[Bug 1949089] Re: systemd randomly fails to activate mount units in Ubuntu Core 18

[Dan Streetman]

> @slyon you might want to see if commit
918e6f1c0151429f5095355f4f3f74f16e79724a fixes this

(and there are a couple follow on commits to this)

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Title:
  systemd randomly fails to activate mount units in Ubuntu Core 18

Status in systemd package in Ubuntu:
  New
Status in systemd source package in Bionic:
  New

Bug description:
  Since a month or so, we've been seeing random failures in our snapd
  spread tests where systemd could not start the mount unit associated
  with a snap because of a failed dependency.

  The issue is described in the comments to PR
  https://github.com/snapcore/snapd/pull/10935, but I'll summarize it
  here.

  When starting a snap, snapd creates a mount unit to mount the snap's
  squashfs (the template is
  https://github.com/snapcore/snapd/blob/release/2.53/systemd/systemd.go#L1186-L1205).
  The snapd asks systemd to reload the configuration, and starts the
  mount unit.

  The failure we've observed is that sometimes systemd decides to stop
  our mount unit (search for "Unmounting Mount unit for test-snapd-svc-
  flip-flop" in the attached log), and then tries to reactivate it
  again, and at that point it fails.

  When I asked for help, Lukas pointed out that the latest update
  contains a patch that is related to reload handling and mount units:
  http://launchpadlibrarian.net/555420796/systemd_237-3ubuntu10.51_237-3ubuntu10.52.diff.gz
  (the patch itself is better visible at
  https://github.com/systemd/systemd/commit/f0831ed2a03fcef582660be1c3b1a9f3e267e656).
  When looking at the systemd git log, though, I noticed another patch
  that was applied shortly after this one, which also seems related but
  was not backported:
  https://github.com/systemd/systemd/commit/04eb582acc203eab0bc5c2cc5e13986f16e09df0

  Since the stopping of our mount unit happens immediately after a
  systemd reload, it actually seems very likely that the inclusion of
  f0831ed2a03fcef582660be1c3b1a9f3e267e656 in the systemd update is what
  causes our woes (though, indeed, the issue is not reliably
  reproducible, so we cannot be sure).

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