plymouth hang - login to console not possible

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Environment

SUSE Linux Enterprise Server 15
SUSE Linux Enterprise Server 15 SP1
SUSE Linux Enterprise Server 12 SP4

Situation

The situation is happening when using a graphical boot target (with GDM) but no display/monitor is connected.
Plymouth might be unable to quit and hangs.
This affects the start of systemd services that are normally started subsequent to Plymouth,
in this case here: systemd starting X Display Manager.

At the end of booting up, the display manager fails to load and console login isn't possible.
Remote login is possible so ssh can be used to login and diagnose the issue.

"zypper ps -s" shows the plymouth-start service hanging, unable to quit:

# zypper ps -s
The following running processes use deleted files:

PID | PPID | UID | User | Command | Service
----+------+-----+------+---------------------+---------------
351 | 1 | 0 | root | plymouthd (deleted) | plymouth-start

The command "systemctl list-jobs" shows the plymouth-quit-wait.service running:
# systemctl list-jobs

JOB      UNIT                        TYPE  STATE  
<job_id> plymouth-quit-wait.service  start running

Resolution

Add plymouth.enable=0 to the kernel boot options to disable plymouth.
After a successful test the option can be added permanently to the grub2 bootloader (through YaST or directly to the GRUB_CMDLINE_LINUX_DEFAULT in "/etc/default/grub").

Another option is to run the command "plymouth quit" when the system is running into this state.
Also connecting the machine to a monitor might be an option for some.

Cause

Plymouth and gdm/X is a race that can't really be fixed.
Plymouth as a graphical boot system and logger isn't really needed, unless a graphical boot animation, splash screen is needed on boot up. Hence disabling the service is considered a useful workaround.

Disclaimer

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