Use WaitableEvents to wake up sync IPC waiting

mojo/public/cpp/bindings/sync_handle_registry.h

 // Copyright 2016 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_HANDLE_REGISTRY_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_SYNC_HANDLE_REGISTRY_H_
 
+#include <map>
 #include <unordered_map>
 
 #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/system/core.h"
 #include "mojo/public/cpp/system/wait_set.h"
 
 namespace mojo {
 
 // SyncHandleRegistry is a thread-local storage to register handles that want to
 // be watched together.
 //
 // This class is not thread safe.
 class MOJO_CPP_BINDINGS_EXPORT SyncHandleRegistry
     : public base::RefCounted<SyncHandleRegistry> {
  public:
   // Returns a thread-local object.
   static scoped_refptr<SyncHandleRegistry> current();
 
   using HandleCallback = base::Callback<void(MojoResult)>;
   bool RegisterHandle(const Handle& handle,
                       MojoHandleSignals handle_signals,
                       const HandleCallback& callback);
 
   void UnregisterHandle(const Handle& handle);
 
-  // Waits on all the registered handles and runs callbacks synchronously for
-  // those ready handles.
+  // Registers a |base::WaitableEvent| which can be used to wake up
+  // Wait() before any handle signals. |event| is not owned, and if it signals
+  // during Wait(), |callback| is invoked. Returns |true| if registered
+  // successfully or |false| if |event| was already registered.
+  bool RegisterEvent(base::WaitableEvent* event, const base::Closure& callback);
+
+  void UnregisterEvent(base::WaitableEvent* event);
+
+  // Waits on all the registered handles and events and runs callbacks
+  // synchronously for any that become ready.
   // The method:
   //   - returns true when any element of |should_stop| is set to true;
   //   - returns false when any error occurs.
-  bool WatchAllHandles(const bool* should_stop[], size_t count);
+  bool Wait(const bool* should_stop[], size_t count);
 
  private:
   friend class base::RefCounted<SyncHandleRegistry>;
 
-  struct HandleHasher {
-    size_t operator()(const Handle& handle) const {
-      return std::hash<uint32_t>()(static_cast<uint32_t>(handle.value()));
-    }
-  };
-  using HandleMap = std::unordered_map<Handle, HandleCallback, HandleHasher>;
-
   SyncHandleRegistry();
   ~SyncHandleRegistry();
 
-  HandleMap handles_;
-
   WaitSet wait_set_;
+  std::map<Handle, HandleCallback> handles_;
+  std::map<base::WaitableEvent*, base::Closure> events_;
 
   base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncHandleRegistry);
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_SYNC_HANDLE_REGISTRY_H_