sane: disabledDefaultBackends does not work reliably

[robinkrahl]

Describe the bug

The /etc/sane-config/dll.conf file lists enabled backends, with one backend per line. The hardware.sane.disabledDefaultBackends option uses string substitution to edit this file. But as it does not take into account line endings, disabling a backend x also affects other backends that contain the string x.

Steps To Reproduce

Steps to reproduce the behavior:

1. Set hardware.sane.enabled = true; in /etc/nixos/configuration.nix and observe the generated dll.conf:

$ realpath /etc/sane-config/dll.conf
/nix/store/hb5yn5874izgjychgfjgkjbf0igz9ivk-sane-config/etc/sane.d/dll.conf

2. Set hardware.sane.disabledDefaultBackends = [ "net" ]; and repeat:

$ realpath /etc/sane-config/dll.conf
/nix/store/116ir7mwgfnwji955sjblyz0fla87933-sane-config/etc/sane.d/dll.conf

3. Compare the two files:

$ diff /nix/store/{hb5yn5874izgjychgfjgkjbf0igz9ivk,116ir7mwgfnwji955sjblyz0fla87933}-sane-config/etc/sane.d/dll.conf
7,8c7,8
< # The next line enables the network backend; comment it out if you don't
< # need to use a remote SANE scanner over the network -- see sane-net(5)
---
> # The next line enables the # net disabled in nixos configwork backend; comment it out if you don't
> # need to use a remote SANE scanner over the # net disabled in nixos configwork -- see sane-# net disabled in nixos config(5)
10c10
< net
---
> # net disabled in nixos config
31c31
< dell1600n_net
---
> dell1600n_# net disabled in nixos config

Expected behavior

Instead of mangling the dell1600n_net driver, only the net driver should be disabled.

Additional context

This is the code that performs the string substition:

nixpkgs/pkgs/applications/graphics/sane/config.nix

Line 33 in b7535c4

substituteInPlace $out/etc/sane.d/dll.conf --replace '${backend}' '# ${backend} disabled in nixos config'

It should probably use something like ^backend$ instead, though I don’t know how this could be expressed here.

On a related note, it would be great to have an option to disable all default backends and manually select the backends to enable.

Notify maintainers

@symphorien (pinging because of c64fb50)

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

$ nix-shell -p nix-info --run "nix-info -m"
 - system: `"x86_64-linux"`
 - host os: `Linux 6.1.76, NixOS, 23.11 (Tapir), 23.11.3920.25e3d4c0d359`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.18.1`
 - channels(root): `"nixos-23.11"`
 - channels(robin): `"home-manager-23.05.tar.gz"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

---

Add a 👍 reaction to issues you find important.

[symphorien]

please test #293022