[Bug 2080972] [NEW] /tmp as non-tmpfs doesn't get cleaned upon reboot

[Skia]

Public bug reported:

On Oracular, it seems `/tmp` doesn't get cleaned on reboot, when the
`/etc/systemd/system/tmp.mount` file is present.

Reproducer:

$ wget https://cloud-images.ubuntu.com/minimal/daily/oracular/20240913/oracular-minimal-cloudimg-amd64.img
$ qemu-img resize oracular-minimal-cloudimg-amd64.img +20G
$ cat >user-data <<EOF
#cloud-config
password: ubuntu
chpasswd: { expire: False }
ssh_pwauth: True
EOF
$ cloud-localds user-data.img user-data
$ kvm -m 2G -smp 2 -hda oracular-minimal-cloudimg-amd64.img -hdb user-data.img

In the VM, logged in with `ubuntu`/`ubuntu`:
$ sudo touch /etc/systemd/system/tmp.mount
$ df -h | grep '/tmp'  # you can verify that /tmp is a tmpfs, because this is the default now
$ sudo systemctl reboot
# Wait for reboot, then login again
$ echo Hello there >/tmp/will_stay
$ df -h | grep '/tmp'  # you can verify that /tmp is not a tmpfs anymore
$ sudo systemctl reboot
# Wait for reboot again, then login again
$ cat /tmp/will_stay
Hello there


At least our auto-upgrade-testing tooling relies on `/tmp` getting cleaned upon reboot, and as a user, this is what I expect too.

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/2080972

Title:
  /tmp as non-tmpfs doesn't get cleaned upon reboot

Status in systemd package in Ubuntu:
  New

Bug description:
  On Oracular, it seems `/tmp` doesn't get cleaned on reboot, when the
  `/etc/systemd/system/tmp.mount` file is present.

  Reproducer:

  $ wget https://cloud-images.ubuntu.com/minimal/daily/oracular/20240913/oracular-minimal-cloudimg-amd64.img
  $ qemu-img resize oracular-minimal-cloudimg-amd64.img +20G
  $ cat >user-data <<EOF
  #cloud-config
  password: ubuntu
  chpasswd: { expire: False }
  ssh_pwauth: True
  EOF
  $ cloud-localds user-data.img user-data
  $ kvm -m 2G -smp 2 -hda oracular-minimal-cloudimg-amd64.img -hdb user-data.img

  In the VM, logged in with `ubuntu`/`ubuntu`:
  $ sudo touch /etc/systemd/system/tmp.mount
  $ df -h | grep '/tmp'  # you can verify that /tmp is a tmpfs, because this is the default now
  $ sudo systemctl reboot
  # Wait for reboot, then login again
  $ echo Hello there >/tmp/will_stay
  $ df -h | grep '/tmp'  # you can verify that /tmp is not a tmpfs anymore
  $ sudo systemctl reboot
  # Wait for reboot again, then login again
  $ cat /tmp/will_stay
  Hello there

  
  At least our auto-upgrade-testing tooling relies on `/tmp` getting cleaned upon reboot, and as a user, this is what I expect too.

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