Re: [PATCH v3 8/8] clone, submodule update: create and check out branches

> From: Glen Choo <chooglen@google.com>
> 
> Teach "git submodule update" to:
> 
> - create the branch with the same name as the current superproject
>   branch when cloning a submodule
> - check out that branch (instead of the commit OID) when updating
>   the submodule worktree
> 
> when submodule branching is enabled (submodule.propagateBranches = true)
> on the superproject and a branch is checked out. "git clone
> --recurse-submodules" also learns this trick because it is implemented
> with "git submodule update --recursive".
> 
> This approach of checking out the branch will not result in a dirty
> worktree for freshly cloned submodules because we can ensure that the
> submodule branch points to the superproject gitlink. In other cases, it
> does not work as well, but we can handle them incrementally:
> 
> - "git pull --recurse-submodules" merges the superproject tree,
>   (changing the gitlink without updating the submodule branches), and
>   runs "git submodule update" to update the worktrees, so it is almost
>   guaranteed to result in a dirty worktree.

> 
>   The implementation of "git pull --recurse-submodules" is likely to
>   change drastically as submodule.propagateBranches work progresses
>   (e.g. "git merge" learns to recurse in to submodules), and we may be
>   able to replace the "git submodule update" invocation, or teach it new
>   tricks that make the update behave well.
> 
> - The user might make changes to the submodule branch without committing
>   them back to superproject. This is primarily affects "git checkout
>   --recurse-submodules", since that is the primary way of switching away
>   from a branch and leaving behind WIP (as opposed to "git submodule
>   update", which is run post-checkout).
> 
>   In a future series, "git checkout --recurse-submodules" will learn to
>   consider submodule branches. We can introduce appropriate guardrails
>   then, e.g. requiring that the superproject working tree is not dirty
>   before switching away.
> 
> Signed-off-by: Glen Choo <chooglen@google.com>
> ---
>  builtin/submodule--helper.c |  37 ++++++++-
>  t/t5617-clone-submodules.sh |  38 +++++++++
>  t/t7406-submodule-update.sh | 156 ++++++++++++++++++++++++++++++++++++
>  3 files changed, 227 insertions(+), 4 deletions(-)
> 

> +# Test the behavior of an already-cloned submodule.
> +# NEEDSWORK When updating with branches, we always use the branch instead of the
> +# gitlink's OID. This results in some imperfect behavior:
> +#
> +# - If the gitlink's OID disagrees with the branch OID, updating with branches
> +#   may result in a dirty worktree
> +# - If the branch does not exist, the update fails.
> +#
> +# We will reevaluate when "git checkout --recurse-submodules" supports branches
> +# For now, just test for this imperfect behavior.

> +test_expect_success 'branches - correct branch checked out, OIDs disagree' '
> +	test_when_finished "rm -fr branch-super-cloned" &&
> +	cp -r branch-super-clean branch-super-cloned &&
> +
> +	git -C branch-super-cloned branch --recurse-submodules new-branch &&
> +	git -C branch-super-cloned checkout new-branch &&
> +	git -C branch-super-cloned/sub1 checkout new-branch &&
> +	test_commit -C branch-super-cloned/sub1 new-commit &&
> +	git -C branch-super-cloned submodule update &&
> +
> +	HEAD_BRANCH1=$(git -C branch-super-cloned/sub1 symbolic-ref HEAD) &&
> +	test $HEAD_BRANCH1 = "refs/heads/new-branch" &&
> +	test_clean_submodule ! branch-super-cloned sub1
> +'

> +
> +test_expect_success 'branches - other branch checked out, correct branch exists, OIDs disagree' '
> +	test_when_finished "rm -fr branch-super-cloned" &&
> +	cp -r branch-super-clean branch-super-cloned &&
> +
> +	git -C branch-super-cloned branch --recurse-submodules new-branch &&
> +	git -C branch-super-cloned checkout new-branch &&
> +	git -C branch-super-cloned/sub1 checkout new-branch &&
> +	test_commit -C branch-super-cloned/sub1 new-commit &&
> +	git -C branch-super-cloned/sub1 checkout main &&
> +	git -C branch-super-cloned submodule update &&
> +
> +	HEAD_BRANCH1=$(git -C branch-super-cloned/sub1 symbolic-ref HEAD) &&
> +	test $HEAD_BRANCH1 = "refs/heads/new-branch" &&
> +	test_clean_submodule ! branch-super-cloned sub1
> +'
> +

> +test_expect_success 'branches - other branch checked out, correct branch does not exist' '
> +	test_when_finished "rm -fr branch-super-cloned" &&
> +	cp -r branch-super-clean branch-super-cloned &&
> +
> +	git -C branch-super-cloned branch new-branch &&
> +	git -C branch-super-cloned checkout new-branch &&
> +	test_must_fail git -C branch-super-cloned submodule update
> +'
> +
>  test_done