gpg-interface: find the last gpg signature line

A signed tag has a detached signature like this:

  object ...
  [...more header...]

  This is the tag body.

  -----BEGIN PGP SIGNATURE-----
  [opaque gpg data]
  -----END PGP SIGNATURE-----

Our parser finds the _first_ line that appears to start a
PGP signature block, meaning we may be confused by a
signature (or a signature-like line) in the actual body.
Let's keep parsing and always find the final block, which
should be the detached signature over all of the preceding
content.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Ben Toews <mastahyeti@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gpg-interface.c

@@ -110,11 +110,17 @@ static int is_gpg_start(const char *line)
 size_t parse_signature(const char *buf, size_t size)
 {
 	size_t len = 0;
-	while (len < size && !is_gpg_start(buf + len)) {
-		const char *eol = memchr(buf + len, '\n', size - len);
+	size_t match = size;
+	while (len < size) {
+		const char *eol;
+
+		if (is_gpg_start(buf + len))
+			match = len;
+
+		eol = memchr(buf + len, '\n', size - len);
 		len += eol ? eol - (buf + len) + 1 : size - len;
 	}
-	return len;
+	return match;
 }
 
 void set_signing_key(const char *key)

t/t7004-tag.sh

@@ -1059,6 +1059,17 @@ test_expect_success GPG \
 	git tag -v blanknonlfile-signed-tag
 '
 
+test_expect_success GPG 'signed tag with embedded PGP message' '
+	cat >msg <<-\EOF &&
+	-----BEGIN PGP MESSAGE-----
+
+	this is not a real PGP message
+	-----END PGP MESSAGE-----
+	EOF
+	git tag -s -F msg confusing-pgp-message &&
+	git tag -v confusing-pgp-message
+'
+
 # messages with commented lines for signed tags:
 
 cat >sigcommentsfile <<EOF