Use WaitableEvents to wake up sync IPC waiting

mojo/public/cpp/bindings/sync_event_watcher.h

Index: mojo/public/cpp/bindings/sync_event_watcher.h
diff --git a/mojo/public/cpp/bindings/sync_event_watcher.h b/mojo/public/cpp/bindings/sync_event_watcher.h
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+// Copyright 2017 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 MOJO_PUBLIC_CPP_BINDINGS_SYNC_EVENT_WATCHER_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_SYNC_EVENT_WATCHER_H_
+
+#include <stddef.h>
+
+#include "base/callback.h"
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/synchronization/waitable_event.h"
+#include "base/threading/thread_checker.h"
+#include "mojo/public/cpp/bindings/bindings_export.h"
+#include "mojo/public/cpp/bindings/sync_handle_registry.h"
+
+namespace mojo {
+
+// SyncEventWatcher supports waiting on a base::WaitableEvent to signal while
+// also allowing other SyncEventWatchers and SyncHandleWatchers on the same
+// thread to wake up as needed.
+//
+// This class is not thread safe.
+class MOJO_CPP_BINDINGS_EXPORT SyncEventWatcher {
+ public:
+  SyncEventWatcher(base::WaitableEvent* event, const base::Closure& callback);
+
+  ~SyncEventWatcher();
+
+  // Registers |event_| with SyncHandleRegistry, so that when others perform
+  // sync watching on the same thread, |event_| will be watched along with them.
+  void AllowWokenUpBySyncWatchOnSameThread();
+
+  // Waits on |event_| plus all other events and handles registered with this
+  // thread's SyncHandleRegistry, running callbacks synchronously for any ready
+  // events and handles.
+  // This method:
+  //   - returns true when |should_stop| is set to true;
+  //   - return false when any error occurs, including this object being
+  //     destroyed during a callback.
+  bool SyncWatch(const bool* should_stop);
+
+ private:
+  void IncrementRegisterCount();
+  void DecrementRegisterCount();
+
+  base::WaitableEvent* const event_;
+  const base::Closure callback_;
+
+  // Whether |event_| has been registered with SyncHandleRegistry.
+  bool registered_ = false;
+
+  // If non-zero, |event_| should be registered with SyncHandleRegistry.
+  size_t register_request_count_ = 0;
+
+  scoped_refptr<SyncHandleRegistry> registry_;
+
+  scoped_refptr<base::RefCountedData<bool>> destroyed_;
+
+  base::ThreadChecker thread_checker_;
+
+  DISALLOW_COPY_AND_ASSIGN(SyncEventWatcher);
+};
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_SYNC_EVENT_WATCHER_H_