How to use locally built packages as dependency for further local package

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1

I’ve written the following flake.nix file:

{
  description = "Just a C++ project";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-23.11";
  };

  outputs = {
    self, nixpkgs
    }: let
      system = "x86_64-linux";
    in {
      defaultPackage.${system} = 
        with import nixpkgs {inherit system;};
        stdenv.mkDerivation {
          name = "dummy-project";
          version = "v1.0.0";
          src = ./.;

          nativeBuildInputs = [ cmake ];

          buildInputs = [
            dummy-dependency        # this causes me a headache
            gtest
            ];
        };
    };
}

As you can see, this flake has a dependency on dummy-dependency. Since dummy-dependency is a company internal dependency, I can’t make it public through git or even publish it to the nixpkgs directory so it is built locally from a locally cloned repository (I know, not ideal, but it’s a first step in testing this package manager without too much work).

The dummy-dependency repository has a default.nix file writte by me, which looks like this:

with import <nixpkgs> {};
stdenv.mkDerivation {
  pname = "dummy-dependency"
  version = "1.0.0";
  src = ./.;

  system = "x86_64-linux";

  nativeBuildInputs = [
    cmake
  ];

  buildInputs = [ 
    boost
    gtest
    ];
}

This package has been built using nix build and it seems like the files have landed correctly in the /nix/store/ directory.

$ find /nix/store/ -type d -name "*dummy-dependency*" -print
/nix/store/l52xdhnbh7l1pd9vzlavwjp9kw7db16h-dummy-dependency
/nix/store/pyfba247zhs377plyy2kkd1vxvc1xsj0-dummy-dependency
/nix/store/6vvd6v3kiascqcb58zb1xch2vk00ians-dummy-dependency

Although building the initially mentioned flake (first field), throws the following error:

error: undefined variable 'dummy-dependency'

How does one build and install packages locally without having to upload / make them public, and at the same time be able to import them as dependencies for further projects?

2

Welcome to reproducibility city
Where the lambda is blue and questions are tricky.

Flakes has a path syntax for absolute path. Maybe you can use it as your input,

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-23.11";
    dumbdep.url = "./../two-live-crew_me-so-dummy";
    dumbdep.flake = false; # because there no flake.nix in your dependency
  };

While this is correct answer to your question about no uploading/make it public.
I think what you should be looking for is private git authorization, because local paths to other dependencies may only work in your machine. Also take a look in substituter - unfortunate name to any ssh/s3/http server - where you can share your builds between machine (like cache.nixos.org but privately).

Once you have the input, you have to change your outputs:

  outputs = { self, nixpkgs, dumbdep }:
     #... too lazy to type
         buildInputs = [
           (callPackage dumbdep {}) # callPackage because is not a flake
           # or dumbdep.packages."${system}".default # if it was a flake.
           gtest
         ];

Note: your flake template is outdated, defaultPackage.system is now packages.system.default