[Bug 2138450] [NEW] Restoring resolv.conf is broken

[Sebastian Weyer]

Public bug reported:

Starting in version 3.10.0 of the ubuntu image snap, restoring the
resolv.conf file at the end of the build has been broken. Instead of
restoring the backed up resolv.conf.tmp file, the host's resolv.conf
file will remain in the image on top of the backed up file.

Since commit 069af78, `RestoreResolvConf` is called an additional time.
The first call to this function works correctly, whereas the second
doesn't.

This is because between the first and the second call to that function,
the file `$chroot/etc/resolv.conf` is converted into a symlink pointing
to `/run/systemd/resolve/stub-resolv.conf` which is a runtime generated
file from systemd. That symlink will be backed up correctly by
`BackupAndCopyResolvConf` but when `setupGrub()` in
`internal/statemachine/bootloader_helper.go` tries to restore it, it
will fail because `RestoreResolvConf` will try to follow the symlink and
check if that file exists by running `if !osutil.FileExists(){}`.

In order to fix this, we should only return nil if the file doesn't
exist and the symlink doesn't exist.

A pull request has been prepared here:
https://github.com/canonical/ubuntu-image/pull/287

This issue is critical for releasing an image for NVIDIA Tegra devices.
The images for these devices are built on builders located in the Boston
lab and contain a DNS configuration with a local address which results
in it being broken on other devices:

nameserver 10.189.128.2
search ppa

** Affects: ubuntu-image
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/2138450

Title:
  Restoring resolv.conf is broken

Status in Ubuntu Image:
  New

Bug description:
  Starting in version 3.10.0 of the ubuntu image snap, restoring the
  resolv.conf file at the end of the build has been broken. Instead of
  restoring the backed up resolv.conf.tmp file, the host's resolv.conf
  file will remain in the image on top of the backed up file.

  Since commit 069af78, `RestoreResolvConf` is called an additional
  time. The first call to this function works correctly, whereas the
  second doesn't.

  This is because between the first and the second call to that
  function, the file `$chroot/etc/resolv.conf` is converted into a
  symlink pointing to `/run/systemd/resolve/stub-resolv.conf` which is a
  runtime generated file from systemd. That symlink will be backed up
  correctly by `BackupAndCopyResolvConf` but when `setupGrub()` in
  `internal/statemachine/bootloader_helper.go` tries to restore it, it
  will fail because `RestoreResolvConf` will try to follow the symlink
  and check if that file exists by running `if !osutil.FileExists(){}`.

  In order to fix this, we should only return nil if the file doesn't
  exist and the symlink doesn't exist.

  A pull request has been prepared here:
  https://github.com/canonical/ubuntu-image/pull/287

  This issue is critical for releasing an image for NVIDIA Tegra
  devices. The images for these devices are built on builders located in
  the Boston lab and contain a DNS configuration with a local address
  which results in it being broken on other devices:

  nameserver 10.189.128.2
  search ppa

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