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@xxxxxxxx> > Linux Sound Maintainer; ALSA Project; Red Hat, Inc. Thanks Takashi Sakamoto
