Oilpan: Move ThreadableWebSocketChannelClientWrapper to Oilpan's heap

Source/core/dom/CrossThreadTask.h

 /*
  * Copyright (C) 2009-2010 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 39 matching lines @@
 };
 
 template<typename T> struct CrossThreadTaskTraits<PassRefPtr<T> > {
     typedef PassRefPtr<T> ParamType;
 };
 
 template<typename T> struct CrossThreadTaskTraits<PassOwnPtr<T> > {
     typedef PassOwnPtr<T> ParamType;
 };
 
+template<typename T> struct CrossThreadTaskTraits<CrossThreadPersistent<T> > {
+    typedef CrossThreadPersistent<T> ParamType;

[haraken, 2014/05/27 09:21:37]

This was the cause of the empty ASSERT error.

This template is expanded as follows:

class ExecutionContextTask {
  CrossThreadPersistent<ThreadableWebSocketChannelClientWrapper> param;
};

// This object is created in a worker thread.
class ThreadableWebSocketChannelClientWrapper : public
GarbageCollectedFinalized<ThreadableWebSocketChannelClientWrapper>{
  Vector<OwnPtr<ExecutionContextTask>> m_pendingTasks;
}

The problem happens in a case where the worker thread shuts down before flushing
m_pendingTasks. In that case, the worker thread calls collectAllGarbage() but it
cannot collect the ThreadableWebSocketChannelClientWrapper, because the
ThreadableWebSocketChannelClientWrapper and the CrossThreadPersistent are
referenced in a cycle.

I tried to explicitly clear m_pendingTasks before shutting down the worker
thread, but I found it's tricky. Also we won't want to add similar hacks to
other caller sites of ExecutionContextTask.

A better approach would be to move ExecutionContextTask to the heap and use a
Member instead of the CrossThreadPersistent. However, this requires a lot of
work.

As a short-term fix, I changed the CrossThreadPersistent to a RawPtr. This is
not safe in general but is OK in this particular case, because the tasks are
guaranteed to be executed in the same worker thread and thus
ThreadableWebSocketChannelClientWrapper is guaranteed to be alive while the
tasks are in the pending queue of the ThreadableWebSocketChannelClientWrapper.

What do you think?

+};
+
 template<typename P1, typename MP1>
 class CrossThreadTask1 : public ExecutionContextTask {
 public:
     typedef void (*Method)(ExecutionContext*, MP1);
     typedef CrossThreadTask1<P1, MP1> CrossThreadTask;
     typedef typename CrossThreadTaskTraits<P1>::ParamType Param1;
 
     static PassOwnPtr<CrossThreadTask> create(Method method, Param1 parameter1)
     {
         return adoptPtr(new CrossThreadTask(method, parameter1));
@@ skipping 398 matching lines @@
                                                        method,
                                                        CrossThreadCopier<P1>::copy(parameter1), CrossThreadCopier<P2>::copy(parameter2),
                                                        CrossThreadCopier<P3>::copy(parameter3), CrossThreadCopier<P4>::copy(parameter4),
                                                        CrossThreadCopier<P5>::copy(parameter5), CrossThreadCopier<P6>::copy(parameter6),
                                                        CrossThreadCopier<P7>::copy(parameter7), CrossThreadCopier<P8>::copy(parameter8));
 }
 
 } // namespace WebCore
 
 #endif // CrossThreadTask_h