Worker: Introduce an observation mechanism for WorkerThread termination

third_party/WebKit/Source/core/workers/WorkerThreadLifecycleObserver.h

Index: third_party/WebKit/Source/core/workers/WorkerThreadLifecycleObserver.h
diff --git a/third_party/WebKit/Source/core/workers/WorkerThreadLifecycleObserver.h b/third_party/WebKit/Source/core/workers/WorkerThreadLifecycleObserver.h
new file mode 100644
index 0000000000000000000000000000000000000000..44cc813e4a1bcff4f20eb30ea1f2c3e5801b0593
--- /dev/null
+++ b/third_party/WebKit/Source/core/workers/WorkerThreadLifecycleObserver.h
@@ -0,0 +1,44 @@
+// Copyright 2016 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 WorkerThreadLifecycleObserver_h
+#define WorkerThreadLifecycleObserver_h
+
+#include "core/CoreExport.h"
+#include "platform/LifecycleObserver.h"
+
+namespace blink {
+
+class WorkerThreadContext;
+
+// An interface for observing worker thread termination from the main thread.
+// This may be useful, for example, when an object living on the main thread
+// needs to release references to objects on the worker thread before it gets
+// terminated.
+//
+// A class that inherits this interface should override
+// LifecycleObserver::contextDestroyed() that is called on the main thread when
+// the worker thread is about to terminate. While contextDestroyed() is called,
+// it is guaranteed that the worker thread is still alive.
+//
+// A newly created observer should firstly check whether the worker thread is
+// alive by wasContextDestroyedBeforeObserverCreation(). If this return true,
+// the worker thread has already been terminated before the observer is created,
+// and contextDestroyed() is never notified.

[haraken, 2016/06/10 00:45:36]

This looks like a strange semantics. Other lifecycle observers don't have
wasContextDestroyedBeforeObserverCreation().

If the context is already gone, LifecycleObserver should not be instantiated in
the first place. In the particular case of the worker context,
WorkerThread::workerThreadContext() should return nullptr after the context is
destroyed. Then WorkerWebSocketChannel should not create a LifecycleObserver if
WorkerThread::workerThreadContext() returns nullptr.

+class CORE_EXPORT WorkerThreadLifecycleObserver : public LifecycleObserver<WorkerThreadContext, WorkerThreadLifecycleObserver> {
+protected:
+    explicit WorkerThreadLifecycleObserver(WorkerThreadContext*);
+
+    bool wasContextDestroyedBeforeObserverCreation() const
+    {
+        return m_wasContextDestroyedBeforeObserverCreation;
+    }
+
+private:
+    const bool m_wasContextDestroyedBeforeObserverCreation;
+};
+
+} // namespace blink
+
+#endif // WorkerThreadLifecycleObserver_h