[Sync] Avoid unregistering object IDs on shutdown

sync/notifier/sync_notifier.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.
 //
 // Interface to the sync notifier, which is an object that receives
 // notifications when updates are available for a set of sync types.
 // All the observers are notified when such an event happens.
 
 #ifndef SYNC_NOTIFIER_SYNC_NOTIFIER_H_
 #define SYNC_NOTIFIER_SYNC_NOTIFIER_H_
 
 #include <string>
 
 #include "sync/internal_api/public/base/model_type.h"
 #include "sync/notifier/invalidation_util.h"
 
 namespace syncer {
 class SyncNotifierObserver;
 
 class SyncNotifier {
  public:
   SyncNotifier() {}
   virtual ~SyncNotifier() {}
 
-  // Updates the set of ObjectIds associated with a given |handler|.
-  // Passing an empty ObjectIdSet will unregister |handler|.
-  // There should be at most one handler registered per object id.
+  // Clients should follow the pattern below:
+  //
+  // When starting the client:
+  //
+  //   notifier->RegisterHandler(client_handler);
+  //
+  // When the set of IDs to register changes for the client during its lifetime
+  // (i.e., between calls to RegisterHandler(client_handler) and
+  // UnregisterHandler(client_handler):
+  //
+  //   notifier->UpdateRegisteredIds(client_handler, client_ids);
+  //
+  // When shutting down the client for browser shutdown:
+  //
+  //   notifier->UnregisterHandler(client_handler);
+  //
+  // Note that there's no call to UpdateRegisteredIds() -- this is because the
+  // invalidation API persists registrations across browser restarts.
+  //
+  // When permanently shutting down the client, e.g. when disabling the related
+  // feature:
+  //
+  //   notifier->UpdateRegisteredIds(client_handler, ObjectIdSet());
+  //   notifier->UnregisterHandler(client_handler);
+
+  // Starts sending notifications to |handler|.  |handler| must not be NULL,
+  // and it must already be registered.
+  virtual void RegisterHandler(SyncNotifierObserver* handler) = 0;
+
+  // 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.
   virtual void UpdateRegisteredIds(SyncNotifierObserver* handler,
                                    const ObjectIdSet& ids) = 0;
 
+  // Stops sending notifications to |handler|.  |handler| must not be NULL, and
+  // it must already be registered.  Note that this doesn't unregister the IDs
+  // associated with |handler|.
+  virtual void UnregisterHandler(SyncNotifierObserver* handler) = 0;
+
   // SetUniqueId must be called once, before any call to
   // UpdateCredentials.  |unique_id| should be a non-empty globally
   // unique string.
   virtual void SetUniqueId(const std::string& unique_id) = 0;
 
   // SetState must be called once, before any call to
   // UpdateCredentials.  |state| may be empty.
   // Deprecated in favour of InvalidationStateTracker persistence.
   virtual void SetStateDeprecated(const std::string& state) = 0;
 
   // The observers won't be notified of any notifications until
   // UpdateCredentials is called at least once. It can be called more than
   // once.
   virtual void UpdateCredentials(
       const std::string& email, const std::string& token) = 0;
 
   // This is here only to support the old p2p notification implementation,
   // which is still used by sync integration tests.
   // TODO(akalin): Remove this once we move the integration tests off p2p
   // notifications.
   virtual void SendNotification(ModelTypeSet changed_types) = 0;
 };
 }  // namespace syncer
 
 #endif  // SYNC_NOTIFIER_SYNC_NOTIFIER_H_