Revert 222154 "sync: Gracefully handle very early shutdown"

trunk/src/sync/internal_api/public/base/cancelation_signal.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_INTERNAL_API_PUBLIC_BASE_CANCELATION_SIGNAL_H_
-#define SYNC_INTERNAL_API_PUBLIC_BASE_CANCELATION_SIGNAL_H_
-
-#include "base/synchronization/lock.h"
-#include "sync/base/sync_export.h"
-
-namespace syncer {
-
-class CancelationObserver;
-
-// This class is used to allow one thread to request that another abort and
-// return early.
-//
-// The signalling thread owns this class and my call RequestStop() at any time.
-// After that call, this class' IsStopRequested() will always return true.  The
-// intended use case is that the task intending to support early exit will
-// periodically check the value of IsStopRequested() to see if it should return
-// early.
-//
-// The receiving task may also choose to register an CancelationObserver whose
-// OnStopRequested() method will be executed on the signaller's thread when
-// RequestStop() is called.  This may be used for sending an early Signal() to a
-// WaitableEvent.  The registration of the handler is necessarily racy.  If
-// RequestStop() is executes before TryRegisterHandler(), TryRegisterHandler()
-// will not perform any registration and return false.  That function's caller
-// must handle this case.
-//
-// This class supports only one handler, though it could easily support multiple
-// observers if we found a use case for such a feature.
-class SYNC_EXPORT_PRIVATE CancelationSignal {
- public:
-  CancelationSignal();
-  ~CancelationSignal();
-
-  // Tries to register a handler to be invoked when RequestStop() is called.
-  //
-  // If RequestStop() has already been called, returns false without registering
-  // the handler.  Returns true when the registration is successful.
-  //
-  // If the registration was successful, the handler must be unregistered with
-  // UnregisterHandler before this CancelationSignal is destroyed.
-  bool TryRegisterHandler(CancelationObserver* handler);
-
-  // Unregisters the abort handler.
-  void UnregisterHandler(CancelationObserver* handler);
-
-  // Returns true if RequestStop() has been called.
-  bool IsStopRequested();
-
-  // Sets the stop_requested_ flag and calls the OnStopRequested() method of the
-  // registered handler, if there is one registered at the time.
-  // OnStopRequested() will be called with the |stop_requested_lock_| held.
-  void RequestStop();
-
- private:
-  // Protects all members of this class.
-  base::Lock stop_requested_lock_;
-
-  // True if RequestStop() has been invoked.
-  bool stop_requested_;
-
-  // The registered abort handler.  May be NULL.
-  CancelationObserver* handler_;
-};
-
-}  // namespace syncer
-
-#endif  // SYNC_INTERNAL_API_PUBLIC_BASE_CANCELATION_SIGNAL_H_