lightdm + systemd: PAM stack is processed twice

[Christian Kastner]

Hi,

pam_mount (from src:libpam-mount) is a PAM module for mounting filesystems.

In #739676 [1], it is evident that when using lightdm and systemd,
pam_mount is executed twice upon login. The first execution is via
lightdm, and it is successful. The second invocation seems to be via
logind, and fails (I believe this is due to pam_mount internals).
However, it also messes with pam_mount's internal login counter, which
confuses pam_mount and results in volumes not being unmounted at logoff
time.

Can anyone enlighten me where this second PAM stack processing is coming
from? And whether this can safely be inhibited?

For the curious, a snippet of an auth.log documenting the above can be
found in the linked bug report.

Regards,
Christian

[1] http://bugs.debian.org/739676