[PATCHv2 2/2] submodule update: allow '.' for branch value

[Stefan Beller <sbeller@google.com>]

Gerrit has a "superproject subscription" feature[1], that triggers a
commit in a superproject that is subscribed to its submodules.
Conceptually this Gerrit feature can be done on the client side with
Git via:

    git -C <superproject> submodule update --remote --force
    git -C <superproject> commit -a -m "Update submodules"
    git -C <superproject> push

for each branch in the superproject. To ease the configuration in Gerrit
a special value of "." has been introduced for the submodule.<name>.branch
to mean the same branch as the superproject[2], such that you can create a
new branch on both superproject and the submodule and this feature
continues to work on that new branch.

Now we have find projects in the wild with such a .gitmodules file.
To have Git working well with these, we imitate the behavior and
look up the superprojects branch name if the submodules branch is
configured to ".". In projects that do not use Gerrit, this value
whould be never configured as "." is not a valid branch name.

[1] introduced around 2012-01, e.g.
    https://gerrit-review.googlesource.com/#/c/30810/
[2] excerpt from [1]:
 > The branch value could be '.' if the submodule project branch
 > has the same name as the destination branch of the commit having
 > gitlinks/.gitmodules file.

CC: Avery Pennarun <apenwarr@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
---

 This comes with another test that I run into while using this code.
 Please replace patch 2 with this v2.
 
 Thanks,
 Stefan

 git-submodule.sh            |  9 ++++++++-
 t/t7406-submodule-update.sh | 42 +++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/git-submodule.sh b/git-submodule.sh
index 4ec7546..1eb33ad 100755
--- a/git-submodule.sh
+++ b/git-submodule.sh
@@ -590,7 +590,6 @@ cmd_update()
 
 		name=$(git submodule--helper name "$sm_path") || exit
 		url=$(git config submodule."$name".url)
-		branch=$(get_submodule_config "$name" branch master)
 		if ! test -z "$update"
 		then
 			update_module=$update
@@ -616,6 +615,14 @@ cmd_update()
 
 		if test -n "$remote"
 		then
+			branch=$(get_submodule_config "$name" branch master)
+			if test "$branch" = "."
+			then
+				if ! branch=$(git symbolic-ref --short -q HEAD)
+				then
+					die "$(eval_gettext "submodule branch configured to inherit branch from superproject, but it's not on any branch")"
+				fi
+			fi
 			if test -z "$nofetch"
 			then
 				# Fetch remote before determining tracking $sha1
diff --git a/t/t7406-submodule-update.sh b/t/t7406-submodule-update.sh
index bd261ac..953c486 100755
--- a/t/t7406-submodule-update.sh
+++ b/t/t7406-submodule-update.sh
@@ -209,9 +209,49 @@ test_expect_success 'submodule update --remote should fetch upstream changes' '
 	)
 '
 
-test_expect_success 'local config should override .gitmodules branch' '
+test_expect_success 'submodule update --remote should fetch upstream changes with .' '
+	(cd super &&
+	 git config -f .gitmodules submodule."submodule".branch "." &&
+	 git add .gitmodules &&
+	 git commit -m "submodules: update from the respective superproject branch"
+	) &&
 	(cd submodule &&
+	 echo line4a >> file &&
+	 git add file &&
+	 test_tick &&
+	 git commit -m "upstream line4a" &&
+	 git checkout -b test-branch &&
+	 test_commit on-test-branch
+	) &&
+	(cd super &&
+	 git submodule update --remote --force submodule &&
+	 (cd submodule &&
+	  test "$(git log -1 --oneline)" = "$(GIT_DIR=../../submodule/.git git log -1 --oneline master)"
+	 ) &&
 	 git checkout -b test-branch &&
+	 git submodule update --remote --force submodule &&
+	 (cd submodule &&
+	  test "$(git log -1 --oneline)" = "$(GIT_DIR=../../submodule/.git git log -1 --oneline test-branch)"
+	 ) &&
+	 git checkout master &&
+	 git branch -d test-branch
+	)
+'
+
+test_expect_success 'branch = . does not confuse the rest of update' '
+	(cd super &&
+	 git checkout --detach &&
+	 # update is not confused by branch="." even if the the superproject
+	 # is not on any branch currently
+	 git submodule update &&
+	 git revert HEAD &&
+	 git checkout master
+	)
+'
+
+test_expect_success 'local config should override .gitmodules branch' '
+	(cd submodule &&
+	 git checkout test-branch &&
 	 echo line5 >> file &&
 	 git add file &&
 	 test_tick &&
-- 
2.9.2.468.g3d9025b