nixos/gdm: Failed to allocate UID for greeter

[zroug]

Nixpkgs version

• Beta (25.11)

Describe the bug

Under the right circumstances, GDM fails to allocate a UID for the greeter user (see steps to reproduce). This appears to be a side effect of the workaround described in #440720 (comment).

The error message found in the log is Gdm: GdmDisplay: Failed to allocate UID for greeter: User 'gdm-greeter-5' not preallocated and system lacks userdb.

Steps to reproduce

This happens for us on a machine configured to use Gnome Remote Desktop with remote login. Depending on the number of users/logins, GDM quickly runs out of the five preallocated users, resulting in this error. Note that this sometimes occurs with fewer than five users. I think GDM recycles users, but sometimes it takes time, so if a user connects and disconnects multiple times, this error occurs too.

Expected behaviour

No error allocating a UID for the greeter.

Screenshots

No response

Relevant log output

Gdm: GdmDisplay: Failed to allocate UID for greeter: User 'gdm-greeter-5' not preallocated and system lacks userdb

Additional context

We worked around this issue by preallocating a bunch of additional users. This configuration is probably not very common, so I don't think this issue is high priority.

Relevant commit: 23a2bfc

System metadata

• system: "x86_64-linux"
• host os: Linux 6.12.59, NixOS, 25.11 (Xantusia), 25.11.20251124.a320ce8
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.31.2
• nixpkgs: /nix/store/6wccvb0wrxy3s56mzwn50q8429ckd6zq-source

Notify maintainers

@bobby285271
@dasj19
@hedning
@jtojnar

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