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Draft: Make unmounting of /proc and /sys more resilient

Marcel Partap requested to merge mpartap-guest/live-build:unmount-resilience into master

.. against concurrent ephemeral accesses (f.e. by git status calls from inside ranger file manager)

I was thinking about limiting the number of unmount attempts, but from my logs, the maximum time it needed was twenty seconds.. Here's an example:

[2020-09-10 16:28:49] lb chroot_proc remove
P: Begin unmounting /proc...
umount: /mnt/stickbuildarea/fsfw-dresden-usb-stick/chroot/proc: target is busy.
                     USER        PID ACCESS COMMAND
/mnt/stickbuildarea/fsfw-dresden-usb-stick/chroot/proc:
                     root     kernel mount /proc
                     rtkit      4872 .rc.. rtkit-daemon
                     empee584  1076570 f.... element-desktop
                     root      1758598 f.... git
                     root      1758850 f.... git
                     empee584  2039370 f.... chromium
umount: /mnt/stickbuildarea/fsfw-dresden-usb-stick/chroot/proc: target is busy.
                     USER        PID ACCESS COMMAND
/mnt/stickbuildarea/fsfw-dresden-usb-stick/chroot/proc:
                     root     kernel mount /proc
                     rtkit      4872 .rc.. rtkit-daemon
                     empee584  1076570 f.... element-desktop
                     root      1759170 f.... git
                     empee584  2039370 f.... chromium
umount: /mnt/stickbuildarea/fsfw-dresden-usb-stick/chroot/proc: target is busy.
                     USER        PID ACCESS COMMAND
/mnt/stickbuildarea/fsfw-dresden-usb-stick/chroot/proc:
                     root     kernel mount /proc
                     rtkit      4872 .rc.. rtkit-daemon
                     empee584  1076570 f.... element-desktop
                     root      1759303 f.... git
                     root      1759362 f.... git
                     empee584  2039370 f.... chromium
umount: /mnt/stickbuildarea/fsfw-dresden-usb-stick/chroot/proc: target is busy.
                     USER        PID ACCESS COMMAND
/mnt/stickbuildarea/fsfw-dresden-usb-stick/chroot/proc:
                     root     kernel mount /proc
                     rtkit      4872 .rc.. rtkit-daemon
                     empee584  1076570 f.... element-desktop
                     empee584  2039370 f.... chromium
umount: /mnt/stickbuildarea/fsfw-dresden-usb-stick/chroot/proc (proc-live) unmounted
[2020-09-10 16:29:17] lb chroot_devpts remove

Hell knows why these other (apart from git) accesses occur, but they do, and they used to break the build for me. The issue occured on about 80% of the builds I have logged since putting this in three months ago.. suggestions welcome of course ; )

Edited by Marcel Partap

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