Refactoring WorkletThreadBackingHolder with template pattern

third_party/WebKit/Source/modules/webaudio/AudioWorkletThread.cpp

Index: third_party/WebKit/Source/modules/webaudio/AudioWorkletThread.cpp
diff --git a/third_party/WebKit/Source/modules/webaudio/AudioWorkletThread.cpp b/third_party/WebKit/Source/modules/webaudio/AudioWorkletThread.cpp
index f59c7373237c228e98fdabb450050777484a7da2..0703cb91ea9a443e24542321a09c0361a5bf9b68 100644
--- a/third_party/WebKit/Source/modules/webaudio/AudioWorkletThread.cpp
+++ b/third_party/WebKit/Source/modules/webaudio/AudioWorkletThread.cpp
@@ -22,63 +22,28 @@ namespace blink {
 
 namespace {
 
-// TODO(hongchan): consider refactoring static methods in this class into
-// a template class.
-class AudioWorkletThreadHolder final : public WorkletBackingThreadHolder {
- public:
-  static AudioWorkletThreadHolder* instance() {
-    MutexLocker locker(holderInstanceMutex());
-    return s_instance;
-  }
-
-  static void ensureInstance() {
-    if (!s_instance)
-      s_instance = new AudioWorkletThreadHolder;
-  }
-
-  static void clear() {
-    MutexLocker locker(holderInstanceMutex());
-    if (s_instance) {
-      s_instance->shutdownAndWait();
-      delete s_instance;
-      s_instance = nullptr;
-    }
-  }
+// AudioWorkletThreadHolder is a singletone container/mananger of a dedicated
+// thread for processing the AudioWorklet.
+class AudioWorkletThreadHolder final
+    : public WorkletThreadHolder<AudioWorkletThreadHolder> {

[nhiroki, 2016/10/19 01:36:49]

I'd prefer to avoid class inheritance as much as possible because WorkerThread
classes already have deep inheritance hierarchy. Instead of inheritance, how
about like this?

1) Make WorkletThreadHolder receive client's WorkerThread name
(AudioWorkletThread) as a template parameter instead of a derived class name
(AudioWorkletThreadHolder).

  WorkletThreadHolder<AudioWorkletThreadHolder>
    -> WorkletThreadHolder<AudioWorkletThread>

2) Define createWorkletThreadHolder() on AudioWorkletThread like this:

  static WorkletThreadHolder<AudioWorkletThread>*
AudioWorkletThread::createWorkletThreadHolder(std::unique_ptr<WorkerBackingThread>
backingThread = nullptr) { ... }

3) Make
WorkletThreadHolder<AudioWorkletThread>::ensureInstance()/createForTest() call
AudioWorkletThread::createWorkletThreadHolder() instead of the derived class's
ctor.

WDYT?

+  friend class WorkletThreadHolder<AudioWorkletThreadHolder>;
 
+ public:
   static void createForTest() {
-    MutexLocker locker(holderInstanceMutex());
-    DCHECK_EQ(nullptr, s_instance);
-    s_instance =
-        new AudioWorkletThreadHolder(WorkerBackingThread::createForTest(
-            "AudioWorkletThread", BlinkGC::PerThreadHeapMode));
+    WorkletThreadHolder<AudioWorkletThreadHolder>::createForTest(
+        WorkerBackingThread::createForTest("AudioWorkletThread",
+                                           BlinkGC::PerThreadHeapMode));
   }
 
  private:
   AudioWorkletThreadHolder(
       std::unique_ptr<WorkerBackingThread> backingThread = nullptr)
-      : WorkletBackingThreadHolder(
-            backingThread
-                ? std::move(backingThread)
-                : WorkerBackingThread::create("AudioWorkletThread",
-                                              BlinkGC::PerThreadHeapMode)) {}
-
-  static Mutex& holderInstanceMutex() {
-    DEFINE_THREAD_SAFE_STATIC_LOCAL(Mutex, holderMutex, new Mutex);
-    return holderMutex;
-  }
-
-  void initializeOnThread() {
-    MutexLocker locker(holderInstanceMutex());
-    DCHECK(!m_initialized);
-    m_thread->initialize();
-    m_initialized = true;
-  }
-
-  static AudioWorkletThreadHolder* s_instance;
+      : WorkletThreadHolder(backingThread ? std::move(backingThread)
+                                          : WorkerBackingThread::create(
+                                                "AudioWorkletThread",
+                                                BlinkGC::PerThreadHeapMode)) {}
 };
 
-AudioWorkletThreadHolder* AudioWorkletThreadHolder::s_instance = nullptr;
-
 }  // namespace
 
 std::unique_ptr<AudioWorkletThread> AudioWorkletThread::create(