在Nix模块中构建Docker镜像Derivation时出现src缺失报错

问题场景 #

你尝试将Docker镜像构建逻辑拆分为模块文件，但执行nix-build时出现错误，而将逻辑放在单个文件中却能正常工作。以下是相关代码与错误信息：

拆分模块的代码 #

services/invidious.nix

{ pkgs, stdenv, ... }:
stdenv.mkDerivation {
  name = "invidious";
  container = pkgs.dockerTools.buildLayeredImage {
    name = "invidious";
    contents = [ pkgs.busybox pkgs.bash pkgs.invidious ];
    config = {
      Cmd = [ "/bin/bash" ];
      Env = [];
      Volumes = {};
    };
  };
}

services.nix

let
  config = import ./config.nix;
  pkgs = config.pkgs;
  invidious = import ./services/invidious.nix;
in rec {
  serviceimages = pkgs.writeText "images.txt" ''
    ${invidious(pkgs)}
  '';
}

config.nix

{
  # nixos-22.05 / https://status.nixos.org/
  pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/d86a4619b7e80bddb6c01bc01a954f368c56d1df.tar.gz") {};
}

执行错误信息 #

nix-build services.nix -A serviceimages
these 2 derivations will be built:
  /nix/store/dbl3bzc05pssq3q9g8wd2i92xpmwf5bb-invidious.drv
  /nix/store/4x31hx9nxcbbksi2hsim08djrsj4h1zh-images.txt.drv
building '/nix/store/dbl3bzc05pssq3q9g8wd2i92xpmwf5bb-invidious.drv'...
unpacking sources
variable $src or $srcs should point to the source
error: builder for '/nix/store/dbl3bzc05pssq3q9g8wd2i92xpmwf5bb-invidious.drv' failed with exit code 1;
       last 2 log lines:
       > unpacking sources
       > variable $src or $srcs should point to the source
       For full logs, run 'nix log /nix/store/dbl3bzc05pssq3q9g8wd2i92xpmwf5bb-invidious.drv'.

正常工作的单文件代码 #

let
  # nixos-22.05 / https://status.nixos.org/
  pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/d86a4619b7e80bddb6c01bc01a954f368c56d1df.tar.gz") {};
in rec {

  docker = pkgs.dockerTools.buildLayeredImage {
    name = "invidious";
    contents = [ pkgs.busybox pkgs.bash pkgs.invidious ];
    config = {
      Cmd = [ "/bin/sh" ];
      Env = [];
      Volumes = {};
    };
  };

  results = pkgs.writeText "images.txt" ''
    ${docker}
  '';
}

问题根源 #

1. 错误使用stdenv.mkDerivation：services/invidious.nix里用stdenv.mkDerivation包裹了dockerTools.buildLayeredImage。stdenv.mkDerivation是用于构建需要编译/处理源码的包，它默认会执行解压源码的步骤，因此必须指定src或srcs参数。但你这里只是生成Docker镜像，根本不需要处理源码，完全没必要用stdenv.mkDerivation包装。
2. 参数传递错误：invidious函数接收的是一个属性集（{ pkgs, ... }），但services.nix里直接传递了pkgs值，而非包含pkgs的属性集。

解决方案 #

1. 修改services/invidious.nix #

去掉多余的stdenv.mkDerivation，直接返回dockerTools.buildLayeredImage的结果：

{ pkgs, ... }:
pkgs.dockerTools.buildLayeredImage {
  name = "invidious";
  contents = [ pkgs.busybox pkgs.bash pkgs.invidious ];
  config = {
    Cmd = [ "/bin/bash" ];
    Env = [];
    Volumes = {};
  };
}

2. 修改services.nix的参数传递 #

将invidious(pkgs)改为传递属性集：

let
  config = import ./config.nix;
  pkgs = config.pkgs;
  invidious = import ./services/invidious.nix;
in rec {
  serviceimages = pkgs.writeText "images.txt" ''
    ${invidious { inherit pkgs; }}
  '';
}

修改后重新执行nix-build services.nix -A serviceimages即可正常构建，生成的镜像路径会写入images.txt文件。

内容的提问来源于stack exchange，提问作者djsumdog