[Invalidations] Add GetInvalidatorState() to Invalidator{,Frontend}

chrome/browser/sync/invalidation_frontend.h

Index: chrome/browser/sync/invalidation_frontend.h
diff --git a/chrome/browser/sync/invalidation_frontend.h b/chrome/browser/sync/invalidation_frontend.h
index 87d6a97ab7aaca44a2b17404208a8c73fbb29870..3076b6138b97e9f75ceaa9c9c18c8169264eb4c2 100644
--- a/chrome/browser/sync/invalidation_frontend.h
+++ b/chrome/browser/sync/invalidation_frontend.h
@@ -13,6 +13,43 @@ class InvalidationHandler;
 
 // Interface for classes that handle invalidation registrations and send out
 // invalidations to register handlers.
+//
+// Invalidation clients should follow the pattern below:
+//
+// When starting the client:
+//
+//   frontend->RegisterInvalidationHandler(client_handler);
+//
+// When the set of IDs to register changes for the client during its lifetime
+// (i.e., between calls to RegisterInvalidationHandler(client_handler) and
+// UnregisterInvalidationHandler(client_handler):
+//
+//   frontend->UpdateRegisteredInvalidationIds(client_handler, client_ids);
+//
+// When shutting down the client for browser shutdown:
+//
+//   frontend->UnregisterInvalidationHandler(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:
+//
+//   frontend->UpdateRegisteredInvalidationIds(client_handler, ObjectIdSet());
+//   frontend->UnregisterInvalidationHandler(client_handler);
+//
+// If an invalidation handler cares about the invalidator state, it should also
+// do the following when starting the client:
+//
+//   invalidator_state = frontend->GetInvalidatorState();
+//
+// It can also do the above in OnInvalidatorStateChange(), or it can use the
+// argument to OnInvalidatorStateChange().
+//
+// NOTE(akalin): Invalidations that come in during browser shutdown may get
+// dropped.  This won't matter once we have an Acknowledge API, though: see
+// http://crbug.com/78462 and http://crbug.com/124149.
 class InvalidationFrontend {
  public:
   // Starts sending notifications to |handler|.  |handler| must not be NULL,
@@ -39,6 +76,11 @@ class InvalidationFrontend {
   virtual void UnregisterInvalidationHandler(
       syncer::InvalidationHandler* handler) = 0;
 
+  // Returns the current invalidator state.  When called from within
+  // InvalidationHandler::OnInvalidatorStateChange(), this must return
+  // the updated state.
+  virtual syncer::InvalidatorState GetInvalidatorState() const = 0;
+
  protected:
   virtual ~InvalidationFrontend() { }
 };