clone: teach --detach option

Teach "git clone" the "--detach" option, which leaves the cloned repo in
detached HEAD (like "git checkout --detach"). In addition, if the clone
is not bare, do not create the local branch pointed to by the remote's
HEAD symref (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 local submodule branches are named after the superproject's
branches, so it makes no sense to create a local branch named after the
submodule's remote's branch.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

3 files changed, 38 insertions, 4 deletions

diff --git a/Documentation/git-clone.txt b/Documentation/git-clone.txt
index d6434d262d..6a4e5d31b4 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 +210,12 @@ 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.
+	Additionally, in a non-bare repository, the corresponding local 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 547d6464b3..e624d3f49a 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -78,6 +78,7 @@ 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 const char *bundle_uri;
 
 static int recurse_submodules_cb(const struct option *opt,
@@ -162,6 +163,8 @@ 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 a local branch")),
 	OPT_STRING(0, "bundle-uri", &bundle_uri,
 		   N_("uri"), N_("a URI for downloading bundles before fetching from origin remote")),
 	OPT_END()
@@ -613,10 +616,12 @@ 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"));
@@ -1357,7 +1362,8 @@ 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 45f0803ed4..418cfd5471 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" &&