Re: [PATCH 6/6] clone, submodule update: check out branches

From: Jonathan Tan <jonathantanmy@google.com>
Date: Sep 1, 2022, 20:00 UTC
Message-ID: <20220901200047.515294-1-jonathantanmy@google.com>
In-Reply-To: <6f7f2f9a3f19b6d874d644b7fb7feb3a72fc6227.1661806456.git.gitgitgadget@gmail.com>

"Glen Choo via GitGitGadget" <gitgitgadget@gmail.com> writes:

Show 5 quoted lines

> From: Glen Choo <chooglen@google.com>
> 
> Teach "git submodule update" to update submodules by creating and
> checking out the current superproject branch when
> "submodule.propagateBranches=true".

"git submodule update" already knows how to update submodules; probably better to say:

  Teach "git submodule update" to create and check out a branch of the
  same name as the current superproject branch when updating a submodule
  if "submodule.propagateBranches=true" is set on the superproject.

Show 7 quoted lines

> With "submodule.propagateBranches=true", submodules are cloned with
> "--detach" so that they do not contain branches from their upstream.
> This prevents conflicts between branch names from the superproject and
> the branch names from the submodule's upstream. Arguably, "--detach"
> should also be the default for "submodule.propagateBranches=false"
> since it doesn't make sense to create a submodule branch when the
> submodule is always expected to be in detached HEAD.

This paragraph made me think of the use case in which we cloned a submodule-using repo, made a commit in a submodule (thus advancing a branch) without a corresponding commit in a superproject, and then recloned our clone, hoping that the state will persist. It would not persist, but as stated here, the existing behavior is already that branches in submodules are not cloned, so retaining this existing behavior is not a problem.

Show 7 quoted lines

> "git submodule update" tries to create the branch as long as it is not
> currently checked out, thus it will fail if the submodule has the
> branch, but it is not checked out. This is fine because the main purpose
> of "git submodule update" is to clone new submodules (which have no
> branches, and will never have this problem). "git checkout" with
> "submodule.propagateBranches" will cover the use case of recursively
> checking out an existing branch.

In regular usage, the user will, as you say, run "git checkout". So when "git submodule update" is run, a submodule will either have no branches (because it was just cloned or because we have never switched to that branch before in the superproject) or it will have the correct branch already checked out, so it would already be considered up to date (no matter whether the commit matches with the superproject's gitlink: only the name of the branch matters).

I'm concerned about the case in which the user, say, has created a branch in a submodule for some reason. E.g.:

  (cd sub; git branch my-branch)
  git checkout my-branch

so this would fail because we wouldn't be able to create "my-branch" in the "sub" submodule. We might need a message explaining what can be done to fix this situation, but for now, maybe a NEEDSWORK will suffice.

Show 6 quoted lines

> @@ -2206,6 +2214,8 @@ static int run_update_command(struct update_data *ud, int subforce)
>  		strvec_pushl(&cp.args, "checkout", "-q", NULL);
>  		if (subforce)
>  			strvec_push(&cp.args, "-f");
> +		if (ud->super_branch)
> +			strvec_pushl(&cp.args, "-b", ud->super_branch, NULL);

Here is where the NEEDSWORK would go.

Show 30 quoted lines

> @@ -106,4 +113,31 @@ test_expect_success '--no-also-filter-submodules overrides clone.filterSubmodule
>  	test_cmp_config -C super_clone3/sub false --default false remote.origin.promisor
>  '
>  
> +test_expect_success 'submodule.propagateBranches checks out branches at correct commits' '
> +	git -C sub checkout -b not-main &&
> +	git -C subsub checkout -b not-main &&
> +	git clone --recurse-submodules \
> +		-c submodule.propagateBranches=true \
> +		"file://$pwd/." super_clone4 &&
> +
> +	# Assert that each repo is pointing to "main"
> +	for REPO in "super_clone4" "super_clone4/sub" "super_clone4/sub/subsub"
> +	do
> +	    HEAD_BRANCH=$(git -C $REPO symbolic-ref HEAD) &&
> +	    test $HEAD_BRANCH = "refs/heads/main" || return 1
> +	done &&
> +
> +	# Assert that the submodule branches are pointing to the right revs
> +	EXPECT_SUB_OID="$(git -C super_clone4 rev-parse :sub)" &&
> +	ACTUAL_SUB_OID="$(git -C super_clone4/sub rev-parse refs/heads/main)" &&
> +	test $EXPECT_SUB_OID = $ACTUAL_SUB_OID &&
> +	EXPECT_SUBSUB_OID="$(git -C super_clone4/sub rev-parse :subsub)" &&
> +	ACTUAL_SUBSUB_OID="$(git -C super_clone4/sub/subsub rev-parse refs/heads/main)" &&
> +	test $EXPECT_SUBSUB_OID = $ACTUAL_SUBSUB_OID &&
> +
> +	# Assert that the submodules do not have branches from their upstream
> +	test_must_fail git -C super_clone4/sub rev-parse not-main &&
> +	test_must_fail git -C super_clone4/sub/subsub rev-parse not-main
> +'

Instead of reusing "main", can we use a branch name that exists in the superproject but not the submodule? Here, we cannot tell the difference between git reusing the referent of submodule's "main" versus git using the gitlink in superproject's "main".

I'll write some more comments on the other patches, but overall this patch set makes sense to me.