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

base/message_loop/message_loop.h

Index: base/message_loop/message_loop.h
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index 9c635eebd73b3507449611a0f03ac40c94dab16d..e66c214239aead73cb8a9942ca912f13d0ae0856 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -12,6 +12,7 @@
 #include "base/basictypes.h"
 #include "base/callback_forward.h"
 #include "base/debug/task_annotator.h"
+#include "base/gtest_prod_util.h"
 #include "base/location.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
@@ -37,6 +38,7 @@
 
 namespace base {
 
+class BindableSingleThreadTaskRunner;
 class HistogramBase;
 class RunLoop;
 class ThreadTaskRunnerHandle;
@@ -296,10 +298,16 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   const std::string& thread_name() const { return thread_name_; }
 
   // Gets the TaskRunner associated with this message loop.
-  // TODO(skyostil): Change this to return a const reference to a refptr
-  // once the internal type matches what is being returned (crbug.com/465354).

[Sami, 2015/06/19 21:17:35]

N.B. This can't return a BindableSingleThreadTaskRunner since it breaks these
types of constructs:

  scoped_refptr<SingleThreadTaskRunner> foo = bar ? message_loop.task_runner() :
something_else.task_runner();

The problem seems to be that the types become ambiguous, but I'm not sure why it
doesn't just default to the common base class:

error: conditional expression is ambiguous; 'const
scoped_refptr<base::SingleThreadTaskRunner>' can be converted to
'scoped_refptr<base::BindableSingleThreadTaskRunner>' and vice versa

[alexclarke, 2015/06/24 10:27:04]

This is kind of an annoying feature of C++.  Automatic upcasts to a base class
are fine, but an automatic downcast seems a bit iffy.  Anyway the usual solution
to this is to add an explicit cast in to coerce the
BindableSingleThreadTaskRunner to a SingleThreadTaskRunner.

Looks like there is only one such cast needed, so I think it's OK.

   scoped_refptr<SingleThreadTaskRunner> task_runner() { return task_runner_; }
 
+  // Sets a new TaskRunner for this message loop and returns the previous one.
+  // If the message loop was already bound to a thread, this function must be
+  // called on that thread and the task runner must also belong to that thread.
+  // Note that changing the task runner will also affect the
+  // ThreadTaskRunnerHandle for the target thread.
+  scoped_refptr<BindableSingleThreadTaskRunner> SwapTaskRunner(
+      scoped_refptr<BindableSingleThreadTaskRunner> task_runner);
+
   // Enables or disables the recursive task processing. This happens in the case
   // of recursive message loops. Some unwanted message loop may occurs when
   // using common controls or printer functions. By default, recursive task
@@ -401,6 +409,7 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   friend class internal::IncomingTaskQueue;
   friend class ScheduleWorkTest;
   friend class Thread;
+  FRIEND_TEST_ALL_PREFIXES(MessageLoopTest, SwapTaskRunnerBeforeBinding);
 
   using MessagePumpFactoryCallback = Callback<scoped_ptr<MessagePump>()>;
 
@@ -415,7 +424,7 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // thread the message loop runs on, before calling Run().
   // Before BindToCurrentThread() is called only Post*Task() functions can
   // be called on the message loop.
-  scoped_ptr<MessageLoop> CreateUnbound(
+  static scoped_ptr<MessageLoop> CreateUnbound(
       Type type,
       MessagePumpFactoryCallback pump_factory);

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

(Aha, how I could miss this's unused)

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

Are you intending to use this, or would you like us to remove it?

[kinuko, 2015/06/25 14:31:12]

I was intending to use this at least when I added, since I thought it'd make the
intention clearer.  I'll create a follow-up CL to use this in base::Thread and
ask reviewer's opinion, so would like you to keep it as is in this CL.

 
@@ -522,7 +531,7 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   scoped_refptr<internal::IncomingTaskQueue> incoming_task_queue_;
 
   // The task runner associated with this message loop.
-  scoped_refptr<internal::MessageLoopTaskRunner> task_runner_;
+  scoped_refptr<BindableSingleThreadTaskRunner> task_runner_;
   scoped_ptr<ThreadTaskRunnerHandle> thread_task_runner_handle_;
 
   template <class T, class R> friend class base::subtle::DeleteHelperInternal;