[PATCH 1/6] clone: teach --detach option

["Glen Choo via GitGitGadget" <gitgitgadget@gmail.com>]

From: Glen Choo <chooglen@google.com>

Teach "git clone" the "--detach" option, which leaves the cloned repo in
detached HEAD (like "git checkout --detach"). If the clone is not bare,
the remote's HEAD branch is also not created (bare clones always copy
all remote branches directly to local branches, so the branch is still
created in the bare case).

This is especially useful in the "submodule.propagateBranches" workflow,
where the submodule branch names match the superproject's branch names,
so it makes no sense to name the branches after the submodule's remote's
branches.

Signed-off-by: Glen Choo <chooglen@google.com>
---
 Documentation/git-clone.txt |  7 ++++++-
 builtin/clone.c             | 12 +++++++++---
 t/t5601-clone.sh            | 22 ++++++++++++++++++++++
 3 files changed, 37 insertions(+), 4 deletions(-)

diff --git a/Documentation/git-clone.txt b/Documentation/git-clone.txt
index 632bd1348ea..a3af90824b6 100644
--- a/Documentation/git-clone.txt
+++ b/Documentation/git-clone.txt
@@ -16,7 +16,7 @@ SYNOPSIS
 	  [--depth <depth>] [--[no-]single-branch] [--no-tags]
 	  [--recurse-submodules[=<pathspec>]] [--[no-]shallow-submodules]
 	  [--[no-]remote-submodules] [--jobs <n>] [--sparse] [--[no-]reject-shallow]
-	  [--filter=<filter> [--also-filter-submodules]] [--] <repository>
+	  [--filter=<filter> [--also-filter-submodules] [--detach]] [--] <repository>
 	  [<directory>]
 
 DESCRIPTION
@@ -210,6 +210,11 @@ objects from the source repository into a pack in the cloned repository.
 	`--branch` can also take tags and detaches the HEAD at that commit
 	in the resulting repository.
 
+--detach::
+	If the cloned repository's HEAD points to a branch, point the newly
+	created HEAD to the branch's commit instead of the branch itself. In a
+	non-bare repository, the branch will not be created.
+
 -u <upload-pack>::
 --upload-pack <upload-pack>::
 	When given, and the repository to clone from is accessed
diff --git a/builtin/clone.c b/builtin/clone.c
index c4ff4643ecd..1bc1807360e 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -77,6 +77,7 @@ static int option_filter_submodules = -1;    /* unspecified */
 static int config_filter_submodules = -1;    /* unspecified */
 static struct string_list server_options = STRING_LIST_INIT_NODUP;
 static int option_remote_submodules;
+static int option_detach;
 
 static int recurse_submodules_cb(const struct option *opt,
 				 const char *arg, int unset)
@@ -160,6 +161,8 @@ static struct option builtin_clone_options[] = {
 		    N_("any cloned submodules will use their remote-tracking branch")),
 	OPT_BOOL(0, "sparse", &option_sparse_checkout,
 		    N_("initialize sparse-checkout file to include only files at root")),
+	OPT_BOOL(0, "detach", &option_detach,
+		 N_("detach HEAD and don't create branch")),
 	OPT_END()
 };
 
@@ -607,10 +610,12 @@ static void update_remote_refs(const struct ref *refs,
 }
 
 static void update_head(const struct ref *our, const struct ref *remote,
-			const char *unborn, const char *msg)
+			const char *unborn, int should_detach,
+			const char *msg)
 {
 	const char *head;
-	if (our && skip_prefix(our->name, "refs/heads/", &head)) {
+	if (our && !should_detach &&
+	    skip_prefix(our->name, "refs/heads/", &head)) {
 		/* Local default branch link */
 		if (create_symref("HEAD", our->name, NULL) < 0)
 			die(_("unable to update HEAD"));
@@ -1339,7 +1344,8 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 			   branch_top.buf, reflog_msg.buf, transport,
 			   !is_local);
 
-	update_head(our_head_points_at, remote_head, unborn_head, reflog_msg.buf);
+	update_head(our_head_points_at, remote_head, unborn_head,
+		    option_detach, reflog_msg.buf);
 
 	/*
 	 * We want to show progress for recursive submodule clones iff
diff --git a/t/t5601-clone.sh b/t/t5601-clone.sh
index cf3be0584f4..1e7e5143a76 100755
--- a/t/t5601-clone.sh
+++ b/t/t5601-clone.sh
@@ -333,6 +333,28 @@ test_expect_success 'clone checking out a tag' '
 	test_cmp fetch.expected fetch.actual
 '
 
+test_expect_success '--detach detaches and does not create branch' '
+	test_when_finished "rm -fr dst" &&
+	git clone --detach src dst &&
+	(
+		cd dst &&
+		test_must_fail git rev-parse main &&
+		test_must_fail git symbolic-ref HEAD &&
+		test_cmp_rev HEAD refs/remotes/origin/HEAD
+	)
+'
+
+test_expect_success '--detach with --bare detaches but creates branch' '
+	test_when_finished "rm -fr dst" &&
+	git clone --bare --detach src dst &&
+	(
+		cd dst &&
+		git rev-parse main &&
+		test_must_fail git symbolic-ref HEAD &&
+		test_cmp_rev HEAD refs/heads/main
+	)
+'
+
 test_expect_success 'set up ssh wrapper' '
 	cp "$GIT_BUILD_DIR/t/helper/test-fake-ssh$X" \
 		"$TRASH_DIRECTORY/ssh$X" &&
-- 
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