[Bug 1789637] Re: Proper support for frontend lock

[Balint Reczey]

There were no relevant changes between .1 and .2 thus marking this bug
as verified on Xenial again.


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