[Invalidations] Require there to be no registered handlers on Invalidator destruction

sync/notifier/invalidator_registrar.h

 // Copyright (c) 2012 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_INVALIDATOR_REGISTRAR_H_
 #define SYNC_NOTIFIER_INVALIDATOR_REGISTRAR_H_
 
 #include <map>
 
 #include "base/basictypes.h"
 #include "base/observer_list.h"
 #include "base/threading/thread_checker.h"
 #include "sync/notifier/invalidation_handler.h"
 #include "sync/notifier/invalidation_util.h"
 #include "sync/notifier/object_id_invalidation_map.h"
 
 namespace invalidation {
 class ObjectId;
 }  // namespace invalidation
 
 namespace syncer {
 
 // A helper class for implementations of the Invalidator interface.  It helps
 // keep track of registered handlers and which object ID registrations are
 // associated with which handlers, so implementors can just reuse the logic
 // here to dispatch invalidations and other interesting notifications.
 class InvalidatorRegistrar {
  public:
   InvalidatorRegistrar();
+
+  // It is an error to have registered handlers on destruction.
   ~InvalidatorRegistrar();
 
   // Starts sending notifications to |handler|.  |handler| must not be NULL,
   // and it must already be registered.
   void RegisterHandler(InvalidationHandler* handler);
 
   // Updates the set of ObjectIds associated with |handler|.  |handler| must
   // not be NULL, and must already be registered.  An ID must be registered for
   // at most one handler.
   void UpdateRegisteredIds(InvalidationHandler* handler,
@@ skipping 44 matching lines @@
   ObserverList<InvalidationHandler> handlers_;
   IdHandlerMap id_to_handler_map_;
   InvalidatorState state_;
 
   DISALLOW_COPY_AND_ASSIGN(InvalidatorRegistrar);
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_NOTIFIER_INVALIDATOR_REGISTRAR_H_