Re: attach/detach by mixer class implementation of alsa-lib mixer API

[Takashi Sakamoto <o-takashi@sakamocchi.jp>]

Hi,

On Sun, Jun 26, 2022 at 07:23:23PM +0200, Jaroslav Kysela wrote:
> On 25. 06. 22 13:28, Takashi Sakamoto wrote:
> > Hi Jaroslav,
> > 
> > Recent years I'm bothered about unexpected abort of pulseaudio and
> > pipewire processes when testing user-defined control element set.
> > They aborts at element removal event.
> > 
> > ```
> > pulseaudio: mixer.c:149: hctl_elem_event_handler: Assertion `bag_empty(bag)' failed.
> > wireplumber: mixer.c:149: hctl_elem_event_handler: Assertion `bag_empty(bag)' failed.
> > ```
> > 
> > Would I ask your opinion about the design of alsa-lib mixer API?
> > 
> > As long as I investigate, these programs seem to have careless coding as
> > alsa-lib mixer API application. Both of them attaches an instance of
> > snd_mixer_elem_t to an instance of snd_hctl_elem_t by calling
> > snd_mixer_elem_attach() when detecting element addition, but never detach
> > it even if detecting element removal.
> > 
> > In the case, the link list (=bag) of mixer API internal never becomes empty.
> > It has link entries as much as the number of registered mixer classes which
> > attaches snd_mixer_elem_t. Then it hits the assertion.
> > 
> > I think the design of alsa-lib mixer API demands mixer class implementation
> > to detach at element removal which attached at element addition. But I have
> > less conviction about it since enough unfamiliar.
> 
> Yes, if the REMOVE event is delivered to the mixer class, the reference to
> the associated hctl element should be removed. The assert does the check for
> this consistency.
> 
> > I'm glad if receiving your opinion about it.
> > 
> > I wrote test program for the issued behaviour:
> >   - https://gist.github.com/takaswie/8378fe3bc04652d83428694cd7573bc0
> > 
> > For test, please use sample script in alsa-gobject project:
> >   - https://github.com/alsa-project/alsa-gobject/blob/master/samples/ctl
> > 
> > The patches for pulseaudio/pipewire are prepared:
> >   - https://gitlab.freedesktop.org/takaswie/pulseaudio/-/commit/topic/fix-wrong-handling-alsa-ctl-event
> >   - https://gitlab.freedesktop.org/takaswie/pipewire/-/commits/topic/fix-wrong-handling-alsa-ctl-event
> 
> Your fixes seem correct. Please, create the PA/PW merge request with this
> code. Please add me (@perexg) to your merge message.
> 
> Thank you for your work on this.
 
Thanks for your confirmation and reviewing.

In my opinion, it's better to have enough explanation about the
postcondition in alsa-lib documentation so that developer for
implementation of mixer class takes care of the postcondition. I filed a
request to alsa-lib repository.

 * https://github.com/alsa-project/alsa-lib/pull/244

Then I filed to each project with reference to your account:

 * https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/728
 * https://gitlab.freedesktop.org/pipewire/pipewire/-/merge_requests/1296

> 						Jaroslav
> 
> 
> -- 
> Jaroslav Kysela <perex@perex.cz>
> Linux Sound Maintainer; ALSA Project; Red Hat, Inc.


Thanks

Takashi Sakamoto