upload-pack: optionally allow fetching from the tips of hidden refs

With uploadpack.allowtipsha1inwant configuration option set, future
versions of "git fetch" that allow an exact object name (likely to
have been obtained out of band) on the LHS of the fetch refspec can
make a request with a "want" line that names an object that may not
have been advertised due to transfer.hiderefs configuration.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2083,7 +2083,13 @@ uploadpack.hiderefs::
 	are under the hierarchies listed on the value of this
 	variable is excluded, and is hidden from `git ls-remote`,
 	`git fetch`, etc.  An attempt to fetch a hidden ref by `git
-	fetch` will fail.
+	fetch` will fail.  See also `uploadpack.allowtipsha1inwant`.
+
+uploadpack.allowtipsha1inwant::
+	When `uploadpack.hiderefs` is in effect, allow `upload-pack`
+	to accept a fetch request that asks for an object at the tip
+	of a hidden ref (by default, such a request is rejected).
+	see also `uploadpack.hiderefs`.
 
 url.<base>.insteadOf::
 	Any URL that starts with this value will be rewritten to

upload-pack.c

@@ -26,6 +26,7 @@ static const char upload_pack_usage[] = "git upload-pack [--strict] [--timeout=<
 #define SHALLOW		(1u << 16)
 #define NOT_SHALLOW	(1u << 17)
 #define CLIENT_SHALLOW	(1u << 18)
+#define HIDDEN_REF	(1u << 19)
 
 static unsigned long oldest_have;
 
@@ -33,6 +34,7 @@ static int multi_ack;
 static int no_done;
 static int use_thin_pack, use_ofs_delta, use_include_tag;
 static int no_progress, daemon_mode;
+static int allow_tip_sha1_in_want;
 static int shallow_nr;
 static struct object_array have_obj;
 static struct object_array want_obj;
@@ -487,6 +489,12 @@ static int get_common_commits(void)
 	}
 }
 
+static int is_our_ref(struct object *o)
+{
+	return o->flags &
+		((allow_tip_sha1_in_want ? HIDDEN_REF : 0) | OUR_REF);
+}
+
 static void check_non_tip(void)
 {
 	static const char *argv[] = {
@@ -523,7 +531,7 @@ static void check_non_tip(void)
 		o = get_indexed_object(--i);
 		if (!o)
 			continue;
-		if (!(o->flags & OUR_REF))
+		if (!is_our_ref(o))
 			continue;
 		memcpy(namebuf + 1, sha1_to_hex(o->sha1), 40);
 		if (write_in_full(cmd.in, namebuf, 42) < 0)
@@ -532,7 +540,7 @@ static void check_non_tip(void)
 	namebuf[40] = '\n';
 	for (i = 0; i < want_obj.nr; i++) {
 		o = want_obj.objects[i].item;
-		if (o->flags & OUR_REF)
+		if (is_our_ref(o))
 			continue;
 		memcpy(namebuf, sha1_to_hex(o->sha1), 40);
 		if (write_in_full(cmd.in, namebuf, 41) < 0)
@@ -566,7 +574,7 @@ error:
 	/* Pick one of them (we know there at least is one) */
 	for (i = 0; i < want_obj.nr; i++) {
 		o = want_obj.objects[i].item;
-		if (!(o->flags & OUR_REF))
+		if (!is_our_ref(o))
 			die("git upload-pack: not our ref %s",
 			    sha1_to_hex(o->sha1));
 	}
@@ -646,7 +654,7 @@ static void receive_needs(void)
 			    sha1_to_hex(sha1_buf));
 		if (!(o->flags & WANTED)) {
 			o->flags |= WANTED;
-			if (!(o->flags & OUR_REF))
+			if (!is_our_ref(o))
 				has_non_tip = 1;
 			add_object_array(o, NULL, &want_obj);
 		}
@@ -725,8 +733,10 @@ static int mark_our_ref(const char *refname, const unsigned char *sha1, int flag
 {
 	struct object *o = lookup_unknown_object(sha1);
 
-	if (ref_is_hidden(refname))
+	if (ref_is_hidden(refname)) {
+		o->flags |= HIDDEN_REF;
 		return 1;
+	}
 	if (!o)
 		die("git upload-pack: cannot find object %s:", sha1_to_hex(sha1));
 	o->flags |= OUR_REF;
@@ -745,9 +755,10 @@ static int send_ref(const char *refname, const unsigned char *sha1, int flag, vo
 		return 0;
 
 	if (capabilities)
-		packet_write(1, "%s %s%c%s%s agent=%s\n",
+		packet_write(1, "%s %s%c%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" : "",
 			     git_user_agent_sanitized());
 	else
@@ -781,6 +792,8 @@ static void upload_pack(void)
 
 static int upload_pack_config(const char *var, const char *value, void *unused)
 {
+	if (!strcmp("uploadpack.allowtipsha1inwant", var))
+		allow_tip_sha1_in_want = git_config_bool(var, value);
 	return parse_hide_refs_config(var, value, "uploadpack");
 }