[Bug 1819728] Re: Please backport "fix race between daemon-reload and other commands #8803" to 16.04 (for UC16) and 18.04 (for UC18)

[Michael Vogt]

** Patch added: "debdiff with a port of the fix in PR#11121 to xenial"
   https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1819728/+attachment/5246546/+files/systemd_229-4ubuntu21.19.debdiff

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Title:
  Please backport "fix race between daemon-reload and other commands
  #8803" to 16.04 (for UC16) and 18.04 (for UC18)

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Xenial:
  Triaged
Status in systemd source package in Bionic:
  Fix Committed

Bug description:
  [Impact]
  On Ubuntu Core we recently hit the a race in daemon-reload and systemctl twice. This race is fixed in systemd upstream: "fix race between daemon-reload and other commands #8803".

  Note that this is a general problem in systemd with daemon-reload and
  systemctl commands, we just happen to hit it more often on Ubuntu Core
  but the test-case below explodes just fine on a normal Ubuntu release
  like 16.04 or 18.04 (not on 18.10+ as its fixed there).

  [TEST CASE]
  To reproduce its enough to run:

  for i in $(seq 50); do
    systemctl daemon-reload &
    systemctl start ssh &
  done

  This will result in "systemctl start ssh" hanging in ppoll. With the
  patch applied the hangs go away.

  [REGRESSION POTENTIAL]
  Low, this change is already in the systemd upstream and in use cosmic and later.

  The upstream fix is https://github.com/systemd/systemd/pull/8803
  Full spread run with the fixed systemd in the "core" snap and a regression test: https://github.com/snapcore/snapd/pull/6595

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