Bug 1955894

Description of problem:
Upon upgrading from Fedora 33 to 34, SDDM will no longer start, instead leaving VT1 displaying the startup log indefinitely.

Version-Release number of selected component (if applicable): 0.19.0-13.fc34.x86_64

How reproducible: Always

Steps to Reproduce:
1. Upgrade SDDM to the latest version on F33
2. Upgrade F33 to F34
3. Boot system configured with SDDM as display manager

Actual results:
SDDM does not start

Expected results:
SDDM starts and shows the login screen

Additional info:
Here is the entirety of the log present in journalctl for sddm.service:

May 01 00:22:35 sheen systemd[1]: Started Simple Desktop Display Manager.
May 01 00:22:35 sheen sddm[7825]: Failed to read display number from pipe
May 01 00:22:35 sheen sddm[7825]: Attempt 1 starting the Display server on vt 1 failed
May 01 00:22:37 sheen sddm[7825]: Failed to read display number from pipe
May 01 00:22:37 sheen sddm[7825]: Attempt 2 starting the Display server on vt 1 failed
May 01 00:22:39 sheen sddm[7825]: Failed to read display number from pipe
May 01 00:22:39 sheen sddm[7825]: Attempt 3 starting the Display server on vt 1 failed
May 01 00:22:39 sheen sddm[7825]: Could not start Display server on vt 1

It seems the display server is not starting. I have no insight into why this is; I have found no traces of a log produced by SDDM anywhere else, nor do I know how SDDM's startup process works. I know SDDM is supposed to use Wayland now, but I have no idea how that is configured, considering it isn't documented on the change page[1] despite it alluding to the ability to switch back to X11. Thus, I can't tell whether SDDM is attempting to use Wayland or X11.

I noticed that SDDM also failed to relaunch when logging out of my Plasma session in F33, prior to preparing for the upgrade. At the time, I dismissed it as a fluke. The journal from that boot is cut off, but "Failed to read display number from pipe" and "Could not start Display server on vt 1" are present. Note that I had not yet upgraded SDDM from its previously installed version at all. (The last package upgrade on this system was over a month ago, while being logged into Plasma for two months straight; it's possible that the SDDM package was upgraded that previous time, but I'm not familiar with its update history.)

Plasma starts and functions normally using both X11 and Wayland. However, selecting "log out" in the Wayland session does not terminate the remaining Wayland clients (or at least the KDE background processes) gracefully, and they start to consume massive amounts of CPU, forcing a `loginctl terminate-session` or a game of htop whack-a-mole. I don't know what's up with that, but it's probably unrelated. Fortunately, getting all aspects of Wayland working is not a goal for me.

[1]: https://fedoraproject.org/wiki/Changes/WaylandByDefaultForPlasma