[Bug 1789637] Re: Proper support for frontend lock

[Brian Murray]

Hello Balint, or anyone else affected,

Accepted unattended-upgrades into xenial-proposed. The package will
build now and be available at https://launchpad.net/ubuntu/+source
/unattended-upgrades/1.1ubuntu1.18.04.7~16.04.0 in a few hours, and then
in the -proposed repository.

Please help us by testing this new package.  See
https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how
to enable and use -proposed.  Your feedback will aid us getting this
update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug,
mentioning the version of the package you tested and change the tag from
verification-needed-xenial to verification-done-xenial. If it does not
fix the bug for you, please add a comment stating that, and change the
tag to verification-failed-xenial. In either case, without details of
your testing we will not be able to proceed.

Further information regarding the verification process can be found at
https://wiki.ubuntu.com/QATeam/PerformingSRUVerification .  Thank you in
advance for helping!

N.B. The updated package will be released to -updates after the bug(s)
fixed by this package have been verified and the package has been in
-proposed for a minimum of 7 days.

** Changed in: unattended-upgrades (Ubuntu Xenial)
       Status: Confirmed => Fix Committed

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

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https://bugs.launchpad.net/bugs/1789637

Title:
  Proper support for frontend lock

Status in unattended-upgrades package in Ubuntu:
  Fix Released
Status in unattended-upgrades source package in Xenial:
  Fix Committed
Status in unattended-upgrades source package in Bionic:
  Fix Released

Bug description:
  [Impact]

  Apt and dpkg implemented the Frontend Locking API and unattended-
  upgrades needs to adopt it to not leave the packaging system unlocked
  while passing control to python-apt and dpkg to perform package
  installations and removals. Leaving the packaging system unlocked
  caused many crashes of u-u when other tools took the lock and in the
  worse case let other package management tools operate on dpkg's
  database breaking systems.

  The change takes advantage of python-apt's new API and keeps the
  frontend lock during the run of u-u and unlocks only the inner locks
  for committing changes.

  [Test case]

  Run strace unattended-upgrades to upgrade several packages and check
  that lock-frontend is acquired at the beginning and not released until
  the end (not reacquired repeatedly).

  Unfixed u-u's output is like this:
  # strace unattended-upgrade --verbose --dry-run 2>&1 | grep lock
  openat(AT_FDCWD, "/var/run/unattended-upgrades.lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 4
  openat(AT_FDCWD, "/var/lib/dpkg/lock-frontend", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 5
  openat(AT_FDCWD, "/var/lib/dpkg/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 6
  openat(AT_FDCWD, "/var/lib/dpkg/lock-frontend", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 5
  openat(AT_FDCWD, "/var/lib/dpkg/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 6
  openat(AT_FDCWD, "/var/cache/apt/archives/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 93
  openat(AT_FDCWD, "/var/lib/dpkg/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 6
  openat(AT_FDCWD, "/var/lib/dpkg/lock-frontend", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 5
  openat(AT_FDCWD, "/var/lib/dpkg/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 6
  openat(AT_FDCWD, "/var/lib/dpkg/lock-frontend", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 5
  openat(AT_FDCWD, "/var/lib/dpkg/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 6
  ...

  Fixed u-u's output is like that:
  # strace   unattended-upgrade --dry-run --verbose 2>&1 | grep lock
  openat(AT_FDCWD, "/var/run/unattended-upgrades.lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 4
  openat(AT_FDCWD, "/var/lib/dpkg/lock-frontend", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 8
  openat(AT_FDCWD, "/var/lib/dpkg/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 10
  openat(AT_FDCWD, "/var/cache/apt/archives/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 57
  openat(AT_FDCWD, "/var/lib/dpkg/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 10
  openat(AT_FDCWD, "/var/cache/apt/archives/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 69
  openat(AT_FDCWD, "/var/lib/dpkg/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 10
  openat(AT_FDCWD, "/var/cache/apt/archives/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 70
  openat(AT_FDCWD, "/var/lib/dpkg/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 10
  ...

  [Regression potential]

  Incorrect lock API usage can make unattended-upgrades to crash, but it
  is unlikely to hang since the lock handling operations are non-
  blocking. Failing to reserve the locks can also cause problems by
  allowing other package management tools to install packages in
  parallel, but this problem existed in the past, too.

  [Additional Info]
  This is part of a wider series of changes 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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