pulse: add a small pulseaudio daemon

Add a small application that loads the pulse server and makes it
listen on the pulseaudio socket.
Add some systemd activation files to start the service.

src/daemon/systemd/user/meson.build

@@ -1,11 +1,19 @@
 systemd_user_services_dir = systemd.get_pkgconfig_variable('systemduserunitdir', define_variable : [ 'prefix', prefix])
 
-install_data(sources : 'pipewire.socket', install_dir : systemd_user_services_dir)
+install_data(
+  sources : ['pipewire.socket', 'pipewire-pulse.socket'],
+  install_dir : systemd_user_services_dir)
 
 systemd_config = configuration_data()
 systemd_config.set('PW_BINARY', join_paths(pipewire_bindir, 'pipewire'))
+systemd_config.set('PW_PULSE_BINARY', join_paths(pipewire_bindir, 'pipewire-pulse'))
 
 configure_file(input : 'pipewire.service.in',
                output : 'pipewire.service',
                configuration : systemd_config,
                install_dir : systemd_user_services_dir)
+
+configure_file(input : 'pipewire-pulse.service.in',
+               output : 'pipewire-pulse.service',
+               configuration : systemd_config,
+               install_dir : systemd_user_services_dir)

src/daemon/systemd/user/pipewire-pulse.service.in

@@ -0,0 +1,25 @@
+[Unit]
+Description=PipeWire PulseAudio
+
+# We require pipewire-pulse.socket to be active before starting the daemon, because
+# while it is possible to use the service without the socket, it is not clear
+# why it would be desirable.
+#
+# A user installing pipewire and doing `systemctl --user start pipewire-pulse`
+# will not get the socket started, which might be confusing and problematic if
+# the server is to be restarted later on, as the client autospawn feature
+# might kick in. Also, a start of the socket unit will fail, adding to the
+# confusion.
+#
+# After=pipewire-pulse.socket is not needed, as it is already implicit in the
+# socket-service relationship, see systemd.socket(5).
+Requires=pipewire-pulse.socket
+
+[Service]
+Type=simple
+ExecStart=@PW_PULSE_BINARY@
+Restart=on-failure
+
+[Install]
+Also=pipewire-pulse.socket
+WantedBy=default.target

src/daemon/systemd/user/pipewire-pulse.socket

@@ -0,0 +1,10 @@
+[Unit]
+Description=PipeWire PulseAudio
+
+[Socket]
+Priority=6
+Backlog=5
+ListenStream=%t/pulse/native
+
+[Install]
+WantedBy=sockets.target