Pass Callback to TaskRunner by value and consume it on invocation

This is a preparation CL for http://crrev.com/2637843002, which replaces
the Callback parameter of TaskRunner::PostTask with OnceCallback.
This one replaces the passed-by-const-ref Callback parameter of
TaskRunner::PostTask() with pass-by-value.

With the pass-by-const-ref manner as the old code does, we can't avoid
leaving a reference to the callback object on the original thread. That
is, the callback object may be destroyed either on the target thread or
the original thread. That's problematic when a non-thread-safe object is
bound to the callback.

Pass-by-value and move() in this CL mitigate the nondeterminism: if the
caller of TaskRunner::PostTask() passes the callback object as rvalue,
TaskRunner::PostTask() leaves no reference on the original thread.
I.e. the reference is not left if the callback is passed directly from
Bind(), or passed with std::move() as below.

  task_runner->PostTask(FROM_HERE, base::Bind(&Foo));

  base::Closure cb = base::Bind(&Foo);
  task_runner->PostTask(FROM_HERE, std::move(cb));

Otherwise, if the caller passes the callback as lvalue, a reference to
the callback is left on the original thread as we do in the previous code.
I.e. a reference is left if the callback is passed from other non-temporary
variable.

  base::Closure cb = base::Bind(&Foo);
  task_runner->PostTask(FROM_HERE, cb);

This is less controversial part of http://crrev.com/2637843002. This CL
is mainly to land it incrementally.

TBR=shrike@chromium.org
BUG=704027
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2726523002
Cr-Commit-Position: refs/heads/master@{#460288}


CrOS-Libchrome-Original-Commit: 070c8ffb44d04075bd7400691a2325cb6233558a

1 files changed, 4 insertions, 3 deletions

diff --git a/base/message_loop/incoming_task_queue.cc b/base/message_loop/incoming_task_queue.cc
index c7faa9a5b0..f0df650adc 100644
--- a/base/message_loop/incoming_task_queue.cc
+++ b/base/message_loop/incoming_task_queue.cc
@@ -5,6 +5,7 @@
 #include "base/message_loop/incoming_task_queue.h"
 
 #include <limits>
+#include <utility>
 
 #include "base/location.h"
 #include "base/message_loop/message_loop.h"
@@ -59,7 +60,7 @@ IncomingTaskQueue::IncomingTaskQueue(MessageLoop* message_loop)
 
 bool IncomingTaskQueue::AddToIncomingQueue(
     const tracked_objects::Location& from_here,
-    const Closure& task,
+    Closure task,
     TimeDelta delay,
     bool nestable) {
   DLOG_IF(WARNING,
@@ -67,8 +68,8 @@ bool IncomingTaskQueue::AddToIncomingQueue(
       << "Requesting super-long task delay period of " << delay.InSeconds()
       << " seconds from here: " << from_here.ToString();
 
-  PendingTask pending_task(from_here, task, CalculateDelayedRuntime(delay),
-                           nestable);
+  PendingTask pending_task(from_here, std::move(task),
+                           CalculateDelayedRuntime(delay), nestable);
 #if defined(OS_WIN)
   // We consider the task needs a high resolution timer if the delay is
   // more than 0 and less than 32ms. This caps the relative error to