Start mpd-alsa docker automatically when a usb sound card is plugged in

[ers4691]

Hi,
I am currently failing at the task of starting the docker container automatically when a usb sound card is plugged in.
In other words, I can create and start the container with a udev rule. So far so good.
However, mpd does not see my sound card. Only when I restart the container does it work properly.

The udev rule is
SUBSYSTEM==‘sound’, ACTION==‘add’, DEVPATH==‘/devices/pci0000:00/0000:00:15.0/usb1/1-3/1-3:1.0/sound/card?’, RUN+="/root/startMusic.sh’

Does anyone have an idea?

[GioF71]

Hello, maybe try to re-create the container when the event occurs (docker-compose up -d --force-recreate), that should do the trick.
However, yes, this is the most annoying issue with using containers for audio applications.
MPD on its own does not need to be restart just because a sound card is not available when it starts. It recognizes it on the fly.
So I am working on another project, player-launchers, that will allow to configure MPD (and other players as well) via environment variables. I like this approach more than working (and then keeping track) of structured configuration files like a typical mpd.conf.
Of course with this project, we trade in some convenience (MPD, or any of the supported players, must be installed, or binaries must be available anyway).

[ers4691]

I think I've found it.
It's not the container, it's udev. :-)

I think it takes udev too long to boot the container.
That's why I used now SYSTEMD_WANTS instead of RUN.

It works

My new rule:
SUBSYSTEM==‘sound’, ACTION==‘add’, DEVPATH==‘/devices/pci0000:00/0000:00:15.0/usb1/1-3/1-3:1.0/sound/card?’, ENV{SYSTEMD_WANTS}+=‘SMSL.service’ TAG+= ‘systemd’

[GioF71]

Nice!
Can you show the systemd unit and other relevant details?

[ers4691]

Here are the details:
Service in /etc/systemd/system/SMSL.service

[Unit]
Description=SMSL is plugged in, just run the script

[Service]
ExecStart=/root/startMusic.sh

----------- /root/startMusic.sh

#! /bin/sh
echo "Starting Music Compose" >> /tmp/Music.log
date >> /tmp/Music.log
cd /srv/dev-disk-by-id-nvme-WDC_WDS500G2B0C-00PXH0_20349R459504-part4/DockerAppData/Musik
docker compose --env-file ../global.env --env-file ./Musik.env  --file ./Musik.yml up -d
echo "Compose started" >> /tmp/Music.log

The Musik.yml is based on you example files; I'm also starting a spotify container and rompr.

[GioF71]

Thank you for sharing. I have a question:

[Service]
ExecStart=/root/startMusic.sh

but there in startMusic.sh, the container is started in detached mode. So I assume that doing a

systemctl stop SMSL

doesn't do actually anything. Am I wrong?

If so, what about creating the service as a "simple" service and just skip that "-d"?

[ers4691]

That's right.
There is still room for improvement.

I still have a rule in the udev-rules for shutting down the container.

SUBSYSTEM=="sound", ACTION=="add", DEVPATH=="/devices/pci0000:00/0000:00:15.0/usb1/1-3/1-3:1.0/sound/card?", ENV{SYSTEMD_WANTS}+="SMSL.service" TAG+="systemd"
SUBSYSTEM=="sound", ACTION=="remove", DEVPATH=="/devices/pci0000:00/0000:00:15.0/usb1/1-3/1-3:1.0/sound/card?", RUN+="/root/stopMusic.sh"

in stopMusic.sh is:

! /bin/sh
echo "Stopping Music Compose" >> /tmp/Music.log
cd /srv/dev-disk-by-id-nvme-WDC_WDS500G2B0C-00PXH0_20349R459504-part4/DockerAppData/Musik
docker compose --env-file ./Musik.env  --file ./Musik.yml down
echo "Compose stoped" >> /tmp/Music.log

I need to read up on the services and the interaction with udev rules.
a systemctl stop SMSL would be much more elegant

[GioF71]

Thank you very much for sharing.
It's a nice solution to an issue that affects all audio players with dac that can be not powered on at boot time when using docker.
Consider starring the repo if you like the work!