expand-on-first-boot and surprising umounts

[Raphael Lisicki <raphael.lisicki@siemens.com>]

Hi everybody,

I am using a debian bullseye based system and use expand-on-first-boot 
to expand the last partition. It is not the root file system but an 
extra ext4 partition to be mounted under /data. The mounting happens 
after expand-on-first boot has succeeded.

Sometimes, on some builds more often than others, /data gets (attempted 
to be) umounted immediately after being mounted and subsequent units 
will fail.

Removing expand-on-first-boot resolves the issue, so does adding 
"ExecStartPost=/usr/bin/udevadm settle" to expand-on-first-boot but I am 
not sure if this is only a sophisticated way of solving a race condition 
with "sleep".

My gut feeling is that after expand-on-first boot finishes, the kernel
still processes block device events, which systemd gets after /data has
already been mounted and then cause it to be umounted, as systemd was
already picky with umounting stuff in the past [1]. Unfortunately I have
no idea how to test this hypothesis.

Did anyone already experience something like this? The racy-ness of the 
issue makes creating a minimal reproduction hard.



[1] https://github.com/systemd/systemd/issues/9869



best regards
raphael