[PATCH v3 3/8] submodule: use a better fall-back for missing remote.<name>.branch

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

When `remote.<name>.branch` is not configured, `git submodule update`
currently falls back to using the branch name `master`. A much better
idea, however, is to use `HEAD`: on all Git servers running reasonably
recent Git versions, the symref `HEAD` points to the main branch.

Note: t7419 demonstrates that there _might_ be use cases out there that
_expect_ `git submodule update --remote` to update submodules to the
remote `master` branch even if the remote `HEAD` points to another
branch. Arguably, this patch makes the behavior more intuitive, but
there is a slight possibility that this might cause regressions in
obscure setups.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 Documentation/git-submodule.txt |  4 ++--
 builtin/submodule--helper.c     |  2 +-
 t/t7406-submodule-update.sh     | 16 ++++++++++++++++
 t/t7419-submodule-set-branch.sh |  7 +++++--
 4 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/Documentation/git-submodule.txt b/Documentation/git-submodule.txt
index c9ed2bf3d5..b20f85e622 100644
--- a/Documentation/git-submodule.txt
+++ b/Documentation/git-submodule.txt
@@ -284,7 +284,7 @@ OPTIONS
 	`.gitmodules` for `update --remote`.  A special value of `.` is used to
 	indicate that the name of the branch in the submodule should be the
 	same name as the current branch in the current repository.  If the
-	option is not specified, it defaults to 'master'.
+	option is not specified, it defaults to 'HEAD'.
 
 -f::
 --force::
@@ -322,7 +322,7 @@ OPTIONS
 	the superproject's recorded SHA-1 to update the submodule, use the
 	status of the submodule's remote-tracking branch.  The remote used
 	is branch's remote (`branch.<name>.remote`), defaulting to `origin`.
-	The remote branch used defaults to `master`, but the branch name may
+	The remote branch used defaults to `HEAD`, but the branch name may
 	be overridden by setting the `submodule.<name>.branch` option in
 	either `.gitmodules` or `.git/config` (with `.git/config` taking
 	precedence).
diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 46c03d2a12..f55f7b7704 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -1981,7 +1981,7 @@ static const char *remote_submodule_branch(const char *path)
 	free(key);
 
 	if (!branch)
-		return "master";
+		return "HEAD";
 
 	if (!strcmp(branch, ".")) {
 		const char *refname = resolve_ref_unsafe("HEAD", 0, NULL, NULL);
diff --git a/t/t7406-submodule-update.sh b/t/t7406-submodule-update.sh
index 4fb447a143..aa19ff3a2e 100755
--- a/t/t7406-submodule-update.sh
+++ b/t/t7406-submodule-update.sh
@@ -70,6 +70,22 @@ test_expect_success 'setup a submodule tree' '
 	)
 '
 
+test_expect_success 'update --remote falls back to using HEAD' '
+	test_create_repo main-branch-submodule &&
+	test_commit -C main-branch-submodule initial &&
+
+	test_create_repo main-branch &&
+	git -C main-branch submodule add ../main-branch-submodule &&
+	git -C main-branch commit -m add-submodule &&
+
+	git -C main-branch-submodule switch -c hello &&
+	test_commit -C main-branch-submodule world &&
+
+	git clone --recursive main-branch main-branch-clone &&
+	git -C main-branch-clone submodule update --remote main-branch-submodule &&
+	test_path_exists main-branch-clone/main-branch-submodule/world.t
+'
+
 test_expect_success 'submodule update detaching the HEAD ' '
 	(cd super/submodule &&
 	 git reset --hard HEAD~1
diff --git a/t/t7419-submodule-set-branch.sh b/t/t7419-submodule-set-branch.sh
index fd25f786a3..3b925c302f 100755
--- a/t/t7419-submodule-set-branch.sh
+++ b/t/t7419-submodule-set-branch.sh
@@ -52,12 +52,13 @@ test_expect_success 'test submodule set-branch --branch' '
 '
 
 test_expect_success 'test submodule set-branch --default' '
+	test_commit -C submodule c &&
 	(cd super &&
 		git submodule set-branch --default submodule &&
 		! grep branch .gitmodules &&
 		git submodule update --remote &&
 		cat <<-\EOF >expect &&
-		a
+		c
 		EOF
 		git -C submodule show -s --pretty=%s >actual &&
 		test_cmp expect actual
@@ -65,6 +66,7 @@ test_expect_success 'test submodule set-branch --default' '
 '
 
 test_expect_success 'test submodule set-branch -b' '
+	test_commit -C submodule b &&
 	(cd super &&
 		git submodule set-branch -b topic submodule &&
 		grep "branch = topic" .gitmodules &&
@@ -78,12 +80,13 @@ test_expect_success 'test submodule set-branch -b' '
 '
 
 test_expect_success 'test submodule set-branch -d' '
+	test_commit -C submodule d &&
 	(cd super &&
 		git submodule set-branch -d submodule &&
 		! grep branch .gitmodules &&
 		git submodule update --remote &&
 		cat <<-\EOF >expect &&
-		a
+		d
 		EOF
 		git -C submodule show -s --pretty=%s >actual &&
 		test_cmp expect actual
-- 
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