[Bug 1778936] Re: please re-add Support-system-image-read-only-etc.patch

[Sergio Cazzolato]

To reproduce the error:

1. checkout this branch:
  https://github.com/sergiocazzolato/snapd/tree/tests-validate-lp-1778936
2. run the test:
  spread -debug google:ubuntu-core-18-64:tests/main/interfaces-hostname-control

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https://bugs.launchpad.net/bugs/1778936

Title:
  please re-add Support-system-image-read-only-etc.patch

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Bionic:
  Fix Committed
Status in systemd source package in Cosmic:
  Fix Released

Bug description:
  [Impact]

   * core18 systems fail to update hostname
   * this happens due to /etc/hostname actually being a symlink to a file under /etc/writable/, adjust hostnamed to take that into account when trying to update /etc/hostname

  [Test Case]

   * run a core18 system, check that dhcp acquire hostname is correctly
  updated in /etc/writable/hostname

  [Regression Potential]

   * This is cherrypick of code that has gone missing since xenial.
   * There is no change of behaviour for the classic systems.
   * Currently, core18 systems simply fail to update hostname/machine-info files, thus the worst case is that they will still fail to do so.

  [Other Info]
   
   * original bug report

  The 16.04 version of systemd had a patch to support the read-only etc.
  For core18 we will also need this change because core18 is still not
  on a fully writable etc.

  I will attach a debdiff against the current bionic version of systemd.

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