host/audio: per-PID PipeWire null-sink + loopback scaffolding

Session 1 of the per-app audio routing feature. Adds host/audio.rs
with a Routing struct that owns the lifecycle of two pactl-loaded
modules: a per-PID null-sink (pixelpass_capture_<pid>) and a loopback
mirroring @DEFAULT_SINK@.monitor into it at 20ms latency. Activated
by PIXELPASS_AUDIO_VIA_NULL_SINK=1 — kept hidden behind an env var
because without per-stream filtering (session 3) the user-facing
behavior of --app foo would be identical to no flag, which would
mislead users about what the flag does.

When the env var is set, wayland::start substitutes the gst pulsesrc
device from {DEFAULT_SINK}.monitor to pixelpass_capture_<pid>.monitor;
audio still works end-to-end via the loopback. CaptureHandle owns the
Routing alongside gst and serve; teardown order is gst → audio → serve
so streams unlink from the null-sink before the sink is destroyed.

Lifecycle is via pactl shell-outs rather than pipewire-rs. Null-sink
+ loopback are one-shot graph mutations with no event subscription;
the libpipewire route would mean dragging a MainLoop thread in for no
benefit until session 3 needs stream events.

Known cosmetic: the null-sink appears in Plasma's audio mixer as a
user-facing volume slider. Pactl's sink_properties= quoting is fiddly
enough that the device.hidden=true fix is parked for a follow-up.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

This commit is contained in:

Mollusk

2026-05-22 05:11:48 -04:00

parent 9625511c65

commit 8d32ded412

3 changed files with 164 additions and 5 deletions

									
										src/host/mod.rs
									
		+1
		
												View File
												
				@@ -1,3 +1,4 @@

				mod audio;

				mod capture;

				mod serve;

				mod wayland;