[webkit-dev] WTF::currentThread() and non-WTF threads

[Dmitry Titov]

Hi webkit-dev!

Trying to add ASSERTS to RefCounted objects (
https://bugs.webkit.org/show_bug.cgi?id=31639) I've added some more calls to
WTF::currentThread() and started to get a lot of assertions here:

static ThreadIdentifier establishIdentifierForPthreadHandle(pthread_t&
pthreadHandle)
{
    ASSERT(!identifierByPthreadHandle(pthreadHandle));

Here is what happens: A thread that is not created by WTF::CreateThread is
calling currentThread() in hope to get a ThreadIdentifier. It doesn't have
it so we create it and put into ThreadMap. Normally, WTF threads call
WTF::detachThread() that removes the ThreadIdentifier from the map. The
'other' threads do not do so - so the TheradIdentifier remains in the map.

On OSX, pthreads reuse system handles when one thread terminates and another
starts... That easily leads to threads that run sequentially to have the
same handle, and the same ThreadIdentifier - since it is still in the
threadMap, which makes currentThread() kind of useless since it returns the
same id for different threads, the very thing the threadMap is tryign to
avoid.

I'm thinking about changing the implementation to stop "auto-register"
non-WTF threads. Instead, lets add a new function, WTF::registerThread()
that would establish the ThreadIdentifier for a thread not created by
WTF::createThread - and assert in currentThread() if the current thread is
unknown to WTF. This way, we could find all the cases of such threads and
equip them with registerThread()/detachThread() pair that will keep the
thread map in a good state.

The currentThread() would look like this:

ThreadIdentifier currentThread()
{
    pthread_t currentThread = pthread_self();
    if (ThreadIdentifier id = identifierByPthreadHandle(currentThread))
        return id;

    ASSERT_NOT_REACHED();

    // Either the thread is not created by WTF::CreateThread() or registered
by WTF::registerThread(), or we are getting
    // a call from thread-specific destructor after WTF::detachThread() was
called and ThreadIdentifier removed.
    // Neither scenario permits reliable thread id tracking, so we can not
return a meaningful ThreadIdentifier here.
    // Normally ThreadIdentifiers are compared so lets generate a fake
one-time ThreadIdentifier to fail comparison, if
    // it is in fact done.
    static ThreadIdentifier fakeId = minFakeIdentifierCount;

    if (fakeId == maxFakeIdentifierCount)
        fakeId = minFakeIdentifierCount;

    return fakeId++;
}

What would you say? Any bad feelings about that?

Dmitry
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