[mojo-edk] Expose notification source to MojoWatch callbacks

mojo/public/cpp/system/tests/watcher_unittest.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 "base/bind.h"
 #include "base/callback.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/thread_task_runner_handle.h"
@@ skipping 184 matching lines @@
   // This should trigger the watcher above...
   b.reset();
   // ...but the watcher is cancelled first.
   b_watcher.Cancel();
 
   EXPECT_FALSE(b_watcher.IsWatching());
 
   base::RunLoop().RunUntilIdle();
 }
 
+TEST_F(WatcherTest, SyncDispatch) {
+  ScopedMessagePipeHandle a, b;
+  CreateMessagePipe(nullptr, &a, &b);
+
+  bool notified = false;
+  Watcher b_watcher;
+  EXPECT_EQ(MOJO_RESULT_OK,
+            b_watcher.Start(b.get(), MOJO_HANDLE_SIGNAL_READABLE,
+                            OnReady([&] (MojoResult result) {
+                              EXPECT_EQ(MOJO_RESULT_OK, result);
+                              notified = true;
+                            })));
+  EXPECT_TRUE(b_watcher.IsWatching());
+
+  b_watcher.SetAllowSyncDispatch(true);
+  EXPECT_EQ(MOJO_RESULT_OK, WriteMessageRaw(a.get(), "hello", 5, nullptr, 0,
+                                            MOJO_WRITE_MESSAGE_FLAG_NONE));
+
+  // We don't spin the MessageLoop but should have been notified anyway.
+
+  EXPECT_TRUE(notified);
+
+  b_watcher.Cancel();
+}
+
 }  // namespace
 }  // namespace mojo