[Bug 277615] /usr/sbin/mixer -a does not display all mixer devices if /dev/mixer0 happens to not exist.

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=277615

--- Comment #2 from commit-hook@FreeBSD.org ---
A commit in branch main references this bug:

URL:
https://cgit.FreeBSD.org/src/commit/?id=5d980fadf73df64a1e0eda40a93170ed76ce6f14

commit 5d980fadf73df64a1e0eda40a93170ed76ce6f14
Author:     Christos Margiolis <christos@FreeBSD.org>
AuthorDate: 2024-05-23 00:57:17 +0000
Commit:     Christos Margiolis <christos@FreeBSD.org>
CommitDate: 2024-05-23 00:57:17 +0000

    sound: Handle unavailable devices in various OSS IOCTLs

    mixer(8)'s -a option is used to print information about all mixer
    devices in the system. To do this, it loops from 0 to
    mixer_get_nmixers(), and tries to open "/dev/mixer%d". However, this
    approach doesn't work when there are disabled/unregistered mixers in the
    system, or when an audio device simply doesn't have a mixer.

    mixer_get_nmixers() calls SNDCTL_SYSINFO and returns
    oss_sysinfo->nummixers, whose value is the number of currently _enabled_
    mixers only. Taking the bug report mentioned below (277615) as an
    example, suppose a system with 8 mixer devices total, but 3 of them are
    either disabled or non-existent, which means they will not show up under
    /dev, meaning we have 5 enabled mixer devices, which is also what the
    value of oss_sysinfo->nummixers will be. What mixer(8) will do is loop
    from 0 to 5 (instead of 8), and start calling mixer_open() on
    /dev/mixer0, up to /dev/mixer4, and as is expected, the first call will
    fail right away, hence the error shown in the bug report.

    To fix this, modify oss_sysinfo->nummixers to hold the value of the
    maximum unit in the system, which, although not necessarily "correct",
    is more intuitive for applications that will want to use this value to
    loop through all mixer devices.

    Additionally, notify applications that a device is
    unavailable/unregistered instead of skipping it. The current
    implementations of SNDCTL_AUDIOINFO, SNDCTL_MIXERINFO and
    SNDCTL_CARDINFO break applications that expect to get information about
    a device that is skipped. Related discussion can be found here:
    https://reviews.freebsd.org/D45135#1029526

    It has to be noted, that other applications, apart from mixer(8), suffer
    from this.

    PR:             277615
    Sponsored by:   The FreeBSD Foundation
    MFC after:      1 day
    Reviewed by:    dev_submerge.ch
    Differential Revision:  https://reviews.freebsd.org/D45256

 lib/libmixer/mixer.3      |   7 +-
 sys/dev/sound/pcm/dsp.c   |  23 +++++-
 sys/dev/sound/pcm/mixer.c | 201 ++++++++++++++++++++++++----------------------
 sys/dev/sound/pcm/mixer.h |   2 -
 sys/dev/sound/pcm/sound.c |  41 ++++++----
 5 files changed, 155 insertions(+), 119 deletions(-)

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