lightdm post-install script doesn't create /var/lib/lightdm if user already exists, preventing lightdm from starting

[Stephen Lin]

I'm not sure if this really counts as a bug or if it's just an unsupported configuration, but the post-install script (lightdm.postinst) doesn't ensure that "/var/lib/lightdm" exists and has the correct permissions if the user already exists, since it is relying upon "adduser" to create the home directory and only does so if the "lightdm" user doesn't already exist. This prevents lightdm from starting and leads to a fallback X session being created instead, which doesn't give any helpful information about the underlying problem.

The reason I already had a lightdm user in passwd is because I was bootstrapping Ubuntu in a chroot and copied passwd/shadow/group/gshadow into it before installing packages, since I wanted to make sure UID/GIDs would be consistent between the two installations. It would be nice if this case could be more gracefully handled, since I don't think it's all that unusual.