qubes-dom0-update: prevent concurrent execution

[iamahuman]

Qubes OS version
R4.0

Affected component(s) or functionality
qubes-core-admin-linux: qubes-dom0-update

Brief summary
Multiple instances of qubes-dom0-update may run concurrently (e.g. qubes-dom0-update --check-only cron job running simultaneously with Qubes Updater), resulting in update cache corruption.

How Reproducible
100%

To Reproduce

Steps to reproduce the behavior:

1. Open terminal in dom0.
2. Start qubes-dom0-update.
3. Open another terminal in dom0.
4. Before the first update command finishes, wait for a while and start another qubes-dom0-update.

Expected behavior
Two instances of update command seralize with respect to each other.

Actual behavior
There are no attempts in synchronization, resulting in potential update data corruption.

Screenshots
TBD

Additional context
TBD

Solutions you've tried
Cleaning up cache every time such race condition happens is cumbersome.

Simplest solution would be to use locks to prevent concurrent invocation of qubes-dom0-update.

Relevant documentation you've consulted

• https://www.qubes-os.org/doc/software-update-dom0/

Related, non-duplicate issues
TBD

[alimirjamali]

PR Submitted. Low priority for review

[apparatius]

The qubes-dom0-update is calling itself from inside when switching the audio:
https://github.com/QubesOS/qubes-core-admin-linux/blob/3f0afb7030276f7afc0212bbaec187abdab72860/dom0-updates/qubes-dom0-update#L433
So this change is breaking the audio switching option:
https://forum.qubes-os.org/t/terrible-audio-quality-after-suspend-switching-to-pipewire-in-dom0-does-not-work/28893
Maybe add an option for qubes-dom0-update to skip the lock file check and use this option for qubes-dom0-update commands called from inside qubes-dom0-update?
Or skip the lock file check if SWITCHING_AUDIO_IN_PROGRESS variable is set.

[alimirjamali]

The qubes-dom0-update is calling itself from inside when switching the audio:
https://github.com/QubesOS/qubes-core-admin-linux/blob/3f0afb7030276f7afc0212bbaec187abdab72860/dom0-updates/qubes-dom0-update#L433
So this change is breaking the audio switching option:
https://forum.qubes-os.org/t/terrible-audio-quality-after-suspend-switching-to-pipewire-in-dom0-does-not-work/28893
Maybe add an option for qubes-dom0-update to skip the lock file check and use this option for qubes-dom0-update commands called from inside qubes-dom0-update?
Or skip the lock file check if SWITCHING_AUDIO_IN_PROGRESS variable is set.

Ops. I did not foresee that. I will work on a fix.

[alimirjamali]

PR Submitted