[Bug 1796808] Re: frontend locking regression: dpkg::post-invoke scripts can't install packages

[Julian Andres Klode]

The specified tests passed on both:

bionic (1.6.6):

(187/248) Testcase test-frontend-lock:  P P P P P P P P P P P P P P P P
P P P P P P P P P P P P P P P P P P P P P P

xenial (1.2.29):

(165/220) Testcase test-frontend-lock:  P P P P P P P P P P P P P P P P
P P P P P P P P P P P P P P P P P P P P P P P


** Tags removed: verification-needed verification-needed-bionic verification-needed-xenial
** Tags added: verification-done verification-done-bionic verification-done-xenial

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https://bugs.launchpad.net/bugs/1796808

Title:
  frontend locking regression: dpkg::post-invoke scripts can't install
  packages

Status in apt package in Ubuntu:
  Fix Released
Status in apt source package in Xenial:
  Fix Committed
Status in apt source package in Bionic:
  Fix Committed
Status in apt source package in Cosmic:
  Fix Released

Bug description:
  [Impact]
  The switch to frontend locking in 1.2.28/1.6.5/1.7~ caused scripts registered in

  DPkg::Pre-Install-Pkgs
  DPkg::Pre-Invoke
  DPkg::Post-Invoke

  To be run with the frontend lock held. This caused problems with some
  installer packages like libdvd-pkg which install a locally built
  package in such a hook.

  To reduce the impact, a fix has been applied in 1.7.0 which exports
  the DPKG_FRONTEND_LOCKED variable when running those scripts, allowing
  dpkg to be run from those scripts.

  [Test case]
  Make sure that the test-frontend-lock test run by autopkgtest succeeds. This checks that:

  (1) the variable is correctly exported to those scripts
  (2) the frontend lock is still held
  (3) dpkg can be run from the post-invoke script

  [Regression potential]
  Depending on the script (for example, if the script starts a daemon manually or something like that), DPKG_FRONTEND_LOCKED might leak to other processes which might survive the apt call and thus revert surviving process to not using frontend locking.

  [Other info]
  This fix is for backwards compatibility only. Starting with the dd cycle, it will be dropped for the Pre-Install-Pkgs and Pre-Invoke scripts. Potentially for post-invoke as well, if we find a better solution for it.

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