[Bug 1829860] Re: APT unlocks in same order as it locks

[Julian Andres Klode]

Same for bionic 1.6.12:

openat(AT_FDCWD, "/var/lib/dpkg/lock-frontend", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 4
fcntl(4, F_SETFD, FD_CLOEXEC)           = 0
fcntl(4, F_SETLK, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0
openat(AT_FDCWD, "/var/lib/dpkg/lock", O_RDWR|O_CREAT|O_NOFOLLOW, 0640) = 5
fcntl(5, F_SETFD, FD_CLOEXEC)           = 0
fcntl(5, F_SETLK, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0


end of run:

close(5) = 0
close(4) = 0
close(3) = 0

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

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https://bugs.launchpad.net/bugs/1829860

Title:
  APT unlocks in same order as it locks

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

Bug description:
  [Impact]
  APT releases the locks in the same order it acquires them, rather than reverse order. Given that we have no waiting for locks, this is not _super_ problematic, but it might be wrong: You'd get a lock failure on dpkg's lock, rather than lock-frontend.

  [Test case]
  Watch lock release with strace and see that it unlocks the right way.

  [Regression potential]
  Some other locking races or something?

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