base: Make it possible to replace the MessageLoop's task runner

base/message_loop/message_loop.cc

Index: base/message_loop/message_loop.cc
diff --git a/base/message_loop/message_loop.cc b/base/message_loop/message_loop.cc
index 0eb24cf7d2f89641950bf36ba7768ca7d2dc7ccc..fc2a61433af3b78ec3deb3884f8bdf692fc8e717 100644
--- a/base/message_loop/message_loop.cc
+++ b/base/message_loop/message_loop.cc
@@ -367,6 +367,7 @@ bool MessageLoop::IsIdleForTesting() {
 
 //------------------------------------------------------------------------------
 
+// static
 scoped_ptr<MessageLoop> MessageLoop::CreateUnbound(
     Type type, MessagePumpFactoryCallback pump_factory) {
   return make_scoped_ptr(new MessageLoop(type, pump_factory));
@@ -403,7 +404,23 @@ void MessageLoop::BindToCurrentThread() {
 
   incoming_task_queue_->StartScheduling();
   task_runner_->BindToCurrentThread();
-  thread_task_runner_handle_.reset(new ThreadTaskRunnerHandle(task_runner_));
+  SwapTaskRunner(task_runner_);
+}
+
+scoped_refptr<BindableSingleThreadTaskRunner> MessageLoop::SwapTaskRunner(
+    scoped_refptr<BindableSingleThreadTaskRunner> task_runner) {
+  // If this message loop was already bound to a thread, ensure the task runner
+  // belongs to the same thread and update the thread task runner handle.
+  // Otherwise the task runner will be bound later in BindToCurrentThread().
+  if (pump_) {
+    DCHECK_EQ(this, current());
+    DCHECK(task_runner->BelongsToCurrentThread());
+    thread_task_runner_handle_.reset();
+    thread_task_runner_handle_.reset(new ThreadTaskRunnerHandle(task_runner));

[kinuko, 2015/06/25 05:58:08]

nit: no need to call reset() twice, line 418 is not necessary?

[alexclarke, 2015/06/25 09:36:51]

Done.

+  }
+  scoped_refptr<BindableSingleThreadTaskRunner> previous_runner = task_runner_;

[kinuko, 2015/06/25 05:58:08]

nit: task_runner_.Pass() might be slightly better

+  task_runner_ = task_runner;
+  return previous_runner;
 }
 
 void MessageLoop::RunHandler() {