Comment 1 for bug 1839431

Tested on Debian Buster. Seems to only happen on a fat client with some
shells. On thin clients, it simply fails to log in. Possibly related to:

  https://bugs.debian.org/490897

Confirmed behavior logging in as root:

  fish, tcsh, csh

Following shells log in as the correct user:

  zsh, dash, /bin/sh (as dash), bash, mksh, ksh

Login hangs due to some unrelated problem:

  sash

With some shells, the LDM_USERNAME environment variable remains unset,
and so in /usr/share/ldm/rc.d/X95-run-x-session ends up calling:

  su - ${LDM_USERNAME} ...

And ends up as root.

The quick fix would be to check if LDM_USERNAME is set and at least
error out if so, rather than granting root.

Still would be better to identify exactly where in the code we're
expecting LDM_USERNAME to be set and fix it or error out there.

live well,
  vagrant