Implement WTF::WeakPtr in terms of base::WeakPtr

third_party/WebKit/Source/core/dom/MainThreadTaskRunner.cpp

 /*
  * Copyright (C) 2013 Google Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
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@@ skipping 22 matching lines @@
 #include "public/platform/Platform.h"
 #include "wtf/Assertions.h"
 
 namespace blink {
 
 MainThreadTaskRunner::MainThreadTaskRunner(ExecutionContext* context)
     : m_context(context)
     , m_pendingTasksTimer(this, &MainThreadTaskRunner::pendingTasksTimerFired)
     , m_suspended(false)
     , m_weakFactory(this)
+    // Bind a WeakPtr now to avoid data races creating a WeakPtr inside postTask.
+    , m_weakPtr(m_weakFactory.createWeakPtr())
 {
 }
 
 MainThreadTaskRunner::~MainThreadTaskRunner()
 {
 }
 
 void MainThreadTaskRunner::postTaskInternal(const WebTraceLocation& location, std::unique_ptr<ExecutionContextTask> task, bool isInspectorTask, bool instrumenting)
 {
     Platform::current()->mainThread()->getWebTaskRunner()->postTask(location, crossThreadBind(
         &MainThreadTaskRunner::perform,
-        m_weakFactory.createWeakPtr(),
+        m_weakPtr,

[esprehn, 2016/09/13 19:44:21]

This means we're sharing a weakptr now instead of making a new one per task?

[dcheng, 2016/09/13 19:51:40]

It's copied, not shared, when it's bound into the task. From an overhead
perspective, we take a small hit, since we have to keep around a WeakPtr and a
WeakPtrFactory.

Once this lands though, I have a patch to reduce the overhead of WeakPtr a
little bit, and I will also explore the feasibility of making it safe to call
GetWeakPtr() on multiple threads if there are no explicit InvalidateWeakPtr()
calls.

         passed(std::move(task)),
         isInspectorTask,
         instrumenting));
 }
 
 void MainThreadTaskRunner::postTask(const WebTraceLocation& location, std::unique_ptr<ExecutionContextTask> task, const String& taskNameForInstrumentation)
 {
     if (!taskNameForInstrumentation.isEmpty())
         InspectorInstrumentation::asyncTaskScheduled(m_context, taskNameForInstrumentation, task.get());
     const bool instrumenting = !taskNameForInstrumentation.isEmpty();
@@ skipping 47 matching lines @@
     while (!m_pendingTasks.isEmpty()) {
         std::unique_ptr<ExecutionContextTask> task = std::move(m_pendingTasks[0].first);
         const bool instrumenting = m_pendingTasks[0].second;
         m_pendingTasks.remove(0);
         InspectorInstrumentation::AsyncTask asyncTask(m_context, task.get(), instrumenting);
         task->performTask(m_context);
     }
 }
 
 } // namespace blink