Nixpkgs: xbacklight reports "No outputs have backlight property"

[Luis-Hebendanz]

Issue description

The xbacklight utility errors out on 19.03 with:

 $ xbacklight -set 800  
No outputs have backlight property

And if I specifically define the Monitor following error happens:

xbacklight -d eDP-1 -set 800                                                                   
RANDR Query Version returned error -1

I noticed that after upgrading to 19.03 my monitor device names changed from eDP1 to eDP-1 and so on. It probably has something to do with that.
I used the 5.0 Kernel and the 4.19.35-hardened Kernel both had the same issue.

On Debian there is anopen issue on that from 2016:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=833508

Technical details

• system: "x86_64-linux"
• host os: Linux 5.0.8, NixOS, 19.03.172286.8ea36d73256 (Koi)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.2.2
• channels(root): "nixos-19.03.172286.8ea36d73256, nixos-unstable-19.09pre175916.1fc591f9a5b"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

[Luis-Hebendanz]

I fixed it. The solution is to use acpilight which has a binary with the same name called xbacklight so the old xorg.xbacklight has to be removed. On top of that hardware.brightnessctl.enable = true; has to be enabled and your user has to be added to the "video" group. After that xbacklight -list should return all controllable displays

[erikarvstedt]

A much faster alternative to the acpilight package is light.

[StephenWithPH]

For future searchers... @Luis-Hebendanz 's comment above got me 90% there, but I initially received the error:

/run/current-system/sw/bin/xbacklight: [Errno 13] Permission denied: '/sys/class/backlight/intel_backlight/brightness'

It was easy enough to chown the file to root:video and chmod it to 0664. This, combined with the above, solved the problem for me.