Change to sssd.nix causes nixos build scripts to fail with error

[benwbooth]

Describe the bug
A clear and concise description of what the bug is.

I created a Dockerfile for building a custom nixos Docker image out of the nixos/nix Docker image on Docker hub. A few weeks ago it was building fine using nixos/nix:2.3.4. But I noticed that with the latest nixos/nix:latest, I was getting the following error message when building nixos:

#10 1.430 error: while evaluating the attribute 'config.system.build.toplevel' at /root/.nix-defexpr/channels/nixpkgs/nixos/modules/system/activation/top-level.nix:280:5:
#10 1.430 while evaluating 'foldr' at /root/.nix-defexpr/channels/nixpkgs/lib/lists.nix:52:20, called from /root/.nix-defexpr/channels/nixpkgs/nixos/modules/system/activation/top-level.nix:130:12:
#10 1.430 while evaluating 'fold'' at /root/.nix-defexpr/channels/nixpkgs/lib/lists.nix:55:15, called from /root/.nix-defexpr/channels/nixpkgs/lib/lists.nix:59:8:
#10 1.430 while evaluating anonymous function at /root/.nix-defexpr/channels/nixpkgs/nixos/modules/system/activation/top-level.nix:123:50, called from undefined position:
#10 1.430 while evaluating the attribute 'assertion' at /root/.nix-defexpr/channels/nixpkgs/nixos/modules/config/nsswitch.nix:101:9:
#10 1.430 while evaluating the attribute 'system.nssModules.path' at /root/.nix-defexpr/channels/nixpkgs/nixos/modules/config/nsswitch.nix:24:11:
#10 1.430 while evaluating 'makeSearchPathOutput' at /root/.nix-defexpr/channels/nixpkgs/lib/strings.nix:142:5, called from /root/.nix-defexpr/channels/nixpkgs/nixos/modules/config/nsswitch.nix:24:18:
#10 1.430 while evaluating 'makeSearchPath' at /root/.nix-defexpr/channels/nixpkgs/lib/strings.nix:122:5, called from /root/.nix-defexpr/channels/nixpkgs/lib/strings.nix:142:11:
#10 1.430 while evaluating the attribute 'mergedValue' at /root/.nix-defexpr/channels/nixpkgs/lib/modules.nix:415:5:
#10 1.430 The option value system.nssModules' in /root/.nix-defexpr/channels/nixpkgs/nixos/modules/services/misc/sssd.nix' is not of type `list of paths'.

#10 ERROR: executor failed running [/bin/sh -c export NIX_PATH=$NIX_PATH:/root/.nix-defexpr/channels:nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels && export NIXOS_CONFIG=/etc/nixos/configuration.nix && nix-env --set -I nixpkgs=$HOME/.nix-defexpr/channels/nixpkgs -f '<nixpkgs/nixos>' -p /nix/var/nix/profiles/system -A system --show-trace]: runc did not terminate sucessfully

Looking at the changes to nixpkgs/nixos/modules/services/misc/sssd.nix, I discovered that changing line 72 from

system.nssModules = pkgs.sssd;

back to

system.nssModules = optional cfg.enable pkgs.sssd;

fixed the problem. It looks like this was changed on May 9 to remove redundant condition:

90bc3ec

But removing the redundant condition causes the assertion in nixpkgs/nixos/modules/config/nsswitch.nix to fail due to the scripts now complaining that system.nssModules is not of type list of paths.

Any chance we can get that line reverted?

Thanks!

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# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module: sssd nsswitch