fix(linbo_partition): retry umount /cache to survive concurrent re-mounts

[helge42]

Symptom

linbo-remote -i <host> -c format (or partition) aborts with:

Cannot unmount /cache.

Running the same operation locally via linbo-wrapper format from the LINBO GUI / shell works. The bug therefore looks client-state-dependent but is in fact a timing race: it hits fast hardware (NVMe) reliably and slower hardware (HDD / older SSD) only sporadically.

Root cause

linbo_gui periodically re-mounts /cache for status polling. On an otherwise idle client this is visible in dmesg as recurring cycles roughly every second:

EXT4-fs (nvme0n1p6): mounted filesystem ... r/w with ordered data mode.
EXT4-fs (nvme0n1p6): unmounting filesystem ...
EXT4-fs (nvme0n1p6): mounted filesystem ... ro with ordered data mode.
EXT4-fs (nvme0n1p6): unmounting filesystem ...

linbo_partition does (lines 213–218):

cd /
mount | grep -q ' /cache ' && umount /cache &> /dev/null && sleep 3
if mount | grep -q ' /cache '; then
  echo "Cannot unmount /cache." >&2
  exit 1
fi

The umount succeeds — but during the sleep 3 the polling re-mounts /cache, and the subsequent mount | grep re-check then aborts. gui_ctl disable, which linbo-remote calls before format, hides the GUI but does not stop the status polling.

Reproduction

• Client sitting at the LINBO GUI, /cache mounted.
• Watch the cycle on the client: while :; do date '+%H:%M:%S'; mount | grep cache; sleep 0.2; done
• Trigger from the server: linbo-remote -i <client> -c format — fails with Cannot unmount /cache.
• For comparison, in a shell on the client (Dropbear, port 2222): linbo-wrapper format — works.

Fix

Replace the single umount + sleep 3 with a short retry loop (up to 10 × 1 s). As soon as one iteration finds /cache unmounted, the loop exits and the existing failure check still fires if /cache is permanently busy. No new dependencies, behavior unchanged for the "not busy at all" case.

Test

Patched linbo_partition via a pre-hook in update-linbofs.pre.d on a production linuxmuster.net 7 server (linbofs64, 4.3.33). After PXE-booting test clients, linbo-remote -c format runs reliably across multiple invocations and on multiple clients. No regression observed for local linbo-wrapper format.