Pipewire fails to reacquire a realtime priority when restarted

[Chris Coulson]

Pipewire tries to acquire a realtime priority when it starts via the libpipewire-module-rt plugin. By default, it prefers to use the org.freedesktop.portal.Realtime interface if xdg-desktop-portal is running (it checks if org.freedesktop.portal.Desktop exists on the session bus). If xdg-desktop-portal isn't running, it uses rtkit directly - see https://gitlab.freedesktop.org/pipewire/pipewire/-/tree/1.2.7/src/modules?ref_type=tags#L932.

I started looking at this because I have an annoying issue with my audio which results in my speakers making popping noises frequently when they're outputting anything. I have another issue where my USB speakers are frequently not detected by pipewire without restarting it.

When pipewire starts with a new session, it acquires a realtime priority by using rtkit directly because xdg-desktop-portal isn't running at this stage. However, if you restart pipewire manually later on when xdg-desktop-portal is running, pipewire uses the portal interface to request a realtime priority, and this fails with xdg-desktop-portal outputting the following error to the journal:

Apr 13 02:58:01 farnsworth xdg-desktop-por[7283]: Realtime error: Could not get pidns: Could not fstatat ns/pid: Not a directory

The libpipewire-module-rt plugin does have an option to disable usage of the portal interface for requesting a realtime priority (rtportal.enabled=false).

This is tested with pipewire 1.2.7 in plucky.