sync: Gracefully handle very early shutdown

sync/internal_api/public/base/cancellation_observer.h

Index: sync/internal_api/public/base/cancellation_observer.h
diff --git a/sync/internal_api/public/base/cancellation_observer.h b/sync/internal_api/public/base/cancellation_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..7ada4ab6474db702ee7a09764bed0696968deba8
--- /dev/null
+++ b/sync/internal_api/public/base/cancellation_observer.h
@@ -0,0 +1,22 @@
+// Copyright (c) 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_INTERNAL_API_PUBLIC_BASE_CANCELLATION_OBSERVER_H_

[akalin, 2013/09/05 16:06:35]

worth pointing out that cancelable etc. seems to be more common in chromium than
cancellable, etc. (I don't care either way.)

[rlarocque, 2013/09/05 22:37:06]

Although it looks very wrong to me, I should be using American spelling.  Fixed.

+#define SYNC_INTERNAL_API_PUBLIC_BASE_CANCELLATION_OBSERVER_H_
+
+#include "sync/base/sync_export.h"
+
+namespace syncer {
+
+// Interface for classes that handle signals from the CancellationSignal.
+class SYNC_EXPORT_PRIVATE CancellationObserver {
+ public:
+  CancellationObserver();
+  virtual ~CancellationObserver() = 0;
+  virtual void OnStopRequested() = 0;
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_INTERNAL_API_PUBLIC_BASE_CANCELLATION_OBSERVER_H_