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

[Wojtek Porczyk]

On Thu, Jan 19, 2017 at 11:56:36AM +0000, Daniel P. Berrange wrote:
> On Thu, Jan 19, 2017 at 12:48:11PM +0100, Wojtek Porczyk wrote:
> > Hello libvirt-list,
(snip)
> > > } 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
> > 
(snip)
> 
> Yes, your analysis looks correct to me. I don't think it is easy to fix
> this in the libvirt-override.c file really, unless we were to spawn a throw
> away thread to call "ff" in. The better approach would be for the user suppied
> event loop impl of removeHandle to arrange to call 'ff' themselves at the
> correct time.

IIUC, this cannot be done without involving user-supplied event loop at all.
Unless I miss something (I'm not a C programmer really), that's the
implication of "This callback needs to be invoked from a clean stack", since
libvirt-override.c has access to "clean stack" only via whatever is
implemented in python. 

That unfortunately means the python API (specifically the expected addHandle
signature) should be amended to include explicit ff callback to be called
directly from the loop after removeHandleFunc returns. That's not as bad as
it sounds, because the argument can be optional in Python, so new Python code
could be compatible with both old and new libvirt-python, and
libvirt-override.c could catch TypeError exception to retry the function with
different signature for existing code. That would still work not worse than
present, since it will still leak memory. In any case, the ff callback should
be dropped from libvirt_virEventRemoveHandleFunc(), because again, I suppose
all currently running implementations leak like a sieve.

I have no idea how to write that in C.


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