Adding TryPostTask to the message loop

base/message_loop.h

 // Copyright (c) 2012 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_H_
 #define BASE_MESSAGE_LOOP_H_
 
 #include <queue>
 #include <string>
 
@@ skipping 26 matching lines @@
 #elif defined(USE_MESSAGEPUMP_LINUX) && !defined(OS_NACL)
 #include "base/message_pump_linux.h"
 #else
 #include "base/message_pump_gtk.h"
 #endif
 
 #endif
 #endif
 
 namespace base {
-
 class HistogramBase;
+class MessageLoopLockTest;
 class RunLoop;
 class ThreadTaskRunnerHandle;
 #if defined(OS_ANDROID)
 class MessagePumpForUI;
 #endif
 
 // 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
@@ skipping 102 matching lines @@
   // The NonNestable variants work similarly except that they promise never to
   // dispatch the task from a nested invocation of MessageLoop::Run.  Instead,
   // such tasks get deferred until the top-most MessageLoop::Run is executing.
   //
   // The MessageLoop takes ownership of the Task, and deletes it after it has
   // been Run().
   //
   // 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,
       base::TimeDelta delay);
 
   void PostNonNestableTask(
       const tracked_objects::Location& from_here,
       const base::Closure& task);
 
   void PostNonNestableDelayedTask(
@@ skipping 208 matching lines @@
 #elif defined(OS_POSIX) && !defined(OS_IOS)
   base::MessagePumpLibevent* pump_libevent() {
     return static_cast<base::MessagePumpLibevent*>(pump_.get());
   }
 #endif
 
   scoped_refptr<base::MessagePump> pump_;
 
  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
   // loop in a SEH try block or not depending on the set_SEH_restoration()
   // flag invoking respectively RunInternalInSEHFrame() or RunInternal().
   void RunHandler();
 
 #if defined(OS_WIN)
   __declspec(noinline) void RunInternalInSEHFrame();
 #endif
 
   // A surrounding stack frame around the running of the message loop that
   // supports all saving and restoring of state, as is needed for any/all (ugly)
   // recursive calls.
   void RunInternal();
 
   // Called to process any delayed non-nestable tasks.
   bool ProcessNextDelayedNonNestableTask();
 
   // Runs the specified PendingTask.
   void RunTask(const base::PendingTask& pending_task);
 
   // Calls RunTask or queues the pending_task on the deferred task list if it
   // cannot be run right now.  Returns true if the task was run.
   bool DeferOrRunPendingTask(const base::PendingTask& pending_task);
 
   // Adds the pending task to delayed_work_queue_.
   void AddToDelayedWorkQueue(const base::PendingTask& pending_task);
 
-  // Adds the pending task to our incoming_queue_.
+  // This function attempts to add pending task to our incoming_queue_.
+  // The append can only possibly fail when |use_try_lock| is true.
   //
-  // 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
+  // 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
   // accessible on this thread.
   void ReloadWorkQueue();
 
   // Delete tasks that haven't run yet without running them.  Used in the
   // destructor to make sure all the task's destructors get called.  Returns
   // true if some work was done.
   bool DeletePendingTasks();
@@ skipping 262 matching lines @@
 
 }  // namespace base
 
 // TODO(brettw) remove this when all users are updated to explicitly use the
 // namespace
 using base::MessageLoop;
 using base::MessageLoopForIO;
 using base::MessageLoopForUI;
 
 #endif  // BASE_MESSAGE_LOOP_H_