gitignoreSource: ensure Nix store path determinism

[lukateras]

This ensures that, when used in situations like gitignoreSource ./., both Hydra and local checkout will have the exact same Nix path, which is important for reproducibility and being able to leverage binary cache.

[lukateras]

Wow, thank you for the fast merge! :)

[yorickvP]

I support having this in here. Actually, I think cleanSourceWith should do this. However,

1. having 'source' as the path name leads to very unclear error messages.
2. This PR breaks composability with other cleanSourceWith's.

My old workaround for this problem, which is composable with other cleanSource's:

let
  toConstantSource = name: src: if src ? _isLibCleanSourceWith then builtins.path {
    inherit name;
    inherit (src) filter;
    path = src.origSrc;
  } else builtins.path { inherit name; path = src; };
  constGitIgnore = name: path: toConstantSource name (gitignoreSource path);

[lukateras]

1. having 'source' as the path name leads to very unclear error messages.

For the record, source is pretty idiomatic. For example, see fetchzip and by extension fetchers built on top of it (like fetchFromGitHub): https://github.com/NixOS/nixpkgs/blob/f3282c8d1e0ce6ba5d9f6aeddcfad51d879c7a4a/pkgs/build-support/fetchzip/default.nix#L14

Which implies that majority of Nixpkgs stdenv derivations have src that uses name = "source". It seems that your concern is similar to that in NixOS/nixpkgs#49862.

2. This PR breaks composability with other cleanSourceWith's.

True. And that was listed as a feature: https://github.com/hercules-ci/gitignore/blob/ec5dd0536a5e4c3a99c797b86180f7261197c124/README.md#features

However, it's still possible to compose with gitignoreFilter.

[lukateras]

@yorickvP: Also, your workaround doesn't seem to work for me as of Nix 2.2.2. When I try to build a derivation that uses src = lib.cleanSource (constGitIgnore "source" ./.), I get:

string '/nix/store/rwlmxrxs27x7lplhgji74rbfjdxl3nla-source' cannot refer to other paths, at /nix/store/jc62lsfijc5qab87yrbi6aag67jd0lr8-source/lib/sources.nix:49:17

Seems to me that's because constGitIgnore in your example returns builtins.path without all of attributes that cleanSourceWith returns. I'd guess the idea is that it only composes to the right, with constGitIgnore always being the final filter.

[yorickvP]

@yegortimoshenko yeah, it terminates composability, but can still be used as the end to a composed set of cleanSource's.

Regarding (1), having package names in the nix store path may be a mistake altogether.