[alsa-devel] [alsa-lib][PATCH] hctl: remove assertion from element event handler because removal event overwrites all of queued events

[Takashi Iwai]

On Sun, 10 Jul 2016 17:12:34 +0200,
Takashi Sakamoto wrote:
> 
> In ALSA control core, when several events occurs for an element, they're
> represented bit mask (struct snd_ctl_event.data.elem.mask). Thus userspace
> applications can handles these events separately in one I/O operation.
> 
> There's an exception; removal event. This is represented by all of bits in
> the mask. Therefore, when a removal event occurs for an element, all of
> queued events for the element are overwritten.
> 
> This works worse for a combination of applications which add/remove control
> elements and applications which keep a cache of elements. For example,
> let's assume a case that:
> 
> 1.some elements are added by ioctl(SNDRV_CTL_IOCTL_ELEM_ADD).
> 2.ALSA control core handles the request and queues 'add' event.
> 3.applications don't read the event yet.
> 4.the elements are removed by ioctl(SNDRV_CTL_IOCTL_ELEM_REMOVE)
> 5.ALSA control core handles the request and queues 'removal' event by
>   overwriting queued events.
> 6.applications read the event at last.
> 7.the applications cannot find removed elements in their cache data.
> 
> Especially, when applications use hctl interface of ALSA userspace library,
> this situation occurs because the interface is designed to keep element
> cache implicitly inner the library. PulseAudio is such an application. When
> executing a test program of ALSA library (test/user-ctl-element-set.c),
> pulseaudio process aborts due to assertion at calls of
> snd_hctl_handle_events().
> 
> The rule to represent events is in ALSA kernel/userspace interfaces
> (UAPI header) and it's difficult to improve the rule. Therefore,
> developers should pay enough attention to the fact that userspace
> applications can receive removal events for elements which are not
> notified with 'add' event.
> 
> For this reason, this commit removes assertion from event handler of ALSA
> hctl API.
> 
> CC: pulseaudio-discuss at lists.freedesktop.org
> Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>

Applied, thanks.


Takashi