mkfs: avoid blockdev failing to re-read partition table

Invoking blockdev --rereadpt straight after creating the file system might fail with:

blockdev: ioctl error on BLKRRPART: Device or resource busy
Unexpected non-zero exit code 1 in /sbin/grml-debootstrap /sbin/grml-debootstrap /sbin/grml-debootstrap at line 1376 2132 0 detected!
last bash command: blockdev --rereadpt "$main_device"

This is caused by udev kicking in and causing a race condition. Let's invoke udevadm settle (which watches the udev event queue, and exits if all current events are handled), and then retry blockdev --rereadpt ... up to 30 times/seconds.

Thanks: Darshaka Pathirana for bug report and initial investigation, and Chris Hofstaedtler for feedback Closes: https://github.com/grml/grml-debootstrap/issues/273

Disclaimer: I am not a fan of sleep loops, but I guess any other solution would need something similar "under the hood".

Just tested the PR and it works for me:

root@grml ~ # ./grml-debootstrap -r bookworm -t /dev/sda1 --grub /dev/sda --hostname grml-debootstrap --mirror http://mirror.hetzner.de/debian/packages

 * EFI support detected, but system seems to be running in BIOS mode.
 * grml-debootstrap [0.108] - Please recheck configuration before execution:

   Target:          /dev/sda1
   Install grub:    /dev/sda
   Install efi:     no
   Using release:   bookworm
   Using hostname:  grml-debootstrap
   Using mirror:    http://mirror.hetzner.de/debian/packages
   Using arch:      amd64
   Config files:    /etc/debootstrap

   Important! Continuing will delete all data from /dev/sda1!

 * Is this ok for you? [y/N] y
[...]
mke2fs 1.47.0 (5-Feb-2023)
/dev/sda1 contains a ext4 file system
        created on Fri Jun  7 09:56:39 2024
Proceed anyway? (y,N) y
Discarding device blocks: done                            
Creating filesystem with 2096896 4k blocks and 524288 inodes
Filesystem UUID: 43bba0a1-121f-4666-95ee-2220b6d31079
Superblock backups stored on blocks: 
        32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632

Allocating group tables: done                            
Writing inode tables: done                            
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done 

 * Disabling automatic filesystem check on /dev/sda1 via tune2fs
tune2fs 1.47.0 (5-Feb-2023)
Setting maximal mount count to -1
Setting interval between checks to 0 seconds
 * Mounting /dev/sda1 to /mnt/debootstrap.9433
mount: (hint) your fstab has been modified, but systemd still uses
       the old version; use 'systemctl daemon-reload' to reload.
 * Identified UUID 43bba0a1-121f-4666-95ee-2220b6d31079 for /dev/sda1
 * Running debootstrap  for release bookworm (amd64) using http://mirror.hetzner.de/debian/packages
 * Executing: debootstrap --arch amd64  bookworm /mnt/debootstrap.9433 http://mirror.hetzner.de/debian/packages
[...]

I can not recall the mount: (hint) your fstab has been modified line, but apart from that, the fix works.

grml / grml-debootstrap

mkfs: avoid blockdev failing to re-read partition table #275