Comment 170 for bug 1197395

With the caveat that I haven't followed the entire thread, I had a similar problem today on an older 14.04 release: the owner of /run/user/$id/pulse would "spontaneously" become root. Comment #34 says this is easily reproducible when running pkexec/synaptic. I believe I can explain - broadly - what is happening by describing what I can confirm.

First, here is one way to reproduce the problem:
*) PulseAudio (v4 in this case) starts as a normal user.
*) As root (via sudo su -), run 'aplay --list-devices'
*) /run/user/$id/pulse is now owned by root.

Tracing 'aplay' I can see that it is dlopen()ing a bunch of pulseaudio support libraries. *Somehow* they (the pulseaudio support libraries, I presume) determine that pusleaudio is running. They extract a cookie from /root/.config/cookie and try to connect to the server via /run/user/$id/pulse/native socket.

At this point, I don't know what the code is trying to do, but what it _does_ do is this:

mkdir("/run/user/1000/pulse", 0700) = -1 EEXIST (File exists)
open("/run/user/1000/pulse", O_RDONLY|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC) = 5
fchown(5, 0, 0) = 0
fchmod(5, 0700) = 0
[...]
getuid() = 0
socket(PF_LOCAL, SOCK_STREAM|SOCK_CLOEXEC, 0) = 5
connect(5, {sa_family=AF_LOCAL, sun_path="/run/user/1000/pulse/native"}, 110) = -1 ENOENT (No such file or directory)
[...]

If my supposition is true that this is in the pulseaudio libraries, any application running as root (like synaptic) is going to disable the sound server when it tries to play audio through pulseaudio.

I scanned the pulseaudio changelogs and didn't see anything mentioning this, but I can also confirm that Ubuntu 17.10 with pulseaudio version 10 and aplay version 1.1.3 does not do this. The older aplay version was 1.0.27.2.

In Ubuntu 17.10 pulseaudio is being run by 'gdm' but the same aplay strace shows that it now looks for the socket in /var/run/pulse/native instead of any particular UID in /run. It's unclear to me if the problem is solved (in pulseaudio, if the culprit) or just band-aided by changing the configuration (perhaps of ALSA integration with pulseaudio).