[PATCH v2 2/6] upload-pack: send symbolic ref information as capability

[Junio C Hamano <gitster@pobox.com>]

One long-standing flaw in the pack transfer protocol was that there
was no way to tell the other end which branch "HEAD" points at.
With a new "sym" capability (e.g. "sym=HEAD:refs/heads/master"; can
be repeated more than once to represent symbolic refs other than
HEAD, such as "refs/remotes/origin/HEAD").

Add an infrastructure to collect symbolic refs, format them as extra
capabilities and put it on the wire.  For now, just send information
on the "HEAD" and nothing else.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 upload-pack.c | 48 +++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 43 insertions(+), 5 deletions(-)

diff --git a/upload-pack.c b/upload-pack.c
index a6e107f..53958b9 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -734,6 +734,16 @@ static int mark_our_ref(const char *refname, const unsigned char *sha1, int flag
 	return 0;
 }
 
+static void format_symref_info(struct strbuf *buf, struct string_list *symref)
+{
+	struct string_list_item *item;
+
+	if (!symref->nr)
+		return;
+	for_each_string_list_item(item, symref)
+		strbuf_addf(buf, " sym=%s:%s", item->string, (char *)item->util);
+}
+
 static int send_ref(const char *refname, const unsigned char *sha1, int flag, void *cb_data)
 {
 	static const char *capabilities = "multi_ack thin-pack side-band"
@@ -745,32 +755,60 @@ static int send_ref(const char *refname, const unsigned char *sha1, int flag, vo
 	if (mark_our_ref(refname, sha1, flag, NULL))
 		return 0;
 
-	if (capabilities)
-		packet_write(1, "%s %s%c%s%s%s agent=%s\n",
+	if (capabilities) {
+		struct strbuf symref_info = STRBUF_INIT;
+
+		format_symref_info(&symref_info, cb_data);
+		packet_write(1, "%s %s%c%s%s%s%s agent=%s\n",
 			     sha1_to_hex(sha1), refname_nons,
 			     0, capabilities,
 			     allow_tip_sha1_in_want ? " allow-tip-sha1-in-want" : "",
 			     stateless_rpc ? " no-done" : "",
+			     symref_info.buf,
 			     git_user_agent_sanitized());
-	else
+		strbuf_release(&symref_info);
+	} else {
 		packet_write(1, "%s %s\n", sha1_to_hex(sha1), refname_nons);
+	}
 	capabilities = NULL;
 	if (!peel_ref(refname, peeled))
 		packet_write(1, "%s %s^{}\n", sha1_to_hex(peeled), refname_nons);
 	return 0;
 }
 
+static int find_symref(const char *refname, const unsigned char *sha1, int flag,
+		       void *cb_data)
+{
+	const char *symref_target;
+	struct string_list_item *item;
+	unsigned char unused[20];
+
+	if ((flag & REF_ISSYMREF) == 0)
+		return 0;
+	symref_target = resolve_ref_unsafe(refname, unused, 0, &flag);
+	if (!symref_target || (flag & REF_ISSYMREF) == 0)
+		die("'%s' is a symref but it is not?", refname);
+	item = string_list_append(cb_data, refname);
+	item->util = xstrdup(symref_target);
+	return 0;
+}
+
 static void upload_pack(void)
 {
+	struct string_list symref = STRING_LIST_INIT_DUP;
+
+	head_ref_namespaced(find_symref, &symref);
+
 	if (advertise_refs || !stateless_rpc) {
 		reset_timeout();
-		head_ref_namespaced(send_ref, NULL);
+		head_ref_namespaced(send_ref, &symref);
 		for_each_namespaced_ref(send_ref, NULL);
 		packet_flush(1);
 	} else {
-		head_ref_namespaced(mark_our_ref, NULL);
+		head_ref_namespaced(mark_our_ref, &symref);
 		for_each_namespaced_ref(mark_our_ref, NULL);
 	}
+	string_list_clear(&symref, 1);
 	if (advertise_refs)
 		return;
 
-- 
1.8.4-585-g8d1dcaf