[Bug 1831981] Re: aptdaemon frontend locking, locking fixes

[Julian Andres Klode]

Regarding the regression report on aptdaemon/armhf/xenial: aptdaemon
test suite hangs on arm* across all releases, but that's not new (it
hang occasionally before, or it just failed earlier, or in xenial's case
had the wrong depends and never ran).

It's working on all other releases, so it's unlikely to be caused by the
locking fix; and running the test suite and failing due to hanging on
one arch and passing on the others is better than failing everywhere :)

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https://bugs.launchpad.net/bugs/1831981

Title:
  aptdaemon frontend locking, locking fixes

Status in aptdaemon package in Ubuntu:
  Fix Released
Status in aptdaemon source package in Xenial:
  Fix Committed
Status in aptdaemon source package in Bionic:
  Fix Committed
Status in aptdaemon source package in Cosmic:
  Fix Committed
Status in aptdaemon source package in Disco:
  Fix Committed
Status in aptdaemon source package in Eoan:
  Fix Released

Bug description:
  [Impact]
  aptdaemon needs to implement frontend locking to avoid it hijacking dpkg status lock from clients that do implement frontend locking; and in order to be protected from apt clients that acquire the status lock (which they won't do if aptdaemon holds the frontend lock).

  This also change the locking order to lock in APT order, and the
  unlock order becomes the reverse of that so that the behavior is sane
  wrt other clients, as it was somewhat broken.

  [Test case]
  Lock /var/lib/dpkg/lock-frontend and then try do so something with aptdaemon, it should not be possible until you release lock-frontend.

  [Regression potential]
  There may be other race conditions appearing now due to the order of (un)locking changing.

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