extra-substituters are not used

[tianze0926]

I have been using NixOS with flakes for a few months, thanks to this amazing book. But recently I noticed that the cachix substituters I put in nixConfig.extra-substituters were totally useless (here is my configuration). The binary cache is not used and Nix starts the building process locallly. Since I use http proxy, I can see the network traffic of Nix, and Nix didn't even access the binary cache such as https://nix-community.cachix.org.

But if I move the substituters to nixConfig.substituters, the binary cache would be used. I am very confused about this. Could anyone help me about it? Thanks~

[ryan4yin]

Good question! I haven't paid much attention to whether this parameter is valid before.
I spent some time studying this problem. Here is a detailed explanation of the background and reasons for this problem.

nixConfig in flake.nix

According to Flake format - Nix Manual:

nixConfig: a set of nix.conf options to be set when evaluating any part of a flake. In the interests of security, only a small set of set of options is allowed to be set without confirmation so long as accept-flake-config is not enabled in the global configuration:

So options in nixConfig will only take effect in the flake itself, and to to specify additional substituters by nixConfig.substituters or nixConfig.extra-substituters, the current user must be setted in trusted-users in the system-level /etc/nix/nix.conf.

nix.settings and /etc/nix/nix.conf

To customize the system-level /etc/nix/nix.conf declaratively, we should add something like this in a nixos module:

{

  nix.settings = {
    # enable flakes globally
    experimental-features = ["nix-command" "flakes"];
    # customize the system-level substituers
    substituters = [
      "https://mirrors.cernet.edu.cn/nix-channels/store"
      "https://cache.nixos.org/"
    ];

    # given the users in this list the right to specify additional substituters via:
    #    1. `nixConfig.substituers` in `flake.nix`
    #    2. command line args `--options substituers http://xxx`
    trusted-users = ["user1" "user2"];
  };
}

extra- prefix in nix.conf

Here is docs about extra- prefix in nix.conf - file format - nix manual

A configuration setting usually overrides any previous value. However, for settings that take a list of items, you can prefix the name of the setting by extra- to append to the previous value.

So the content in nixConfig.extra-substituers or nix.setting.extra-substituers should be appended to the substituers option in the current scope.

But why did nixConfig.extra-substituers is ignored by nix? I found several related issues:

• extra-* settings are order depedendent: extra-foo option is a noop if used before foo in the configuration file NixOS/nix#9487
• NIX_CONFIG variable ignores -extra options NixOS/nix#7560
• Flake: nixConfig options don't get used by nix run NixOS/nix#6170

As the issues showed here, the extra-substituers options may be ignored in many cases.

I would recommend using nix.settings.substituers to set the system-level generic cache servers, and nixConfig.extra-substituers for the flake-only cache servers.

But if you encounter any problem with nixConfig.extra-substituers being ignored, you can specify all the cache servers you need to solve the problem directly through nixConfig.substituers.

[ryan4yin]

Some new content has been added to this: 8d3ec99