nixos/gdm: UID mismatch of gdm-greeter-* users due to update

Nixpkgs version

• Unstable (26.11)

Describe the bug

I get a weird warning of not applying an UID change of the users gdm-greeter-N:

# nixos-rebuild test
building the system configuration...
Checking switch inhibitors... done
stopping the following units: accounts-daemon.service, systemd-tmpfiles-resetup.service
activating the configuration...
warning: not applying UID change of user ‘gdm-greeter-2’ (60580 -> 60579) in /etc/passwd
warning: not applying UID change of user ‘gdm-greeter-3’ (60581 -> 60580) in /etc/passwd
warning: not applying UID change of user ‘gdm-greeter-4’ (60582 -> 60581) in /etc/passwd
removing user ‘gdm-greeter-1’
setting up /etc...
reloading user units for johannes...
reloading the following user units: dbus-broker.service
restarting sysinit-reactivation.target
reloading the following units: dbus-broker.service
restarting the following units: polkit.service
starting the following units: accounts-daemon.service, systemd-tmpfiles-resetup.service
the following new units were started: NetworkManager-dispatcher.service
Done. The new configuration is /nix/store/p2zz15dcghcx66s92rw064mdjmrl9bh7-nixos-system-johannes-pc-26.05.20260523.64c08a7

To track the warning down I have performed a git bisect of my configuration and the issue was introduced by this bad 'nix flake update' commit.

The bug was probably introduced by commit 3fb5cf873d8eab026954ca596e4e662fb96efbd8.

Steps to reproduce

1. Switch to nixpkgs revision 29916453413845e54a65b8a1cf996842300cd299
2. Enable GDM and run nixos-rebuild
3. Switch to nixpkgs revision 64c08a7ca051951c8eae34e3e3cb1e202fe36786
4. nixos-rebuild prints a warning about unapplied UID changes of gdm-greetrer-N users

Expected behaviour

Updating shall not (try to) change the UID of users created by the GDM module.

Screenshots

No response

Relevant log output

Additional context

No response

System metadata

• system: "x86_64-linux"
• host os: Linux 7.0.10, NixOS, 26.05 (Yarara), 26.05.20260523.64c08a7
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.34.7
• channels(root): "nixos"
• nixpkgs: /nix/store/81sr43harc753claf8bzyv3mrnjzq652-source

Notify maintainers

@bobby285271 @dasj19 @hedning @jtojnar

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Reproduce the warning by switching between the two nixpkgs revisions listed in the steps and running nixos-rebuild. Compare the GDM module changes introduced by commit 3fb5cf873d8eab026954ca596e4e662fb96efbd8 and verify that updating does not attempt to change the UIDs of gdm-greeter-N users.