Local live-build fails to build images due to chroot/dev/* umount failures

[Łukasz Zemczak]

[Impact]

When running image builds locally by either running live-build directly or using `ubuntu-image classic`, the builds will almost always fail at one stage with an error similar to this:

umount: <PATH>/chroot/dev/shm: target is busy.

This is due to the fact that chroot/dev is bind-mounted, and during unmount, when someone is physically using the build host, umount operations on this tree can be impossible.
Without this basically using ubuntu-image locally to build your own image and/or running test automation on certain infrastructure bits can be impossible.

[Test Case]

(best done in a VM)
* On a focal machine, install ubuntu-image + livecd-rootfs from -proposed.
* Get the gadget: git clone https://github.com/snapcore/pc-amd64-gadget.git -b classic
* Prime the gadget: cd pc-amd64-gadget/; snapcraft prime
* Start the image build: cd ../; sudo ubuntu-image classic -d -O out/ -p ubuntu-cpc -a amd64 -s focal
* Wait for the image to finish building, observe that that the image build succeeded and image files are available in the out/ directory

Without the fix the build will not succeed.

[Where problems could occur]

I can't think of a scenario that could go wrong here. If there is any regression, it would most likely manifest as another image build failure. Regression potential is low here, especially that livecd-rootfs is only used for image builds.

[Łukasz Zemczak (sil2100)]

Released for impish with 2.731.

[Launchpad Janitor (janitor)]

This bug was fixed in the package livecd-rootfs - 2.664.27

---------------
livecd-rootfs (2.664.27) focal; urgency=medium

  * And whoops, we missed adding ARCH in the SUBARCH ubuntu-image handling for
    intel-iot.

livecd-rootfs (2.664.26) focal; urgency=medium

  * Revert previous change of fixing /dev sharing - this causes weird
    autopkgtest issues.

livecd-rootfs (2.664.25) focal; urgency=medium

  [ Brian Murray ]
  * Add support for creating images (ubuntu-core and classic) with a kernel
    optimized for Intel IoT devices. (LP: #1938338)

  [ Michael Hudson-Doyle ]
  * Simplify how the subiquity client is run on the serial console in the live
    server environment, breaking a unit cycle that sometimes prevents
    subiquity from starting up at all. (LP: #1888497)
  * Do not set the password for the installer user via cloud-init as subiquity
    can now do this itself. (LP: #1933523)

  [ Łukasz 'sil2100' Zemczak ]
  * Fix sharing of the /dev tree to make sure we can safely umount the chroot
    when needed. This fixes local non-livefs-builder image builds.
    (LP: #1938414)

 -- Łukasz 'sil2100' Zemczak <email address hidden> Wed, 04 Aug 2021 17:32:37 +0200