Use WaitableEvents to wake up sync IPC waiting

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

+// 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.
+
+#include "mojo/public/cpp/bindings/sync_event_watcher.h"
+
+#include "base/logging.h"
+
+namespace mojo {
+
+SyncEventWatcher::SyncEventWatcher(base::WaitableEvent* event,
+                                   const base::Closure& callback)
+    : event_(event),
+      callback_(callback),
+      registry_(SyncHandleRegistry::current()),
+      destroyed_(new base::RefCountedData<bool>(false)) {}
+
+SyncEventWatcher::~SyncEventWatcher() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  if (registered_)
+    registry_->UnregisterEvent(event_);
+  destroyed_->data = true;
+}
+
+void SyncEventWatcher::AllowWokenUpBySyncWatchOnSameThread() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  IncrementRegisterCount();
+}
+
+bool SyncEventWatcher::SyncWatch(const bool* should_stop) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  IncrementRegisterCount();
+  if (!registered_) {
+    DecrementRegisterCount();
+    return false;
+  }
+
+  // This object may be destroyed during the Wait() call. So we have to preserve
+  // the boolean that Wait uses.
+  auto destroyed = destroyed_;
+  const bool* should_stop_array[] = {should_stop, &destroyed->data};
+  bool result = registry_->Wait(should_stop_array, 2);
+
+  // This object has been destroyed.
+  if (destroyed->data)
+    return false;
+
+  DecrementRegisterCount();
+  return result;
+}
+
+void SyncEventWatcher::IncrementRegisterCount() {
+  register_request_count_++;
+  if (!registered_)
+    registered_ = registry_->RegisterEvent(event_, callback_);
+}
+
+void SyncEventWatcher::DecrementRegisterCount() {
+  DCHECK_GT(register_request_count_, 0u);
+  register_request_count_--;
+  if (register_request_count_ == 0 && registered_) {
+    registry_->UnregisterEvent(event_);
+    registered_ = false;
+  }
+}
+
+}  // namespace mojo