Cloning empty repository uses locally configured default branch name

[Jonathan Tan <jonathantanmy@google.com>]

When cloning an empty repository, a local branch is created. But its
name is not the name of the branch that the remote HEAD points to - it
is the locally configured default branch name. This issue arose at
$DAYJOB and, from my memory, it is also not an uncommon workflow to
configure things online on a repo host and then use "git clone" so that
things like remotes are automatically configured.

Has anyone looked into solutions for this? Both protocol v0 and v2 do
not send symref information about unborn branches (v0 because, as
protocol-capabilities.txt says, "servers SHOULD include this capability
for the HEAD symref if it is one of the refs being sent"; v2 because
a symref is included only if it refers to one of the refs being sent).
In protocol v2, this could be done by adding a capability to ls-remote
(maybe, "unborn"), and in protocol v0, this could be done either by
updating the existing "symref" capability to be written even when the
target branch is unborn (which is potentially backwards incompatible) or
introducing a new capability which is like "symref".

A small issue is that upload-pack protocol v0 doesn't even write the
blank ref line ("000...000 capabilities^{}") if HEAD points to an unborn
branch, but that can be fixed as in the patch below.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
 upload-pack.c | 40 +++++++++++++++++++++++++++-------------
 1 file changed, 27 insertions(+), 13 deletions(-)

diff --git a/upload-pack.c b/upload-pack.c
index 1006bebd50..d2359a8560 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -1179,18 +1179,15 @@ static void format_symref_info(struct strbuf *buf, struct string_list *symref)
 		strbuf_addf(buf, " symref=%s:%s", item->string, (char *)item->util);
 }
 
-static int send_ref(const char *refname, const struct object_id *oid,
-		    int flag, void *cb_data)
+static const char *capabilities = "multi_ack thin-pack side-band"
+	" side-band-64k ofs-delta shallow deepen-since deepen-not"
+	" deepen-relative no-progress include-tag multi_ack_detailed";
+
+static void write_ref_lines(const char *refname_nons,
+			    const struct object_id *oid,
+			    const struct object_id *peeled,
+			    struct upload_pack_data *data)
 {
-	static const char *capabilities = "multi_ack thin-pack side-band"
-		" side-band-64k ofs-delta shallow deepen-since deepen-not"
-		" deepen-relative no-progress include-tag multi_ack_detailed";
-	const char *refname_nons = strip_namespace(refname);
-	struct object_id peeled;
-	struct upload_pack_data *data = cb_data;
-
-	if (mark_our_ref(refname_nons, refname, oid))
-		return 0;
 
 	if (capabilities) {
 		struct strbuf symref_info = STRBUF_INIT;
@@ -1213,8 +1210,23 @@ static int send_ref(const char *refname, const struct object_id *oid,
 		packet_write_fmt(1, "%s %s\n", oid_to_hex(oid), refname_nons);
 	}
 	capabilities = NULL;
-	if (!peel_ref(refname, &peeled))
-		packet_write_fmt(1, "%s %s^{}\n", oid_to_hex(&peeled), refname_nons);
+	if (peeled)
+		packet_write_fmt(1, "%s %s^{}\n", oid_to_hex(peeled), refname_nons);
+}
+
+static int send_ref(const char *refname, const struct object_id *oid,
+		    int flag, void *cb_data)
+{
+	const char *refname_nons = strip_namespace(refname);
+	struct object_id peeled;
+	struct upload_pack_data *data = cb_data;
+
+	if (mark_our_ref(refname_nons, refname, oid))
+		return 0;
+	write_ref_lines(refname_nons,
+			oid,
+			peel_ref(refname, &peeled) ? NULL : &peeled,
+			data);
 	return 0;
 }
 
@@ -1332,6 +1344,8 @@ void upload_pack(struct upload_pack_options *options)
 		reset_timeout(data.timeout);
 		head_ref_namespaced(send_ref, &data);
 		for_each_namespaced_ref(send_ref, &data);
+		if (capabilities)
+			write_ref_lines("capabilities^{}", &null_oid, NULL, &data);
 		advertise_shallow_grafts(1);
 		packet_flush(1);
 	} else {
-- 
2.29.2.576.ga3fc446d84-goog