dbus: ignores configuration in `/usr/share/dbus-1`

Nixpkgs version

• Stable (25.05)

Describe the bug

dbus ignores any unit files in /usr/share/dbus-1. This means that using nixpkgs dbus outside of NixOS requires awkward contortions to place all the unit files in /etc, regardless of where the upstream puts them and regardless of whether they should be located there.

I have not checked dbus-broker but I believe the same applies there.

Steps to reproduce

Use nixpkgs dbus on a system where system-provided unit files are in /usr/lib as per FHS and systemd documentation. The dbus daemon will not start because it cannot find its configuration file.

Expected behaviour

Bus daemon starts.

Screenshots

No response

Relevant log output

Additional context

This is due to a nixpkgs downstream patch. The below system metadata is not fully correct as I am actually using Spectrum OS’s pinned nixpkgs and I found the bug on a separate machine.

System metadata

• system: "x86_64-linux"
• host os: `Linux 6.12.41, NixOS, 25.11 (Xantusia), 25.11pre844474.fbcf476f790d
• multi-user?: yes
• sandbox: yes
• version: `nix-env (Lix, like Nix) 2.91.3
  System type: x86_64-linux
  Additional system types: i686-linux, x86_64-v1-linux, x86_64-v2-linux, x86_64-v3-linux
  Features: gc, signed-caches
  System configuration file: /etc/nix/nix.conf
  User configuration files: (long list omitted as they are not relevant and I am having to type this in by hand)
  Store directory: /nix/store
  State directory: /nix/var/nix:
  Data directory: /nix/store/(long hash)-lix-2.91.3/share
• channels(root): "nixos"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

Notify maintainers

@jtojnar

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Start by locating the nixpkgs dbus package and its downstream patch that controls configuration lookup, then reproduce the issue on Linux with unit files under /usr/share/dbus-1 or /usr/lib. Done when the daemon starts and accepts configuration from /usr/share/dbus-1 without requiring files to be copied into /etc; also check whether dbus-broker is affected.