Adding TryPostTask to the message loop

base/message_loop.h

Index: base/message_loop.h
diff --git a/base/message_loop.h b/base/message_loop.h
index dbe75a57f9f410ae1a672745c1daffdb8ee4ce37..eb0885ed5b0bbf0413ed3fa370510db4ce828060 100644
--- a/base/message_loop.h
+++ b/base/message_loop.h
@@ -44,8 +44,8 @@
 #endif
 
 namespace base {
-
 class HistogramBase;
+class MessageLoopLockTest;
 class RunLoop;
 class ThreadTaskRunnerHandle;
 #if defined(OS_ANDROID)
@@ -168,12 +168,20 @@ class BASE_EXPORT MessageLoop : public base::MessagePump::Delegate {
   // PostTask(from_here, task) is equivalent to
   // PostDelayedTask(from_here, task, 0).
   //
+  // The TryPostTask is meant for the cases where the calling thread cannot
+  // block. If posting the task will block, the call returns false, the task
+  // is not posted but the task is consumed anyways.
+  //
   // NOTE: These methods may be called on any thread.  The Task will be invoked
   // on the thread that executes MessageLoop::Run().
   void PostTask(
       const tracked_objects::Location& from_here,
       const base::Closure& task);
 
+  bool TryPostTask(
+      const tracked_objects::Location& from_here,
+      const base::Closure& task);
+
   void PostDelayedTask(
       const tracked_objects::Location& from_here,
       const base::Closure& task,
@@ -402,6 +410,7 @@ class BASE_EXPORT MessageLoop : public base::MessagePump::Delegate {
 
  private:
   friend class base::RunLoop;
+  friend class base::MessageLoopLockTest;
 
   // A function to encapsulate all the exception handling capability in the
   // stacks around the running of a main message loop.  It will run the message
@@ -431,13 +440,21 @@ class BASE_EXPORT MessageLoop : public base::MessagePump::Delegate {
   // Adds the pending task to delayed_work_queue_.
   void AddToDelayedWorkQueue(const base::PendingTask& pending_task);
 
-  // Adds the pending task to our incoming_queue_.
-  //
-  // Caller retains ownership of |pending_task|, but this function will
-  // reset the value of pending_task->task.  This is needed to ensure
-  // that the posting call stack does not retain pending_task->task
+  // This function attempts to add pending task to our incoming_queue_.
+  // The append can only possibly fail when |use_try_lock| is true.
+  //
+  // When |use_try_lock| is true, then this call will avoid blocking if
+  // the related lock is already held, and will in that case (when the
+  // lock is contended) fail to perform the append, and will return false.
+  //
+  // If the call succeeds to append to the queue, then this call
+  // will return true.
+  //
+  // In all cases, the caller retains ownership of |pending_task|, but this
+  // function will reset the value of pending_task->task.  This is needed to
+  // ensure that the posting call stack does not retain pending_task->task
   // beyond this function call.
-  void AddToIncomingQueue(base::PendingTask* pending_task);
+  bool AddToIncomingQueue(base::PendingTask* pending_task, bool use_try_lock);
 
   // Load tasks from the incoming_queue_ into work_queue_ if the latter is
   // empty.  The former requires a lock to access, while the latter is directly