variable.

[tim (not reviewing), 2013-10-30 02:03:07]

LGTM, one nit

https://codereview.chromium.org/40303005/diff/120001/sync/notifier/dropped_in...
File sync/notifier/dropped_invalidation_tracker.h (right):

https://codereview.chromium.org/40303005/diff/120001/sync/notifier/dropped_in...
sync/notifier/dropped_invalidation_tracker.h:37: // Called by
Invalidation::Drop() to keep track of a drop event.
Explain parameter or refer to comment at drop_ack_handler_ .

[commit-bot: I haz the power, 2013-10-31 00:42:05]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rlarocque@chromium.org/40303005/350001

[commit-bot: I haz the power, 2013-10-31 03:59:39]

Change committed as 231983