Use WaitableEvents to wake up sync IPC waiting

mojo/public/cpp/bindings/sync_handle_registry.h

Index: mojo/public/cpp/bindings/sync_handle_registry.h
diff --git a/mojo/public/cpp/bindings/sync_handle_registry.h b/mojo/public/cpp/bindings/sync_handle_registry.h
index 3cf0492b76a8bb4be2d7a8c3414e990b1a086796..afb3b56bf41b751d7dcbb43bad22116484179206 100644
--- a/mojo/public/cpp/bindings/sync_handle_registry.h
+++ b/mojo/public/cpp/bindings/sync_handle_registry.h
@@ -5,11 +5,13 @@
 #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"
@@ -34,29 +36,30 @@ class MOJO_CPP_BINDINGS_EXPORT SyncHandleRegistry
 
   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_;