[Bug 1796081] Re: dpkg frontend locking

[Jarno Suni]

I think it can be made to work. I do not know how apt/dpkg is
implemented, but I made an example by Bash:

script.sh file:

#!/bin/bash
#
exec {var}>/tmp/lock 
flock -n $var || { echo already locked; exit 1; }
LOCK= /path/to/subscript.sh  # edit this path

subscript.sh file:

#!/bin/bash
#
[[ -v LOCK ]] && { kill $PPID; sleep 3; } # kill the caller
exec {var}>/tmp/lock
flock -n $var && echo was not locked || echo was locked


The subscript.sh above kills the caller (if LOCK variable is set in its environment). The subscript shows the lock is set even thereafter until the subscript finishes.

Anyway I am missing something: Even if I lock /var/lib/dpkg/lock or
/var/lib/dpkg/lock-frontend this way (instead of locking /tmp/lock),
that does not prevent installing a package by apt install or dpkg
--install. But if I run Synaptic package manager by pkexec
"/usr/sbin/synaptic", it locks the files and prevents installing by apt
or dpkg from outside.

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https://bugs.launchpad.net/bugs/1796081

Title:
  dpkg frontend locking

Status in dpkg package in Ubuntu:
  Fix Released
Status in dpkg source package in Xenial:
  Fix Released
Status in dpkg source package in Bionic:
  Fix Released
Status in dpkg source package in Cosmic:
  Fix Released

Bug description:
  [Impact]
  Frontends of dpkg such as apt and programs using the apt libraries currently acquire the dpkg "lock" lock file. They need to release it before running dpkg, as dpkg also acquires it. Therefore, there is a race condition: In case the application needs to run dpkg multiple times, another application could steal the lock from under it, and the running application would fail in the middle of the install, potentially rendering the system broken.

  This fixes the problem by introducing an additional "lock-frontend"
  file that frontends do not release when calling dpkg. When dpkg is not
  called by a frontend using that file, it will try to acquire the
  frontend lock as well, preventing it from interfering with such
  frontends.

  [Test case]
  1. Hold lock, check that dpkg fails to run
  2. Hold frontend lock, check that dpkg fails to run
  3. Hold frontend lock, run dpkg with DPKG_FRONTEND_LOCKED set, it should succeed

  [Regression potential]
  This is an isolated change adding a new lock file. Therefore, regressions can only be expected in the form of that locking failing.

  [Other info]
  This is part of a wider series of SRUs for frontend locking

  - dpkg (bug 1796081)
  - apt (bug 1781169)
  - python-apt (bug 1795407)
  - packagekit (bug 1795614)
  - unattended-upgrades (bug 1789637)
  - aptdaemon (no bug filed yet)

  Further details about frontend locking can be found in
  https://lists.debian.org/debian-dpkg/2017/01/msg00044.html

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