bug#35874: “guix pull” fails on setlocale

[Ricardo Wurmus <ricardo.wurmus@mdc-berlin.de>]

This is a store corruption bug.

The problem appears to be that I accidentally ran the daemon with the
wrong GUIX_DATABASE_DIRECTORY.  The localstatedir is /gnu/var, not /var.

In an attempt to simplify my complicated cluster setup, I wanted to
switch from the git checkout to “guix pull”.  I was able to use the Guix
client from “guix pull”, but not the daemon, because of the
localstatedir difference.

When I started the daemon from “guix pull” without having set
GUIX_DATABASE_DIRECTORY and I asked Guix to build something I noticed
this error message:

   guix pull: error: cannot unlink `/gnu/store/h90vnqw0nwd0hhm1l5dgxsdrigddfmq4-glibc-2.28/lib/gconv': Directory not empty

Wait, “unlink”?  Of course: when a build is not found in the database,
but the store contains an item of the same name the daemon will remove
the existing directory.

In my case, the daemon did not realize that it couldn’t ever find
anything interesting in the database, because it looked in the wrong
localstate directory.  So it partially removed store items and then
aborted, leaving the store in a broken state.

Can we make the daemon detect that its understanding of the site differs
from that of the Guix client?

--
Ricardo