[Bug 1547033] Re: please allow /etc/mtab to be a pre-existing link on readonly filesystems

[Martin Pitt]

tmpfiles already checks if the symlink is the same if it already exists.
However, in this case it's not literally the same: your's points to
/proc/mounts while /usr/lib/tmpfiles.d/debian.conf makes it point to
../proc/self/mounts. If these were the same, this error would disappear.

Now, /etc/mtab → /proc/mounts is a waste as /proc/mounts is already a
symlink to self/mounts, so you need to resolve symlinks twice. The "../"
is so that this works with different --root arguments, i. e. if you want
to do things with such an image when you mount it somewhere.

Can I convince you to change the snappy image to have /etc/mtab →
../proc/self/mounts ? If not, could we at least agree to →
/proc/self/mounts to avoid the double resolution?

** Summary changed:

- please allow /etc/mtab to be a pre-existing link on readonly filesystems
+ make snappy's /etc/mtab symlink agree to tmpfiles.d/debian.conf

** Changed in: systemd (Ubuntu)
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1547033

Title:
  make snappy's /etc/mtab symlink agree to tmpfiles.d/debian.conf

Status in Snappy:
  New
Status in systemd package in Ubuntu:
  Incomplete

Bug description:
  booting a snappy system which comes with a readonly file system that
  carries /etc/mtab as pre-created link in the readonly area of the fs
  (pointing to /proc/mounts), we always get an error message on boot
  when systemd-tmpfiles tries to re-create the mtab link.

  systemd-tmpfiles should instead check if /etc/mtab is already there,
  is a link and points to the desired target. in this case it should
  just skip the creation attempt gracefully without error.

  Feb 17 18:56:51 localhost systemd[1]: Starting Create Volatile Files and Directories...
  Feb 17 18:56:51 localhost systemd-tmpfiles[631]: [/usr/lib/tmpfiles.d/var.conf:14] Duplicate line for path "/var/log"
  , ignoring.
  Feb 17 18:56:51 localhost systemd-tmpfiles[631]: symlink(../proc/self/mounts, /etc/mtab) failed: Read-only file syste
  m
  Feb 17 18:56:51 localhost systemd[1]: systemd-tmpfiles-setup.service: Main process exited, code=exited, status=1/FAIL
  URE

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