Add code for new invalidation local ack system

sync/notifier/dropped_invalidation_tracker.h

+// Copyright 2013 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_NOTIFIER_DROPPED_INVALIDATION_TRACKER_H_
+#define SYNC_NOTIFIER_DROPPED_INVALIDATION_TRACKER_H_
+
+#include "google/cacheinvalidation/include/types.h"
+#include "sync/base/sync_export.h"
+#include "sync/internal_api/public/base/ack_handle.h"
+#include "sync/internal_api/public/util/weak_handle.h"
+#include "sync/notifier/ack_handler.h"
+
+namespace syncer {
+
+class Invalidation;
+
+// Helps InvalidationHandlers keep track of dropped invalidations for a given
+// ObjectId.
+//
+// The intent of this class is to hide some of the implementation details around
+// how the invalidations system manages dropping and drop recovery.  Any
+// invalidation handler that intends to buffer and occasionally drop
+// invalidations should keep one instance of it per registered ObjectId.
+//
+// When an invalidation handler wishes to drop an invalidation, it must provide
+// an instance of this class to that Invalidation's Drop() method.  In order to
+// indicate recovery from a drop, the handler can call this class'
+// RecordRecoveryFromDropEvent().
+class SYNC_EXPORT DroppedInvalidationTracker {
+ public:
+  explicit DroppedInvalidationTracker(const invalidation::ObjectId& id);
+  ~DroppedInvalidationTracker();
+
+  const invalidation::ObjectId& object_id() const;
+
+  // Called by Invalidation::Drop() to keep track of a drop event.

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

Explain parameter or refer to comment at drop_ack_handler_ .

+  void RecordDropEvent(WeakHandle<AckHandler> handler, AckHandle handle);
+
+  // Returns true if we're still recovering from a drop event.
+  bool IsRecoveringFromDropEvent() const;
+
+  // Called by the InvalidationHandler when it recovers from the drop event.
+  void RecordRecoveryFromDropEvent();
+
+ private:
+  invalidation::ObjectId id_;
+  AckHandle drop_ack_handle_;
+
+  // A WeakHandle to the enitity responsible for persisting invalidation
+  // acknowledgement state on disk.  We can get away with using a WeakHandle
+  // because we don't care if our drop recovery message doesn't gets delivered
+  // in some shutdown cases.  If that happens, we'll have to process the
+  // invalidation state again on the next restart.  It would be a waste of time
+  // and resources, but otherwise not particularly harmful.
+  WeakHandle<AckHandler> drop_ack_handler_;
+
+  DISALLOW_COPY_AND_ASSIGN(DroppedInvalidationTracker);
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_NOTIFIER_DROPPED_INVALIDATION_TRACKER_H_