sync: Gracefully handle very early shutdown

sync/internal_api/public/base/cancelation_signal.cc

Index: sync/internal_api/public/base/cancelation_signal.cc
diff --git a/sync/internal_api/public/base/cancelation_signal.cc b/sync/internal_api/public/base/cancelation_signal.cc
new file mode 100644
index 0000000000000000000000000000000000000000..1ce920e7846442ccfd9a3a23a4f8f0272432b5ee
--- /dev/null
+++ b/sync/internal_api/public/base/cancelation_signal.cc
@@ -0,0 +1,52 @@
+// 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.
+
+#include "sync/internal_api/public/base/cancelation_signal.h"
+
+#include "base/logging.h"
+#include "sync/internal_api/public/base/cancelation_observer.h"
+
+namespace syncer {
+
+CancelationSignal::CancelationSignal()
+  : stop_requested_(false),
+    handler_(NULL) { }
+
+CancelationSignal::~CancelationSignal() {
+  DCHECK(!handler_);
+}
+
+bool CancelationSignal::TryRegisterHandler(CancelationObserver* handler) {
+  base::AutoLock lock(stop_requested_lock_);
+  DCHECK(!handler_);
+
+  if (stop_requested_)
+    return false;
+
+  handler_ = handler;
+  return true;
+}
+
+void CancelationSignal::UnregisterHandler(CancelationObserver* handler) {
+  base::AutoLock lock(stop_requested_lock_);
+  DCHECK_EQ(handler_, handler);
+  handler_ = NULL;
+}
+
+bool CancelationSignal::IsStopRequested() {
+  base::AutoLock lock(stop_requested_lock_);
+  return stop_requested_;
+}
+
+void CancelationSignal::RequestStop() {
+  base::AutoLock lock(stop_requested_lock_);
+  DCHECK(!stop_requested_);
+
+  stop_requested_ = true;
+  if (handler_) {
+    handler_->OnStopRequested();
+  }
+}
+
+}  // namespace syncer