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..482e3b10e10b634b60bb30b35c65033d7364abf5 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"
@@ -34,6 +36,14 @@ class MOJO_CPP_BINDINGS_EXPORT SyncHandleRegistry
 
   void UnregisterHandle(const Handle& handle);
 
+  // Registers a |base::WaitableEvent| which can be used to wake up
+  // WatchAllHandles() before any handle signals. |event| is not owned, and if
+  // it signals during WatchAllHandles(), |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 runs callbacks synchronously for
   // those ready handles.
   // The method:
@@ -57,6 +67,7 @@ class MOJO_CPP_BINDINGS_EXPORT SyncHandleRegistry
   HandleMap handles_;
 
   WaitSet wait_set_;
+  std::map<base::WaitableEvent*, base::Closure> events_;

[yzshen1, 2017/03/23 20:15:50]

It is a little weird to use unordered_map for one thing and map for the other.
Can we unify them? I am fine with either one (without any data support).

[Ken Rockot(use gerrit already), 2017/03/23 22:04:20]

Done. Based on recent discussions making me feel sad about unordered_map
insertion behavior, and the fact that these are small maps with fast lookup in
any case, they are now both map.

 
   base::ThreadChecker thread_checker_;