Comment 0 for bug 1654600

The systemd unit file unattended-upgrades.service is used to stop a running unattended-upgrade
process during shutdown. This unit file is running together with all filesystem
unmount services.

The unattended-upgrades service checks if the lockfile for unattended-upgrade
(in /var/run) exists, and if it does, there is an unattended-upgrade in progress
and the service will wait until it finishes (and therefore automatically wait at
shutdown).

However, if /var is a separate filesystem, it will get unmounted even though /var/run
is a tmpfs that's still mounted on top of the /var/run directory in the /var filesystem.
The unattended-upgrade script will fail to find lockfile, sleeps for 5 seconds, and
tries to check the lockfile again. After 10 minutes (the default timeout), it will finally
exit and the system will continue shutdown.

The problem is the error handling in /usr/share/unattended-upgrades/unattended-upgrade-shutdown
where it tries to lock itself:

    while True:
        res = apt_pkg.get_lock(options.lock_file)
        logging.debug("get_lock returned %i" % res)
        # exit here if there is no lock
        if res > 0:
            logging.debug("lock not taken")
            break
        lock_was_taken = True

The function apt_pkg.get_lock() either returns a file descriptor, or -1 on an error.
File descriptors are just C file descriptors, so they are always positive integers.
The code should check the result to be negative, not positive. I have attached a patch
to reverse the logic.

Additional information:

1)
Description: Ubuntu 16.04.1 LTS
Release: 16.04

2)
unattended-upgrades:
  Installed: 0.90ubuntu0.3
  Candidate: 0.90ubuntu0.3
  Version table:
 *** 0.90ubuntu0.3 500
        500 http://nl.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        500 http://nl.archive.ubuntu.com/ubuntu xenial-updates/main i386 Packages
        100 /var/lib/dpkg/status
     0.90 500
        500 http://nl.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
        500 http://nl.archive.ubuntu.com/ubuntu xenial/main i386 Packages
3)
Fast reboot
4)
Very slow reboot (after a 10 minutes timeout)