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

base/message_loop/message_loop.h

 // Copyright 2013 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.
 
 #ifndef BASE_MESSAGE_LOOP_MESSAGE_LOOP_H_
 #define BASE_MESSAGE_LOOP_MESSAGE_LOOP_H_
 
 #include <queue>
 #include <string>
 
 #include "base/base_export.h"
 #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"
 #include "base/message_loop/incoming_task_queue.h"
 #include "base/message_loop/message_loop_task_runner.h"
 #include "base/message_loop/message_pump.h"
 #include "base/message_loop/timer_slack.h"
 #include "base/observer_list.h"
 #include "base/pending_task.h"
 #include "base/sequenced_task_runner_helpers.h"
 #include "base/synchronization/lock.h"
 #include "base/time/time.h"
 #include "base/tracking_info.h"
 
 // TODO(sky): these includes should not be necessary. Nuke them.
 #if defined(OS_WIN)
 #include "base/message_loop/message_pump_win.h"
 #elif defined(OS_IOS)
 #include "base/message_loop/message_pump_io_ios.h"
 #elif defined(OS_POSIX)
 #include "base/message_loop/message_pump_libevent.h"
 #endif
 
 namespace base {
 
+class BindableSingleThreadTaskRunner;
 class HistogramBase;
 class RunLoop;
 class ThreadTaskRunnerHandle;
 class WaitableEvent;
 
 // A MessageLoop is used to process events for a particular thread.  There is
 // at most one MessageLoop instance per thread.
 //
 // Events include at a minimum Task instances submitted to PostTask and its
 // variants.  Depending on the type of message pump used by the MessageLoop
@@ skipping 239 matching lines @@
   Type type() const { return type_; }
 
   // Optional call to connect the thread name with this loop.
   void set_thread_name(const std::string& thread_name) {
     DCHECK(thread_name_.empty()) << "Should not rename this thread!";
     thread_name_ = thread_name;
   }
   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
   // processing is disabled.
   //
   // Please utilize |ScopedNestableTaskAllower| instead of calling these methods
   // directly.  In general nestable message loops are to be avoided.  They are
   // dangerous and difficult to get right, so please use with extreme caution.
   //
   // The specific case where tasks get queued is:
@@ skipping 81 matching lines @@
 
   //----------------------------------------------------------------------------
  protected:
   scoped_ptr<MessagePump> pump_;
 
  private:
   friend class RunLoop;
   friend class internal::IncomingTaskQueue;
   friend class ScheduleWorkTest;
   friend class Thread;
+  FRIEND_TEST_ALL_PREFIXES(MessageLoopTest, SwapTaskRunnerBeforeBinding);
 
   using MessagePumpFactoryCallback = Callback<scoped_ptr<MessagePump>()>;
 
   // Creates a MessageLoop without binding to a thread.
   // If |type| is TYPE_CUSTOM non-null |pump_factory| must be also given
   // to create a message pump for this message loop.  Otherwise a default
   // message pump for the |type| is created.
   //
   // It is valid to call this to create a new message loop on one thread,
   // and then pass it to the thread where the message loop actually runs.
   // The message loop's BindToCurrentThread() method must be called on the
   // 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.

 
   // Common private constructor. Other constructors delegate the initialization
   // to this constructor.
   MessageLoop(Type type, MessagePumpFactoryCallback pump_factory);
 
   // Configure various members and bind this message loop to the current thread.
   void BindToCurrentThread();
 
   // Invokes the actual run loop using the message pump.
   void RunHandler();
@@ skipping 84 matching lines @@
 
   RunLoop* run_loop_;
 
   ObserverList<TaskObserver> task_observers_;
 
   debug::TaskAnnotator task_annotator_;
 
   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;
   template <class T, class R> friend class base::subtle::ReleaseHelperInternal;
 
   void DeleteSoonInternal(const tracked_objects::Location& from_here,
                           void(*deleter)(const void*),
                           const void* object);
   void ReleaseSoonInternal(const tracked_objects::Location& from_here,
                            void(*releaser)(const void*),
@@ skipping 137 matching lines @@
 
 // Do not add any member variables to MessageLoopForIO!  This is important b/c
 // MessageLoopForIO is often allocated via MessageLoop(TYPE_IO).  Any extra
 // data that you need should be stored on the MessageLoop's pump_ instance.
 COMPILE_ASSERT(sizeof(MessageLoop) == sizeof(MessageLoopForIO),
                MessageLoopForIO_should_not_have_extra_member_variables);
 
 }  // namespace base
 
 #endif  // BASE_MESSAGE_LOOP_MESSAGE_LOOP_H_