Nix doesn't fetch repositories shallowly when told to

[L-as]

Describe the bug

nix build 'git+https://github.com/NixOS/nixpkgs?shallow=1#hello' doesn't fetch Nixpkgs shallowly.

Steps To Reproduce

rm -rf "$HOME/.cache/nix/gitv3"
time nix build 'git+https://github.com/NixOS/nixpkgs?shallow=1#hello' # 9 s for me
time git clone https://github.com/NixOS/nixpkgs --depth 1 --single-branch --branch=master # 250 s for me

Since Nix runs git internally, it should take around the same time, disregarding the time it takes to fetch hello from Hydra.

[AleXoundOS]

Interesting, when did it break? Or was the bug around since the shallow attribute has been introduced?

[yajo]

I think the use case is not reproducible and might be a misconception. Maybe hello or nixpkgs is already on the nix store and there's no need to fetch any sources?

[yajo]

I have been able to check it and it's definitely happening. Big problem here 😢

It seems this test just checks that nix derivation doesn't contain commit count, not that it's being really shallow:

nix/tests/fetchGit.sh

Line 204 in d354fc3

[[ $(nix eval --impure --expr "(builtins.fetchTree { type = \"git\"; url = \"file:https://$TEST_ROOT/shallow\"; ref = \"dev\"; shallow = true; }).revCount or 123") == 123 ]]

[ImBearChild]

Well, maybe this parameter can only determine whether shallow clone is "allowed".

nix/src/libexpr/primops/fetchTree.cc

Lines 411 to 413 in e92cac7

	- `shallow` (default: `false`)
	A Boolean parameter that specifies whether fetching a shallow clone is allowed.

According to the code, it just checks the local git repository if is "shallow repository" or not.

nix/src/libfetchers/git.cc

Lines 586 to 590 in e92cac7

	bool isShallow = chomp(runProgram("git", true, { "-C", repoDir, "--git-dir", gitDir, "rev-parse", "--is-shallow-repository" })) == "true";
	if (isShallow && !shallow)
	throw Error("'%s' is a shallow Git repository, but shallow repositories are only allowed when `shallow = true;` is specified.", actualUrl);

And this parameter is never used when calling "git fetch". So it have no impact on how git fetches remote content.

nix/src/libfetchers/git.cc

Line 568 in e92cac7

runProgram("git", true, { "-C", repoDir, "--git-dir", gitDir, "fetch", "--quiet", "--force", "--", actualUrl, fmt("%s:%s", fetchRef, fetchRef) }, {}, true);

[poelzi]

nix run also ignores the shallow argument.

When I tried nix shell "git+https://github.com/r-ryantm/nixpkgs.git?ref=auto-update/rt-tests?shallow=true#rt-tests", it downloaded the full nixpkgs repo. When shallow is supported there, it becomes much easier to test PRs