Bug#905774: jackd2: jackd no longer starts, since device is already in use; but who's using it?

[Francesco Poli (wintermute)]

Package: jackd2
Version: 1.9.12~dfsg-2
Severity: important

Hello,
a couple of weeks ago I began experiencing an awkward behavior of jackd
on one of my Debian testing machines (I run jackd with almost identical
configuration on two other boxes, without issues).

Basically, jackd no longer wants to start at the beginning of my
X session and also refuses to start, when I try to start it manually
from an xterm with:

  $ jackd --realtime -d alsa --device hw:0 \
    --softmode --hwmeter --rate 44100 &

The output I get is:

  jackdmp 1.9.12
  Copyright 2001-2005 Paul Davis and others.
  Copyright 2004-2016 Grame.
  Copyright 2016-2017 Filipe Coelho.
  jackdmp comes with ABSOLUTELY NO WARRANTY
  This is free software, and you are welcome to redistribute it
  under certain conditions; see the file COPYING for details
  no message buffer overruns
  no message buffer overruns
  no message buffer overruns
  JACK server starting in realtime mode with priority 10
  self-connect-mode is "Don't restrict self connect requests"
  audio_reservation_init
  Acquire audio card Audio0
  creating alsa driver ... hw:0|hw:0|1024|2|44100|0|0|nomon|hwmeter|soft-mode|32bit
  
  
  ATTENTION: The playback device "hw:0" is already in use. Please stop the application using it and run JACK again
  Released audio card Audio0
  audio_reservation_finish
  Cannot initialize driver
  JackServer::Open failed with -1
  Failed to open server
  
  [1]+  Exit 255                jackd --realtime -d alsa --device hw:0 --softmode --hwmeter --rate 44100


OK, so there's another application which is using hw:0 and won't
release the resource.

The problem is: which application is using hw:0 ?!?
jackd does not tell me!

The weird thing is that I searched the web and found some jackd
output transcripts where jackd actually tells the user which
application is using the soundcard with something like:

  ATTENTION: The playback device "hw:0" is already in use. The following
  applications are using your soundcard(s) so you should check them and stop
  them as necessary before trying to start JACK again:
  
  $APPLICATION (process ID $PID)

See for instance http://www.tedfelix.com/linux/linux-midi.html

Why my jackd does not tell me which application is using hw:0 ?!?
How can I figure out which application is getting in the way?


Please help me to investigate and please improve the usefulness of
jackd error messages.


Thanks for your time and for any help you may provide.
Bye!


-- System Information:
Debian Release: buster/sid
  APT prefers testing
  APT policy: (800, 'testing'), (500, 'unstable')
Architecture: amd64 (x86_64)

Kernel: Linux 4.17.0-1-amd64 (SMP w/2 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=en_US:en (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages jackd2 depends on:
ii  coreutils              8.28-1
ii  debconf [debconf-2.0]  1.5.69
ii  libasound2             1.1.6-1
ii  libc6                  2.27-5
ii  libdbus-1-3            1.12.10-1
ii  libexpat1              2.2.5-3
ii  libgcc1                1:8.2.0-1
ii  libjack-jackd2-0       1.9.12~dfsg-2
ii  libopus0               1.3~beta+20180518-1
ii  libreadline7           7.0-5
ii  libsamplerate0         0.1.9-2
ii  libsndfile1            1.0.28-4
ii  libstdc++6             8.2.0-1
ii  python                 2.7.15-3
ii  python-dbus            1.2.8-2+b1

Versions of packages jackd2 recommends:
ii  jackd2-firewire  1.9.12~dfsg-2
ii  libpam-modules   1.1.8-3.7
ii  qjackctl         0.5.0-1

Versions of packages jackd2 suggests:
pn  jack-tools   <none>
pn  meterbridge  <none>

-- debconf information:
* jackd/tweak_rt_limits: true