lightdm login failure after timeout/suspend

[Russell Green]

up to date cosmic, the install is fresh and configuration is stock outside of what is mentioned in reproduction steps. lightdm fails to login when the user password has been removed and reauthentication is required after suspend or timeout. the desktop will flash on screen before failing back to lightdm greeter.

steps to reproduce:

prerequisite -

$ passwd -d <user>

(the root account was also unlocked at this point.)

1. boot machine
2. at lightdm greeter login in to <user> account
3. success
3. suspend session or await timeout.
4. at lightdm greeter login in to <user> account
5. login fails, desktop flashes, fallback to lightdm

workaround:

1. restart lightdm

This bug is not triggered by loging out and back in. Lightdm will also accept any input as a password on step 2 and successfully authenticate, unsure if that is expected behaviour.

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: lightdm 1.26.0-0ubuntu1 [modified: usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf]
ProcVersionSignature: Ubuntu 4.15.0-29.31-generic 4.15.18
Uname: Linux 4.15.0-29-generic x86_64
ApportVersion: 2.20.10-0ubuntu7
Architecture: amd64
CurrentDesktop: LXQt
Date: Wed Aug 8 00:13:46 2018
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)