gpg-interface: remove other signature headers before verifying

When we have a multiply signed commit, we need to remove the signature
in the header before verifying the object, since the trailing signature
will not be over both pieces of data.  Do so, and verify that we
validate the signature appropriately.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gpg-interface.c

@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "commit.h"
 #include "config.h"
 #include "run-command.h"
 #include "strbuf.h"
@@ -366,6 +367,7 @@ int parse_signature(const char *buf, size_t size, struct strbuf *payload, struct
 	size_t match = parse_signed_buffer(buf, size);
 	if (match != size) {
 		strbuf_add(payload, buf, match);
+		remove_signature(payload);
 		strbuf_add(signature, buf + match, size - match);
 		return 1;
 	}

t/t7004-tag.sh

@@ -17,6 +17,13 @@ tag_exists () {
 	git show-ref --quiet --verify refs/tags/"$1"
 }
 
+test_expect_success 'setup' '
+	test_oid_cache <<-EOM
+	othersigheader sha1:gpgsig-sha256
+	othersigheader sha256:gpgsig
+	EOM
+'
+
 test_expect_success 'listing all tags in an empty tree should succeed' '
 	git tag -l &&
 	git tag
@@ -1371,6 +1378,24 @@ test_expect_success GPG \
 	'test_config gpg.program echo &&
 	 test_must_fail git tag -s -m tail tag-gpg-failure'
 
+# try to produce invalid signature
+test_expect_success GPG 'git verifies tag is valid with double signature' '
+	git tag -s -m tail tag-gpg-double-sig &&
+	git cat-file tag tag-gpg-double-sig >tag &&
+	othersigheader=$(test_oid othersigheader) &&
+	sed -ne "/^\$/q;p" tag >new-tag &&
+	cat <<-EOM >>new-tag &&
+	$othersigheader -----BEGIN PGP SIGNATURE-----
+	 someinvaliddata
+	 -----END PGP SIGNATURE-----
+	EOM
+	sed -e "1,/^tagger/d" tag >>new-tag &&
+	new_tag=$(git hash-object -t tag -w new-tag) &&
+	git update-ref refs/tags/tag-gpg-double-sig $new_tag &&
+	git verify-tag tag-gpg-double-sig &&
+	git fsck
+'
+
 # try to sign with bad user.signingkey
 test_expect_success GPGSM \
 	'git tag -s fails if gpgsm is misconfigured (bad key)' \