[PyQt] [BUG] Use of QSGThreadedRenderLoop in QML can lead to deadlock (PyQt 5.4.2)

[Phil Thompson]

On 18/06/2015 8:06 pm, Vladimir Rutsky wrote:
> Hello,
> 
> I described this issue in comment to other bug report
> (http://www.riverbankcomputing.com/pipermail/pyqt/2015-June/036048.html),
> and now want to open separate thread for it, since it's not just
> "application hangs on exit", but real deadlock issue that can occur in
> many cases of QML usage (e.g. when window is shown or hidden).
> 
> The deadlock is reproduced when QML rendering is done using threaded
> render loop (QSGThreadedRenderLoop), which is default on modern
> GNU/Linux systems (Windows always use single threaded rendering in Qt
> 5.4).
> 
> As I understand from GDB stack traces, the deadlock happens in the
> following way.
> Sometimes QQuickView starting to wait rendering thread for some
> operation (using mutex). Rendering thread can request operation that
> is implemented in Python code, this operation will try to lock Python
> GIL in rendering thread, but GIL is not released in main thread, and
> never be, since main thread waits rendering thread in Qt.
> 
> One of the situations in which QQuickView waits rendering thread is
> when QQuickView is being destroyed: it waits while rendering thread
> will release all resources.
> This case can be easily reproduced on GNU/Linux in this example:
> https://gist.github.com/rutsky/d6332e3354972997e938
> The example also have GDB stack traces at the moment when application
> hanged on exit.
> 
> Workaround for this issue is to force Qt to use single threaded
> rendering by setting environment variable QSG_RENDER_LOOP to "basic".

I've added /ReleaseGIL/ to all the QQuickWindow and QQuickView methods 
that seem to depend on the internal mutex.

Phil