Upgrading from Kubuntu 14.10 to 15.10 - can't log in

School has been intense and I haven't felt comfortable making the switch from Kubuntu 14.10 to 15.10 on my main laptop.  This involves transitioning from KDE4 to KDE5 with all of the attendant breakages that will inevitably occur.

But, since 14.10 support ended almost 6 months ago (Canonical only promises 9 months of package updates for non-LTS releases) and I'm going to end up on KDA/Plasma5 no matter what, I decided to take the plunge and do the in-place distribution upgrade.  I have a separate /home partition so I could have simply wiped / and reinstalled directly, which is the recommended process.  Eh, call me a masochist.

Note that this ALSO switched from upstart (a SysV-based init daemon) to systemd.  And a new session manager, SDDM, away from lightDM.  Lots of scary changes at once.

Note that you can't go from 14.10 to 15.10, you MUST go thru 15.04 (Vivid).  The upgrade to 15.04 went well, surprisingly, and I was able to boot back into the new system with no problems.  I didn't fiddle around in 15.04 - I just went straight into the next distribution upgrade to 15.10 (Wily - sorry, Vivid, I hardly knew ye).

Then the pain began.  It wouldn't boot for one thing - just came up in a black screen.  I ended up doing a recovery mode boot and could at least get the recovery menu to come up.  Whew.

Most of the problems for this laptop are related to the Optimus configuration.  So, I tried switching back to the intel driver (prime-select intel).  Still no joy.

I ended up uninstalling xorg-edgers and graphics-drivers PPAs and trying various incarnations of the Ubuntu-packaged nVIDIA drivers (nvidia-35[258]).  After much fiddling, I ended up being able to get a greeter, but only in intel mode.

However, I couldn't log in.  The greeter would go away and give me a black screen, and then 10s later the greeter would come back.  No change in the "failsafe" login.

I couldn't find any obvious issues in the logs, but I knew that updating KDE can be fragile and previous configurations can confuse newer components.  So, I tried creating  new user via useradd and seeing what would happen.

After a reboot, the greeter came up and I was able to log in via the new user.  HALLELUJAH!

At that point I went back to my original user account and began bisecting all of my configuration, specifically the entries in .local, .config, and .kde.  I would move a configuration directory into a backup area and reboot.  If plasma came up, then I'd try a different "configuration" of configuration directories until I found a config directory that broke the login.

After much directory moving and renaming, I ended up narrowing it down to kded5 in .local/share.  So, I simply removed that config and I was able to get back in.

WHEW.

I still can't re-enable the nvidia driver.  prime-select nvidia causes the same login problem, except that instead of looping back to the greeter, it just hangs.  Joy.  More testing to come.