Release upgrade fails due to apt-btrfs-snapshot not being able to take a snapshot

[Kees Bakker]

Upgrading 16.04, with btrfs and apt-btrfs-snapshot fails with
a message that it cannot create a snapshot.

The package apt-btrfs-snapshot (0.3.4.2) is installed. Notice that
on Xenial you need to create a symlink /sbin/btrfs -> /bin/btrfs in
order to get the snapshooting to work with apt.

From the error messages it looks like it is trying to create a second
snapshot with the same name. Perhaps it's a race condition.

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Upgrading
Create a snapshot of '/tmp/apt-btrfs-snapshot-mp-s2c_k3qa/@' in '/tmp/apt-btrfs-snapshot-mp-s2c_k3qa/@apt-snapshot-release-upgrade-bionic-2018-05-17_13:59:08'
Fetched 0 B in 0s (0 B/s)
Supported
Create a snapshot of '/tmp/apt-btrfs-snapshot-mp-m5onzpjo/@' in '/tmp/apt-btrfs-snapshot-mp-m5onzpjo/@apt-snapshot-2018-05-17_13:59:09'
Supported
Create a snapshot of '/tmp/apt-btrfs-snapshot-mp-l8vw4ipx/@' in '/tmp/apt-btrfs-snapshot-mp-l8vw4ipx/@apt-snapshot-2018-05-17_13:59:09/@'
Supported
Create a snapshot of '/tmp/apt-btrfs-snapshot-mp-p4ob7l8q/@' in '/tmp/apt-btrfs-snapshot-mp-p4ob7l8q/@apt-snapshot-2018-05-17_13:59:09/@'
ERROR: cannot snapshot '/tmp/apt-btrfs-snapshot-mp-p4ob7l8q/@': File exists
Exception during pm.DoInstall(): E:Problem executing scripts DPkg::Pre-Invoke 'if [ -x /usr/bin/apt-btrfs-snapshot ] && apt-btrfs-snapshot supported; then apt-btrfs-snapshot snapshot; fi ', E:Sub-process returned an error code
No pending crash reports. Try --help for more information.

Could not install the upgrades

The upgrade has aborted. Your system could be in an unusable state. A
recovery will run now (dpkg --configure -a).

Upgrade complete

The upgrade has completed but there were errors during the upgrade
process.

To continue please press [ENTER]
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[Kees Bakker (keestux)]

The quick workaround is to remove the symlink in /sbin.

You can/should make a snapshot before removing the symlink.