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[Launchpad Bug Tracker <1795407@xxxxxxxxxxxxxxxxxx>]

This bug was fixed in the package python-apt - 1.6.3

---------------
python-apt (1.6.3) bionic; urgency=medium

  * Frontend locking and related locking improvements (LP: #1795407)
    - apt.Cache: Keep / Re-establish the system lock in commit()
    - apt.Cache: Keep archive locked during commit()/ in fetch_archives()
    - apt.Cache: Reinstate locks in a finally / run dpkg inside try
    - Introduce frontend locking
    - Convert apt.Cache.commit and apt_pkg.DepCache.commit to FE lock
  * Other changes to make that work:
    - Cherry-pick apt_pkg.Error type hint from 1.7
    - travis CI: bionic only; stretch does not have FE locking nor PPA
  * As always, updated mirror lists

 -- Julian Andres Klode <juliank@xxxxxxxxxx>  Mon, 01 Oct 2018 16:00:14
+0200

** Changed in: python-apt (Ubuntu Bionic)
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1795407

Title:
  python-apt frontend locking

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

Bug description:
  [Impact]
  Add support to python-apt for frontend locking. This is a bit more complicated, and also requires some other restructuring:

  (1) The archives lock was only taken for a short time, when it should
  have been kept for the duration of an installation, as otherwise debs
  could disappear from archives/ while the install is running.

  (2) There was no lock handling at all. Tools essentially acquire the
  lock, and then release it before commit().

  The fix is based on the apt fix, which makes apt_pkg.SystemLock()
  manage both the frontend and the normal lock, and allows code to call
  apt_pkg.pkgsystem_unlock_inner() and apt_pkg.pkgsystem_lock_inner()
  around dpkg invocations to release the inner lock.

  Cache.commit() takes care of locking itself now. It releases the inner
  lock as needed for dpkg invocations. It also now requires
  apt_pkg.SystemLock to be hold - if it is not, it will acquire it
  itself.

  [Test case]
  1. Mark a package in the cache for install/removal and commit() - FE lock should be taken while dpkg is running.
  2. Mark a package in the cache for install/removal and commit() while holding the lock. Releasing the lock afterwards should still work correctly, and the FE lock should never be released
  3. Observe that the archive lock is not released until the last dpkg run

  The locking behavior can be observed via strace.

  [Regression potential]
  The dpkg lock changed the most: Some lock counting might go wrong and programs might fail as a result. Any problems will be isolated, though - dpkg still acquires its locks, and if a lock miscount happens, apt would not tell dpkg to skip acquiring the frontend lock, so even if we were losing the lock anywhere, dpkg would still try to acquire it and not run unlocked.

  The archive lock done by commit()/fetch_archives() in apt.Cache() is
  now hold as long as the fetcher object exists though (as it uses the
  fetcher's get_lock method), which might cause unexpected behavior in
  apps not expecting that.

  The lists/ lock is unaffected by the changes, so there should not be
  any regressions when it comes to updating index files.

  [Other info (1)]
  CI changes: There were some improvements to type hints for mypy needed to make type checks pass, and changes to the CI's Dockerfile to pull a new apt from the apt stable PPA (as a new enough apt is only in unapproved atm). Neither of those files are used anywhere outside of the package, so they should not cause any problems.

  [Other info (2)]
  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

  - This SRU depends on the apt SRU above, and breaks unattended-
  upgrades in bionic without its SRU due to a bug in the u-u code in
  handling the new API.

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