Comment 10 for bug 1336663

FWIW, this issue is also present using gdm3 in Ubuntu 16.04. With pam_krb5's debug option set, I see the following during initial login (with successful credential cache construction):

gdm-password]: pam_krb5(gdm-password:auth): pam_sm_authenticate: entry
gdm-password]: pam_krb5(gdm-password:auth): (user cgallek) attempting authentication as <email address hidden>
gdm-password]: pam_krb5(gdm-password:auth): user cgallek authenticated as <email address hidden>
gdm-password]: pam_krb5(gdm-password:auth): (user cgallek) temporarily storing credentials in /tmp/krb5cc_pam_LB8CeL
gdm-password]: pam_krb5(gdm-password:auth): pam_sm_authenticate: exit (success)
gdm-password]: pam_krb5(gdm-password:setcred): pam_sm_setcred: entry (establish)
gdm-password]: pam_krb5(gdm-password:setcred): (user cgallek) initializing ticket cache FILE:/tmp/krb5cc_1000_3BiTY0
gdm-password]: pam_krb5(gdm-password:setcred): pam_sm_setcred: exit (success)

When unlocking the screen, I see the following successful credential refresh, but to the wrong cache filename (/tmp/krb5cc_0):

gdm-password]: pam_krb5(gdm-password:auth): pam_sm_authenticate: entry
gdm-password]: pam_krb5(gdm-password:auth): (user cgallek) attempting authentication as <email address hidden>
gdm-password]: pam_krb5(gdm-password:auth): user cgallek authenticated as <email address hidden>
gdm-password]: pam_krb5(gdm-password:auth): (user cgallek) temporarily storing credentials in /tmp/krb5cc_pam_Dkg5Ip
gdm-password]: pam_krb5(gdm-password:auth): pam_sm_authenticate: exit (success)
gdm-password]: pam_krb5(gdm-password:setcred): pam_sm_setcred: entry (reinit)
gdm-password]: pam_krb5(gdm-password:setcred): (user cgallek) refreshing ticket cache /tmp/krb5cc_0
gdm-password]: pam_krb5(gdm-password:setcred): pam_sm_setcred: exit (success)

The _0 cache that is created has the correct new credential in it, but is obviously not known by any other process as it does not match the earlier version of $KRB5CCNAME. I imagine this is the same issue described above where the $KRB5CCNAME environment variable is not available in the gdm context which unlocks the screen.

It's worth noting that in the GDM case, there appears to be a per-session helper process (gdm-session-worker) used to communicate with the GDM service. This process _does not_ have the $KRB5CCNAME variable set in its environment. Would storing the value in this processes environment fix the problem?

~: ps -ef | grep gdm-session-worker
root 11364 11203 0 Jun25 ? 00:00:00 gdm-session-worker [pam/gdm-password]

~: cat /proc/11364/environ
LANG=en_US.UTF-8PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/binGDM_SESSION_DBUS_ADDRESS=unix:abstract=/tmp/dbus-oTnmcExV