alsa-mixer: avoid assertion at alsa-lib mixer API when element removal
Cc: @perexg
PulseAudio v5.99 or later hits assertion at alsa-lib mixer API due to wrong handling of removal event for mixer element.
pulseaudio: mixer.c:149: hctl_elem_event_handler: Assertion `bag_empty(bag)' failed.
The removal event is defined as '~0U', thus it's not distinguished from the other type of event just by bitwise operator.
At the removal event, class implementator for mixer API should detach mixer element from hcontrol element in callback handler since alsa-lib has assertion to check the list of mixer elements for a hcontrol element is empty or not after calling all of handlers. In detail, please refer to MR to alsa-lib:
This commit fixes the above two issues. The issue can be regenerated by
samples/ctl Python 3 script of alsa-gobject.
It adds some user-defined elements into sound card 0. When terminated by SIGINT signal, it removes the elements. Then PulseAudio dies due to the assertion.
Fixes: 1fd8848e ("alsa-util: Add functions for accessing mixer elements through mixer class")