MAN. That took hours to figure out. I simply ran a
dnf upgrade on my Fedora 25 system (using an NVidia GeForce GTX 960 with the proprietary drivers) and, when it rebooted, I was greeted with a failure to start GDM. The system would simply hang at the text list of services being started.
Switching into another terminal, I found error messages such as these repeated with every effort to start GDM:
Dec 01 15:07:04 hostname gdm: GLib: g_hash_table_find: assertion ‘version == hash_table->version’ failed
Dec 01 15:06:51 hostname abrt-hook-ccpp: Process 1455 (gnome-session-failed) of user 42 killed by SIGSEGV – dumping core
Being segmentation faults, I suspected I needed to reinstall my NVidia drivers (which seems to happen once in a while with kernel or mesa updates). I tried this to no avail. Many times.
Because I’m a moron and was filtering my journal for error-level messages only, I didn’t notice this helpful message was also being logged:
Dec 01 15:06:51 hostname kernel: gnome-session-f: segfault at 0 ip 00007f23d0001579 sp 00007ffd1a6248c0 error 4 in libgtk-3.so.0.2200.4[7f23cfd23000+6f0000]
So I started checking around for that same segmentation fault in other users, and lo and behold I hit upon this over at the Arch Linux forums. If I’m reading the tail end of the related bug report properly, it looks like a bug was fixed upstream, but given that Fedora 25 is still hosting mutter-3.22.2-1.fc25 for download, we may still be waiting on the package which includes the fix. Or, it may be that the fix included in package 3.22.2-1 is incomplete (a user at the tail end of the forum link reports just a few days ago using the package which includes the apparent fix to no avail).
So anyway, if you, too, are experiencing GDM/GNOME session failures with the proprietary NVidia drivers on Fedora 25, try simply downgrading mutter:
[you@yourplace ~]$ sudo dnf downgrade mutter
That brought me back to mutter-3.22.1-8.fc25.x86_64 and all is well for me.
Now to get rid of lightdm and roll back all my insane troubleshooting installations…