Change:
I just upgraded to nixos 21.11, and gnome-flashback core dumps on login. Steps to reproduce the behavior: using "Gnome Flashback (Metacity)" to witness the coredump Coredump: If I had to guess, the first stack trace seem to blame: The other are just waiting on stuff. I tried to get debug symbols using the instructions in , but it does not seem to work. Please run and paste the result.
By the way, same thing on unstable.
Do you mean you tried adding the following to your configuration, tried logging into a Flashback session and then looked at the appropriate trace by and then entered ? What specificity does not work?

Discussion:
gitolite-fcgiwrap = handleTest ./gitolite-fcgiwrap.nix {}; glusterfs = handleTest ./glusterfs.nix {}; gnome = handleTest ./gnome.nix {}; <ins> gnome-flashback = handleTest ./gnome-flashback.nix {}; </ins> gnome-xorg = handleTest ./gnome-xorg.nix {}; go-neb = handleTest ./go-neb.nix {}; gobgpd = handleTest ./gobgpd.nix {};

<ins> import ./make-test-python.nix ({ pkgs, lib, ...} : { name = "gnome-flashback"; meta = with lib; { maintainers = teams.gnome.members ++ [ maintainers.chpatrick ]; }; nodes.machine = { nodes, ... }: let user = nodes.machine.config.users.users.alice; in { imports = [ ./common/user-account.nix ]; services.xserver.enable = true; services.xserver.displayManager = { gdm.enable = true; gdm.debug = true; autoLogin = { enable = true; user = user.name; }; }; services.xserver.desktopManager.gnome.enable = true; services.xserver.desktopManager.gnome.debug = true; services.xserver.desktopManager.gnome.flashback.enableMetacity = true; services.xserver.displayManager.defaultSession = "gnome-flashback-metacity"; }; testScript = { nodes, ... }: let user = nodes.machine.config.users.users.alice; uid = toString user.uid; xauthority = "/run/user/${uid}/gdm/Xauthority"; in '' with subtest("Login to GNOME Flashback with GDM"): machine.wait_for_x() # Wait for alice to be logged in" machine.wait_for_unit("default.target", "${user.name}") machine.wait_for_file("${xauthority}") machine.succeed("xauth merge ${xauthority}") # Check that logging in has given the user ownership of devices assert "alice" in machine.succeed("getfacl -p /dev/snd/timer") with subtest("Wait for Metacity"): machine.wait_until_succeeds( "pgrep metacity" ) machine.sleep(20) machine.screenshot("screen") ''; }) </ins>

dontWrapGApps = true; # We want to do the wrapping ourselves. # gnome-flashback and gnome-panel need to be added to XDG_DATA_DIRS so that their .desktop files can be found by gnome-session. <ins> # We need to pass the --builtin flag so that gnome-session invokes gnome-session-binary instead of systemd. # If systemd is used, it doesn't use the environment we set up here and so it can't find the .desktop files. </ins> preFixup = '' makeWrapper ${gnome-session}/bin/gnome-session $out  <del> --add-flags "--session=gnome-flashback-${wmName}"  </del> <ins> --add-flags "--session=gnome-flashback-${wmName} --builtin"  </ins> --set-default XDG_CURRENT_DESKTOP 'GNOME-Flashback:GNOME'  --prefix XDG_DATA_DIRS : '${lib.makeSearchPath "share" ([ wmApplication gnomeSession gnome-flashback ] ++ lib.optional enableGnomePanel gnome-panel)}'  "''${gappsWrapperArgs[@]}"