Comment 1 for bug 2058629

I was previously able to reproduce reliably this behavior but I am not anymore, which is at the same time a good and a bad news. Meanwhile, I now it a similar issue, but at the update_bootloader stage.

[18] update_bootloader
Adding 'local diversion of /etc/grub.d/30_os-prober to /etc/grub.d/30_os-prober.dpkg-divert'
Sourcing file `/etc/default/grub'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-6.5.0-9-generic
Found initrd image: /boot/initrd.img-6.5.0-9-generic
Adding boot menu entry for UEFI Firmware Settings ...
done
Removing 'local diversion of /etc/grub.d/30_os-prober to /etc/grub.d/30_os-prober.dpkg-divert'
umount: /home/paul/scratch/debootstrap-2/scratch/loopback/proc: target is busy.
umount: /home/paul/scratch/debootstrap-2/scratch/loopback: target is busy.
duration: 1.053857891s
Error: teardown failed: teardown command "/usr/bin/umount --recursive /home/paul/scratch/debootstrap-2/scratch/loopback/proc" failed. Output:
umount: /home/paul/scratch/debootstrap-2/scratch/loopback/proc: target is busy.

teardown command "/usr/bin/umount /home/paul/scratch/debootstrap-2/scratch/loopback" failed. Output:
umount: /home/paul/scratch/debootstrap-2/scratch/loopback: target is busy.

This is also not always happening and mounted dirs can be umounted manually after the build failed. So I suspect a race condition again.