remote: make match_refs() copy src ref before assigning to peer_ref

In some instances, match_refs() sets the peer_ref field of refs in the
dst list such that it points to a ref in the src list. This prevents
callers from freeing both the src and dst lists, as doing so would cause
a double-free since free_refs() frees the peer_ref.

As well, the following configuration causes two refs in the dst list to
have the same peer_ref, which can also lead to a double-free:

  push = refs/heads/master:refs/heads/backup
  push = refs/heads/master:refs/heads/master

Existing callers of match_heads() call it only once and then terminate,
w/o ever bothering to free the src or dst lists, so this is not
currently a problem.

This patch modifies match_refs() to first copy any refs it plucks from
the src list before assigning them as a peer_ref. This allows
builtin-remote, a future caller, to free the src and dst lists.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.c

@@ -936,6 +936,7 @@ static int match_explicit(struct ref *src, struct ref *dst,
 			  struct refspec *rs)
 {
 	struct ref *matched_src, *matched_dst;
+	int copy_src;
 
 	const char *dst_value = rs->dst;
 	char *dst_guess;
@@ -946,6 +947,7 @@ static int match_explicit(struct ref *src, struct ref *dst,
 	matched_src = matched_dst = NULL;
 	switch (count_refspec_match(rs->src, src, &matched_src)) {
 	case 1:
+		copy_src = 1;
 		break;
 	case 0:
 		/* The source could be in the get_sha1() format
@@ -955,6 +957,7 @@ static int match_explicit(struct ref *src, struct ref *dst,
 		matched_src = try_explicit_object_name(rs->src);
 		if (!matched_src)
 			return error("src refspec %s does not match any.", rs->src);
+		copy_src = 0;
 		break;
 	default:
 		return error("src refspec %s matches more than one.", rs->src);
@@ -1000,7 +1003,7 @@ static int match_explicit(struct ref *src, struct ref *dst,
 		return error("dst ref %s receives from more than one src.",
 		      matched_dst->name);
 	else {
-		matched_dst->peer_ref = matched_src;
+		matched_dst->peer_ref = copy_src ? copy_ref(matched_src) : matched_src;
 		matched_dst->force = rs->force;
 	}
 	return 0;
@@ -1108,7 +1111,7 @@ int match_refs(struct ref *src, struct ref *dst, struct ref ***dst_tail,
 			dst_peer = make_linked_ref(dst_name, dst_tail);
 			hashcpy(dst_peer->new_sha1, src->new_sha1);
 		}
-		dst_peer->peer_ref = src;
+		dst_peer->peer_ref = copy_ref(src);
 		dst_peer->force = pat->force;
 	free_name:
 		free(dst_name);