Add support to the ALSA device provider to enumerate PCM outputs that do
not correspond to a physical sound device i.e. they are "virtual" sinks,
like the plug, dmix, or softvol PCM outputs that can be setup in the ALSA
configuration files.
The main use-case for this is allowing usage of GstDeviceMonitor in setups
where there is no audio server and have custom ALSA audio configurations.
As those are likely to be uncommon, the feature is opt-in: a list of device
names and wildcard patterns separated by semicolons must be assigned to the
GST_ALSA_PCM_ALLOW environment variable before such PCM outputs will be
enumerated by the ALSA device provider. This allows either scanning all
PCM outputs, listing individual outputs, providing simple patterns with
'*' wildcards (which match only at the start or end of the name), or
a combination of them:
GST_ALSA_PCM_ALLOW=1 # Enable listing PCM outputs.
GST_ALSA_PCM_ALLOW='*' # Same, using a wildcard.
GST_ALSA_PCM_ALLOW='out_1;out_1' # Exact listing.
GST_ALSA_PCM_ALLOW='out_*' # Using a wildcard.
GST_ALSA_PCM_ALLOW='out_*;other_*;line_out' # Multiple items.
The main motivation for this patch is supporting enumeration of PCM outputs
in the WebKit GTK and WPE ports, which use GstDeviceMonitor to determine
which devices may be chosen for sound output. While on desktops typically
PulseAudio or PipeWire are used nowadays, on embedded devices it is often
desirable to avoid them and use custom configurations that perform audio
routing and processing using only ALSA.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8831>
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