[Bug 1407841] [NEW] docker.io package fails install if docker group is pre-existing

[Charles Butler]

Public bug reported:

When installing the docker.io package, it fails during installation with
a message akin to the following:

stderr: addgroup: The group `docker' already exists and is not a system group. Exiting.
dpkg: error processing package docker.io (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 docker.io
E: Sub-process /usr/bin/dpkg returned an error code (1)
 
This was originally discovered during charm development where a user can switch from the canonical docker package to the docker maintained package 'lxc-docker'. With the upstream package, they are not maintaining a docker-group in the charm, and allows for installation regardless of the presence of the group. While the docker.io package will halt installation in a panic if it exists - regardless if its a system group or not. Just the presence alone appears to be the key factor in the failure.

I don't think this behavior is correct - as a user may have created a
docker group themselves, and expect to install the docker.io package
from the repository after removing the latest - due to a bug or any
number of reasons.

I'm linking the commit to the charm and the user stack trace for
additional validation/investigation if required:

https://github.com/chuckbutler/docker-
charm/commit/59946a7bd6bfa3cc050de3b27e62d3207c71be47

https://gist.github.com/slatunje/735e1e5132de02a3ad1d

** Affects: docker.io (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: papercut

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https://bugs.launchpad.net/bugs/1407841

Title:
  docker.io package fails install if docker group is pre-existing

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