fetchers/git: make relative path absolute for local repo (backport #12107)

[mergify]

Motivation

Closes #9708 (again).

This patch:

• (naively) Resolves relative paths to local git repos, returning absolute paths, thus allowing flake URIs such as git+file:./${submodule} to continue working.
• Adds tests to prevent future breakages.

Context

Flake URIs such as git+file:./${submodule} used to work before 2.18, but broke in 2.19, was fixed later, and broke again recently since 3e0129c, resulting in a core-dumped assertion error in #9708 (comment).

This PR suppresses the error and allows git+file:./${submodule}to continue working. However, this fix is not ideal and should potentially be superseded by a better submodule implementation in the future. See:

• Flake input git+file:./${submodule} no longer works #9708
• libutil/url: fix git+file:./ parse error #9897

A better fix is outlined by @roberth in https://discourse.nixos.org/t/57783, namely:

This bug seems to have allowed relative git flake inputs to be resolved against the current working directory (as in POSIX), and this tends to work out ok in the context of flakes, but is the wrong behavior, as it should resolve against the flake.nix base directory instead.
Supporting relative paths in fetchTree would add significant value.
This behavior was used as a workaround for lacking relative flake input or “subflake” support.

---

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• @DamienCassou and @aij for testing

---

This is an automatic backport of pull request #12107 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 96bd9ba has failed:

On branch mergify/bp/2.24-maintenance/pr-12107
Your branch is up to date with 'origin/2.24-maintenance'.

You are currently cherry-picking commit 96bd9bad2.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/libfetchers/git.cc

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of 37ac18d has failed:

On branch mergify/bp/2.24-maintenance/pr-12107
Your branch is ahead of 'origin/2.24-maintenance' by 2 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 37ac18d1d.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   tests/functional/flakes/flake-in-submodule.sh

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[bryango]

@roberth proposed resolutions of backport conflicts!

[bryango]

Suggested change

	<<<<<<< HEAD
	repoInfo.url = repoInfo.isLocal ? url.path : url.base;
	=======
	//
	// FIXME: here we turn a possibly relative path into an absolute path.
	// This allows relative git flake inputs to be resolved against the
	// **current working directory** (as in POSIX), which tends to work out
	// ok in the context of flakes, but is the wrong behavior,
	// as it should resolve against the flake.nix base directory instead.
	//
	// See: https://discourse.nixos.org/t/57783 and #9708
	//
	repoInfo.url = repoInfo.isLocal ? std::filesystem::absolute(url.path).string() : url.to_string();
	>>>>>>> 96bd9bad2 (fetchers/git: make path absolute for local repo)
	//
	// FIXME: here we turn a possibly relative path into an absolute path.
	// This allows relative git flake inputs to be resolved against the
	// **current working directory** (as in POSIX), which tends to work out
	// ok in the context of flakes, but is the wrong behavior,
	// as it should resolve against the flake.nix base directory instead.
	//
	// See: https://discourse.nixos.org/t/57783 and #9708
	//
	repoInfo.url = repoInfo.isLocal ? std::filesystem::absolute(url.path).string() : url.to_string();

[bryango]

Suggested change

	<<<<<<< HEAD
	=======
	# Test that `nix flake metadata` parses `submodule` correctly.
	cat > "$rootRepo"/flake.nix <<EOF
	{
	outputs = { self }: {
	};
	}
	EOF
	git -C "$rootRepo" add flake.nix
	git -C "$rootRepo" commit -m "Add flake.nix"
	storePath=$(nix flake metadata --json "$rootRepo?submodules=1" | jq -r .path)
	[[ -e "$storePath/submodule" ]]
	# The root repo may use the submodule repo as an input
	# through the relative path. This may change in the future;
	# see: https://discourse.nixos.org/t/57783 and #9708.
	cat > "$rootRepo"/flake.nix <<EOF
	{
	inputs.subRepo.url = "git+file:./submodule";
	outputs = { ... }: { };
	}
	EOF
	git -C "$rootRepo" add flake.nix
	git -C "$rootRepo" commit -m "Add subRepo input"
	(
	cd "$rootRepo"
	# The submodule must be locked to the relative path,
	# _not_ the absolute path:
	[[ $(nix flake metadata --json | jq -r .locks.nodes.subRepo.locked.url) = "file:./submodule" ]]
	)
	>>>>>>> 37ac18d1d (tests/flake-in-submodule: git+file:./* input)
	# Test that `nix flake metadata` parses `submodule` correctly.
	cat > "$rootRepo"/flake.nix <<EOF
	{
	outputs = { self }: {
	};
	}
	EOF
	git -C "$rootRepo" add flake.nix
	git -C "$rootRepo" commit -m "Add flake.nix"
	storePath=$(nix flake metadata --json "$rootRepo?submodules=1" | jq -r .path)
	[[ -e "$storePath/submodule" ]]
	# The root repo may use the submodule repo as an input
	# through the relative path. This may change in the future;
	# see: https://discourse.nixos.org/t/57783 and #9708.
	cat > "$rootRepo"/flake.nix <<EOF
	{
	inputs.subRepo.url = "git+file:./submodule";
	outputs = { ... }: { };
	}
	EOF
	git -C "$rootRepo" add flake.nix
	git -C "$rootRepo" commit -m "Add subRepo input"
	(
	cd "$rootRepo"
	# The submodule must be locked to the relative path,
	# _not_ the absolute path:
	[[ $(nix flake metadata --json | jq -r .locks.nodes.subRepo.locked.url) = "file:./submodule" ]]
	)

[edolstra]

To be honest, I'd prefer to hold off on backporting this PR, since the behaviour of making the fetcher depend on the cwd seems extremely questionable to me. (See #12107 (comment).)

[bryango]

Agree, let's see how:

• Warn against the use of relative 'git+file:' flake inputs #12277
• Don't absolutize 'git+file:' in parseURLFlakeRef() #12280

... are finalized and then decide what to do.