Make this idle event per-partition.

Eliminate global variables, convert them to MixerMemento fields.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1583

If a mixer is not open, rethrow the original exception each time
setting the volume is requested.  This further improves error messages
sent to MPD clients.

If a mixer is "global", it is available even if the output isn't
open.  However, since the check was changed from IsEnabled() to
IsReallyEnabled(), enabled outputs have not yet been used have not
been "really" enabled yet, preventing using the mixer.

Fixes a regression by commit 35dbc1a9
(part of https://github.com/MusicPlayerDaemon/MPD/pull/1480).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1563

This reveals more about the nature of an error instead of just
returning "problems setting volume".

Previous versions of MPD would call SetVolume on enabled outputs before
they are ready, causing all of MPD to crash. Checking the really_enabled
flag prevents this, though it also prevents setting volume before the
player starts.

Before (with the PipeWire output):
  [i] ~$ mpc clear
  volume: 81%   repeat: off   random: off   single: off   consume: off
  [i] ~$ systemctl --user restart mpd.service
  [i] ~$ mpc volume 100
  MPD error: Connection closed by the server
  [i] ~ 1 $

After:
  [i] ~$ # mpd is freshly started w/o anything in the queue
  [i] ~$ mpc
  volume:100%   repeat: off   random: off   single: off   consume: off
  [i] ~$ mpc volume 80
  MPD error: problems setting volume
  [i] ~ 1 $ mpc
  volume:100%   repeat: off   random: off   single: off   consume: off
  [i] ~$

Move audio output state check ahead of mixer check and force volume
applying even for disabled software mixed outputs.

This fixes incorrect software mixer volume that used to occur when
volume was changed while output being disabled.

This is easily reproduced with following sequence of commands on
multi-output software mixed MPD setup.

 mpc volume 38; mpc disable 3; mpc volume 88; mpc enable 3

On current MPD, following commands would result in output 3 playing at
volume 38, while all other enabled outputs would play at volume
88. Moreover, global volume would display average of outputs real
volumes. In my case, it's 75.

After applying this patch, following commands would produce expected
behavior. All outputs play at expected (88) volume. And volume is
correctly displayed as 88.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1423

Signed-off-by: Vitaly Ostrosablin tmp6154@yandex.ru
Signed-off-by: Vitaly Ostrosablin <tmp6154@yandex.ru>

Signed-off-by: Rosen Penev <rosenp@gmail.com>

SonarLint reports the latter to be better:

std::scoped_lock basically provides the same feature as std::lock_guard,
but is more generic: It can lock several mutexes at the same time, with a
deadlock prevention mechanism (see {rule:cpp:S5524}). The equivalent code
to perform simultaneous locking with std::lock_guard is significantly more
complex. Therefore, it is simpler to use std::scoped_lock all the time,
even when locking only one mutex (there will be no performance impact).
Signed-off-by: Rosen Penev <rosenp@gmail.com>

This replaces 967af603 with a more
effective workaround.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/822

snd_mixer_handle_events() has already been called by
DispatchSockets().  This way, we can also skip the exception handler.

Found with cppcoreguidelines-interfaces-global-init
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with cppcoreguidelines-special-member-functions
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with cppcoreguidelines-special-member-functions
Signed-off-by: Rosen Penev <rosenp@gmail.com>

A few exceptions are needed for sources which include COM header
(directly or indirectly).

This fixes lots of shadow warnings, see
https://github.com/MusicPlayerDaemon/MPD/issues/1150

None of the current FormatHResultError() callers need the format string.

Reduce header dependencies.