log-tree: show mergetag in log --show-signature output

A commit object that merges a signed tag records the "mergetag" extended
header. Check the validity of the GPG signature on it, and show it in a
way similar to how "gpgsig" extended header is shown.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

log-tree.c

@@ -443,6 +443,78 @@ static void show_signature(struct rev_info *opt, struct commit *commit)
 	strbuf_release(&signature);
 }
 
+static int which_parent(const unsigned char *sha1, const struct commit *commit)
+{
+	int nth;
+	const struct commit_list *parent;
+
+	for (nth = 0, parent = commit->parents; parent; parent = parent->next) {
+		if (!hashcmp(parent->item->object.sha1, sha1))
+			return nth;
+		nth++;
+	}
+	return -1;
+}
+
+static void show_one_mergetag(struct rev_info *opt,
+			      struct commit_extra_header *extra,
+			      struct commit *commit)
+{
+	unsigned char sha1[20];
+	struct tag *tag;
+	struct strbuf verify_message;
+	int status, nth;
+	size_t payload_size, gpg_message_offset;
+
+	hash_sha1_file(extra->value, extra->len, typename(OBJ_TAG), sha1);
+	tag = lookup_tag(sha1);
+	if (!tag)
+		return; /* error message already given */
+
+	strbuf_init(&verify_message, 256);
+	if (parse_tag_buffer(tag, extra->value, extra->len))
+		strbuf_addstr(&verify_message, "malformed mergetag\n");
+	else if ((nth = which_parent(tag->tagged->sha1, commit)) < 0)
+		strbuf_addf(&verify_message, "tag %s names a non-parent %s\n",
+				    tag->tag, tag->tagged->sha1);
+	else
+		strbuf_addf(&verify_message,
+			    "parent #%d, tagged '%s'\n", nth + 1, tag->tag);
+	gpg_message_offset = verify_message.len;
+
+	payload_size = parse_signature(extra->value, extra->len);
+	if ((extra->len <= payload_size) ||
+	    (verify_signed_buffer(extra->value, payload_size,
+				  extra->value + payload_size,
+				  extra->len - payload_size,
+				  &verify_message) &&
+	     verify_message.len <= gpg_message_offset)) {
+		strbuf_addstr(&verify_message, "No signature\n");
+		status = -1;
+	}
+	else if (strstr(verify_message.buf + gpg_message_offset,
+			": Good signature from "))
+		status = 0;
+	else
+		status = -1;
+
+	show_sig_lines(opt, status, verify_message.buf);
+	strbuf_release(&verify_message);
+}
+
+static void show_mergetag(struct rev_info *opt, struct commit *commit)
+{
+	struct commit_extra_header *extra, *to_free;
+
+	to_free = read_commit_extra_headers(commit, NULL);
+	for (extra = to_free; extra; extra = extra->next) {
+		if (strcmp(extra->key, "mergetag"))
+			continue; /* not a merge tag */
+		show_one_mergetag(opt, extra, commit);
+	}
+	free_commit_extra_headers(to_free);
+}
+
 void show_log(struct rev_info *opt)
 {
 	struct strbuf msgbuf = STRBUF_INIT;
@@ -554,8 +626,10 @@ void show_log(struct rev_info *opt)
 		}
 	}
 
-	if (opt->show_signature)
+	if (opt->show_signature) {
 		show_signature(opt, commit);
+		show_mergetag(opt, commit);
+	}
 
 	if (!commit->buffer)
 		return;