[PATCH v3 3/3] clone: respect remote unborn HEAD

[Jonathan Tan <jonathantanmy@google.com>]

Teach Git to use the "unborn" feature introduced in a previous patch as
follows: Git will always send the "unborn" argument if it is supported
by the server. During "git clone", if cloning an empty repository, Git
will use the new information to determine the local branch to create. In
all other cases, Git will ignore it.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
 Documentation/config/init.txt |  2 +-
 builtin/clone.c               | 19 ++++++++++++++++---
 connect.c                     | 28 ++++++++++++++++++++++++----
 t/t5606-clone-options.sh      |  7 ++++---
 t/t5702-protocol-v2.sh        |  9 +++++++++
 5 files changed, 54 insertions(+), 11 deletions(-)

diff --git a/Documentation/config/init.txt b/Documentation/config/init.txt
index dc77f8c844..79c79d6617 100644
--- a/Documentation/config/init.txt
+++ b/Documentation/config/init.txt
@@ -4,4 +4,4 @@ init.templateDir::
 
 init.defaultBranch::
 	Allows overriding the default branch name e.g. when initializing
-	a new repository or when cloning an empty repository.
+	a new repository.
diff --git a/builtin/clone.c b/builtin/clone.c
index 70f9450db4..217c87fddf 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -980,6 +980,7 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	int submodule_progress;
 
 	struct strvec ref_prefixes = STRVEC_INIT;
+	char *unborn_head_target = NULL;
 
 	packet_trace_identity("clone");
 
@@ -1264,7 +1265,8 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	if (!option_no_tags)
 		strvec_push(&ref_prefixes, "refs/tags/");
 
-	refs = transport_get_remote_refs(transport, &ref_prefixes, NULL);
+	refs = transport_get_remote_refs(transport, &ref_prefixes,
+					 &unborn_head_target);
 
 	if (refs) {
 		int hash_algo = hash_algo_by_ptr(transport_get_hash_algo(transport));
@@ -1323,10 +1325,20 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 		remote_head = NULL;
 		option_no_checkout = 1;
 		if (!option_bare) {
-			const char *branch = git_default_branch_name();
-			char *ref = xstrfmt("refs/heads/%s", branch);
+			const char *branch;
+			char *ref;
+
+			if (unborn_head_target &&
+			    skip_prefix(unborn_head_target, "refs/heads/", &branch)) {
+				ref = unborn_head_target;
+				unborn_head_target = NULL;
+			} else {
+				branch = git_default_branch_name();
+				ref = xstrfmt("refs/heads/%s", branch);
+			}
 
 			install_branch_config(0, branch, remote_name, ref);
+			create_symref("HEAD", ref, "");
 			free(ref);
 		}
 	}
@@ -1373,6 +1385,7 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	strbuf_release(&key);
 	junk_mode = JUNK_LEAVE_ALL;
 
+	free(unborn_head_target);
 	strvec_clear(&ref_prefixes);
 	return err;
 }
diff --git a/connect.c b/connect.c
index 99d9052365..3c35324b4c 100644
--- a/connect.c
+++ b/connect.c
@@ -376,7 +376,8 @@ struct ref **get_remote_heads(struct packet_reader *reader,
 }
 
 /* Returns 1 when a valid ref has been added to `list`, 0 otherwise */
-static int process_ref_v2(struct packet_reader *reader, struct ref ***list)
+static int process_ref_v2(struct packet_reader *reader, struct ref ***list,
+			  char **unborn_head_target)
 {
 	int ret = 1;
 	int i = 0;
@@ -397,6 +398,25 @@ static int process_ref_v2(struct packet_reader *reader, struct ref ***list)
 		goto out;
 	}
 
+	if (!strcmp("unborn", line_sections.items[i].string)) {
+		i++;
+		if (unborn_head_target &&
+		    !strcmp("HEAD", line_sections.items[i++].string)) {
+			/*
+			 * Look for the symref target (if any). If found,
+			 * return it to the caller.
+			 */
+			for (; i < line_sections.nr; i++) {
+				const char *arg = line_sections.items[i].string;
+
+				if (skip_prefix(arg, "symref-target:", &arg)) {
+					*unborn_head_target = xstrdup(arg);
+					break;
+				}
+			}
+		}
+		goto out;
+	}
 	if (parse_oid_hex_algop(line_sections.items[i++].string, &old_oid, &end, reader->hash_algo) ||
 	    *end) {
 		ret = 0;
@@ -489,6 +509,8 @@ struct ref **get_remote_refs(int fd_out, struct packet_reader *reader,
 	if (!for_push)
 		packet_write_fmt(fd_out, "peel\n");
 	packet_write_fmt(fd_out, "symrefs\n");
+	if (server_supports_feature("ls-refs", "unborn", 0))
+		packet_write_fmt(fd_out, "unborn\n");
 	for (i = 0; ref_prefixes && i < ref_prefixes->nr; i++) {
 		packet_write_fmt(fd_out, "ref-prefix %s\n",
 				 ref_prefixes->v[i]);
@@ -497,9 +519,7 @@ struct ref **get_remote_refs(int fd_out, struct packet_reader *reader,
 
 	/* Process response from server */
 	while (packet_reader_read(reader) == PACKET_READ_NORMAL) {
-		if (unborn_head_target)
-			BUG("NEEDSWORK: provide unborn HEAD target to caller while reading refs");
-		if (!process_ref_v2(reader, &list))
+		if (!process_ref_v2(reader, &list, unborn_head_target))
 			die(_("invalid ls-refs response: %s"), reader->line);
 	}
 
diff --git a/t/t5606-clone-options.sh b/t/t5606-clone-options.sh
index 7f082fb23b..67a4c6d05b 100755
--- a/t/t5606-clone-options.sh
+++ b/t/t5606-clone-options.sh
@@ -102,11 +102,12 @@ test_expect_success 'redirected clone -v does show progress' '
 '
 
 test_expect_success 'chooses correct default initial branch name' '
-	git init --bare empty &&
+	git -c init.defaultBranch=foo init --bare empty &&
+	test_config -C empty lsrefs.allowUnborn true &&
 	GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME= \
 	git -c init.defaultBranch=up clone empty whats-up &&
-	test refs/heads/up = $(git -C whats-up symbolic-ref HEAD) &&
-	test refs/heads/up = $(git -C whats-up config branch.up.merge)
+	test refs/heads/foo = $(git -C whats-up symbolic-ref HEAD) &&
+	test refs/heads/foo = $(git -C whats-up config branch.foo.merge)
 '
 
 test_expect_success 'guesses initial branch name correctly' '
diff --git a/t/t5702-protocol-v2.sh b/t/t5702-protocol-v2.sh
index 7d5b17909b..4fbbe5aff5 100755
--- a/t/t5702-protocol-v2.sh
+++ b/t/t5702-protocol-v2.sh
@@ -209,6 +209,15 @@ test_expect_success 'clone with file:// using protocol v2' '
 	grep "ref-prefix refs/tags/" log
 '
 
+test_expect_success 'clone of empty repo propagates name of default branch' '
+	git -c init.defaultBranch=mydefaultbranch init file_empty_parent &&
+	test_config -C file_empty_parent lsrefs.allowUnborn true &&
+
+	git -c init.defaultBranch=main -c protocol.version=2 \
+		clone "file://$(pwd)/file_empty_parent" file_empty_child &&
+	grep "refs/heads/mydefaultbranch" file_empty_child/.git/HEAD
+'
+
 test_expect_success 'fetch with file:// using protocol v2' '
 	test_when_finished "rm -f log" &&
 
-- 
2.29.2.729.g45daf8777d-goog