Potential issue with myMPD custom script file permissions

[cbrenberg]

Hello. I've been chatting with the maintainer of myMPD in his mympd-scripts repository about a somewhat recent issue that popped up related to myMPD custom lua scripts being unable to access certain cache files/directories, and he's been unable to reproduce the issue, which makes me think it might be specific to DietPi or my individual setup.

Here's the original issue for context: jcorporation/mympd-scripts#35

I'm hoping to see if somebody running myMPD on a DietPi system can add the two album art scripts from the link at the bottom of this question into myMPD and see if they can reproduce these same error logs I've been getting, which will help me determine whether it's a dietpi thing or if there's something else in my environment causing permissions problems:

Apr 05 15:59:45 LivingRoom mympd[42460]: ERROR    scripts_worker  Can not open file descriptor "/var/cache/mympd/misc/qRuVEvtUxf.tmp" for write
Apr 05 15:59:45 LivingRoom mympd[42460]: ERROR    scripts_worker  File exists
Apr 05 15:59:45 LivingRoom mympd[42460]: ERROR    scripts_worker  Lua - http_download: out is NULL

Here are the lua scripts to add (they attempt to fetch album art dynamically at runtime from one or more third party providers): https://github.com/jcorporation/mympd-scripts/tree/main/Albumart

Thanks for the help!

[MichaIng]

Hmm, the myMPD service should create this dir with correct permissions automatically, using a dynamically creates user, if things did not change.

I'll have a look.

[MichaIng]

Hmm, we do only minimal changes on top of what the official APT package does, pregeneraring a config with the same dynamic user the official systemd service uses. The private symlink or cache and states dir shows that it has been setup as intended 🤔.

Have you updated to the latest myMPD version in the meantime? The dietpi-update updates the APT key for the repo, to allow the upgrade.

[cbrenberg]

I did update to the latest dietpi 12.0 a couple days ago, but I'm still seeing the same error. Linux file permission errors probably would not use the term 'file descriptor', so based on the error message that's actually being logged, I wonder if the actual problem is somewhere in a lua library or helper file being called by myMPD.

I even tried granting the dynamic myMPD user explicit rwx permissions on both the private symlink and its target directory, but even that didn't resolve the issue, which also makes me think the problem could be something other than a linux file permission problem.

The only symptom is that a small number of album covers fail to load in the myMPD UI, so it's definitely not a high priority. It's just something that's been bothering me, so I thought I'd ask just in case somebody had an easy answer.

[cbrenberg]

Update on this:

I've been discussing this issue with Jurgen in the myMPD repo, and we finally figured out the root cause. It has nothing to do with linux file permissions or the mympd dynamic user. The real issue was a couple different things related to how lua scripts are executed concurrently by myMPD and a couple of bugs in the custom lua scripts I'm using that will be resolved in the next release of myMPD.

I'll close this discussion now that we have answers.

[MichaIng]

Thanks for checking back. I was already confused how our setup could cause the issue 😅.