Use WaitableEvents to wake up sync IPC waiting

mojo/public/cpp/bindings/lib/sync_handle_registry.cc

 // 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.
 
 #include "mojo/public/cpp/bindings/sync_handle_registry.h"
 
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/stl_util.h"
 #include "base/threading/thread_local.h"
@@ skipping 37 matching lines @@
 void SyncHandleRegistry::UnregisterHandle(const Handle& handle) {
   DCHECK(thread_checker_.CalledOnValidThread());
   if (!base::ContainsKey(handles_, handle))
     return;
 
   MojoResult result = wait_set_.RemoveHandle(handle);
   DCHECK_EQ(MOJO_RESULT_OK, result);
   handles_.erase(handle);
 }
 
+bool SyncHandleRegistry::RegisterEvent(base::WaitableEvent* event,
+                                       const base::Closure& callback) {
+  auto result = events_.insert({event, callback});
+  DCHECK(result.second);
+  MojoResult rv = wait_set_.AddEvent(event);
+  if (rv == MOJO_RESULT_OK)
+    return true;
+  DCHECK_EQ(MOJO_RESULT_ALREADY_EXISTS, rv);
+  return false;
+}
+
+void SyncHandleRegistry::UnregisterEvent(base::WaitableEvent* event) {
+  auto it = events_.find(event);
+  DCHECK(it != events_.end());
+  events_.erase(it);
+  MojoResult rv = wait_set_.RemoveEvent(event);
+  DCHECK_EQ(MOJO_RESULT_OK, rv);
+}
+
 bool SyncHandleRegistry::WatchAllHandles(const bool* should_stop[],
                                          size_t count) {
   DCHECK(thread_checker_.CalledOnValidThread());
 
   size_t num_ready_handles;
   Handle ready_handle;
   MojoResult ready_handle_result;
 
   scoped_refptr<SyncHandleRegistry> preserver(this);
   while (true) {
     for (size_t i = 0; i < count; ++i)
       if (*should_stop[i])
         return true;
 
     // TODO(yzshen): Theoretically it can reduce sync call re-entrancy if we
     // give priority to the handle that is waiting for sync response.
+    base::WaitableEvent* ready_event = nullptr;
     num_ready_handles = 1;
-    wait_set_.Wait(&num_ready_handles, &ready_handle, &ready_handle_result);
-    DCHECK_EQ(1u, num_ready_handles);
+    wait_set_.Wait(&ready_event, &num_ready_handles, &ready_handle,
+                   &ready_handle_result);
+    if (num_ready_handles) {
+      DCHECK_EQ(1u, num_ready_handles);
+      const auto iter = handles_.find(ready_handle);
+      iter->second.Run(ready_handle_result);
+    }
 
-    const auto iter = handles_.find(ready_handle);
-    iter->second.Run(ready_handle_result);
+    if (ready_event) {
+      const auto iter = events_.find(ready_event);
+      DCHECK(iter != events_.end());
+      iter->second.Run();
+    }
   };
 
   return false;
 }
 
 SyncHandleRegistry::SyncHandleRegistry() {
   DCHECK(!g_current_sync_handle_watcher.Pointer()->Get());
   g_current_sync_handle_watcher.Pointer()->Set(this);
 }
 
 SyncHandleRegistry::~SyncHandleRegistry() {
   DCHECK(thread_checker_.CalledOnValidThread());
 
   // This object may be destructed after the thread local storage slot used by
   // |g_current_sync_handle_watcher| is reset during thread shutdown.
   // For example, another slot in the thread local storage holds a referrence to
   // this object, and that slot is cleaned up after
   // |g_current_sync_handle_watcher|.
   if (!g_current_sync_handle_watcher.Pointer()->Get())
     return;
 
   // If this breaks, it is likely that the global variable is bulit into and
   // accessed from multiple modules.
   DCHECK_EQ(this, g_current_sync_handle_watcher.Pointer()->Get());
 
   g_current_sync_handle_watcher.Pointer()->Set(nullptr);
 }
 
 }  // namespace mojo