[libvirt] libvirt-python bug: custom event loop impl calls ff callback from *Remove(Handle|Timeout)Func

[Wojtek Porczyk]

Hello libvirt-list,

As of current libvirt-python.git, according to libvirt-override.c, if
implementing custom event loop in Python, ff callback is called from
libvirt_virEventRemoveHandleFunc, which is a C glue between
virEventRegisterImpl and actual removeHandle function written in Python:

> result = PyEval_CallObject(removeHandleObj, pyobj_args);
> if (!result) {
>     PyErr_Print();
>     PyErr_Clear();
> } else if (!PyTuple_Check(result) || PyTuple_Size(result) != 3) {
>     DEBUG("%s: %s must return opaque obj registered with %s"
>           "to avoid leaking libvirt memory\n",
>           __FUNCTION__, NAME(removeHandle), NAME(addHandle));
> } else {
>     opaque = PyTuple_GetItem(result, 1);
>     ff = PyTuple_GetItem(result, 2);
>     cff = PyvirFreeCallback_Get(ff);
>     if (cff)
>         (*cff)(PyvirVoidPtr_Get(opaque));
>     retval = 0;
> }

This is exactly what one shoud not be doing according to documentation [1]:

> If the opaque user data requires free'ing when the handle is unregistered,
> then a 2nd callback can be supplied for this purpose. This callback needs to
> be invoked from a clean stack. If 'ff' callbacks are invoked directly from the
> virEventRemoveHandleFunc they will likely deadlock in libvirt.

[1] https://libvirt.org/html/libvirt-libvirt-event.html#virEventAddHandleFunc

This is true, the deadlock occurs. When the "result" tuple is mangled to have
None as third item ("ff"), then cff = PyvirFreeCallback_Get(ff) is NULL and
the deadlock does not happen.

That's also why script examples/event-test.py does not deadlock, because it
does not return anything (that is, returns None) from Python, so the second if
block happens and the ff callback, if any, is not executed (and probably
something leaks, but I didn't check for that). 


Everything also applies to to timeouts (libvirt_virEventRemoteTimeoutFunc).


-- 
pozdrawiam / best regards       _.-._
Wojtek Porczyk               .-^'   '^-.
Invisible Things Lab         |'-.-^-.-'|
                             |  |   |  |
 I do not fear computers,    |  '-.-'  |
 I fear lack of them.        '-._ :  ,-'
    -- Isaac Asimov             `^-^-_>
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