fast-import: add 'abort-if-invalid' mode to '--signed-tags=<mode>'

In git-fast-import(1), the 'abort-if-invalid' mode for the
'--signed-commits' option verifies commit signatures during import and
aborts the entire operation when verification fails. Extend the same
behavior to signed tag objects by introducing an 'abort-if-invalid' mode
for the '--signed-tags' option.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fast-import.c

diff --git a/builtin/fast-import.c b/builtin/fast-import.c
index 783e0e7..cd11810 100644
--- a/builtin/fast-import.c
+++ b/builtin/fast-import.c
@@ -3109,6 +3109,9 @@ static void handle_tag_signature_if_invalid(struct strbuf *buf,
 	if (!check_signature(&sigc, signature.buf, signature.len))
 		goto out;
 
+	if (signed_tag_mode == SIGN_ABORT_IF_INVALID)
+		die(_("aborting due to invalid signature"));
+
 	strbuf_setlen(msg, sig_offset);
 
 	if (signed_tag_mode == SIGN_SIGN_IF_INVALID) {
@@ -3156,6 +3159,7 @@ static void handle_tag_signature(struct strbuf *buf, struct strbuf *msg, const c
 		/* Truncate the buffer to remove the signature */
 		strbuf_setlen(msg, sig_offset);
 		break;
+	case SIGN_ABORT_IF_INVALID:
 	case SIGN_SIGN_IF_INVALID:
 	case SIGN_STRIP_IF_INVALID:
 		handle_tag_signature_if_invalid(buf, msg, sig_offset);
@@ -3165,9 +3169,6 @@ static void handle_tag_signature(struct strbuf *buf, struct strbuf *msg, const c
 	case SIGN_ABORT:
 		die(_("encountered signed tag; use "
 		      "--signed-tags=<mode> to handle it"));
-	case SIGN_ABORT_IF_INVALID:
-		die(_("'abort-if-invalid' is not a valid mode for "
-		      "git fast-import with --signed-tags=<mode>"));
 	default:
 		BUG("invalid signed_tag_mode value %d from tag '%s'",
 		    signed_tag_mode, name);

t/t9306-fast-import-signed-tags.sh

diff --git a/t/t9306-fast-import-signed-tags.sh b/t/t9306-fast-import-signed-tags.sh
index bb4c800..ec2b241 100755
--- a/t/t9306-fast-import-signed-tags.sh
+++ b/t/t9306-fast-import-signed-tags.sh
@@ -77,7 +77,7 @@
 	test_grep ! "SSH SIGNATURE" out
 '
 
-for mode in strip-if-invalid sign-if-invalid
+for mode in strip-if-invalid sign-if-invalid abort-if-invalid
 do
 	test_expect_success GPG "import tag with no signature with --signed-tags=$mode" '
 		test_when_finished rm -rf import &&
@@ -112,6 +112,14 @@
 		# `data <length>` command would have to be changed too.
 		sed "s/OpenPGP signed tag/OpenPGP forged tag/" output >modified &&
 
+		if test "$mode" = abort-if-invalid
+		then
+			test_must_fail git -C import fast-import --quiet \
+				--signed-tags=$mode <modified >log 2>&1 &&
+			test_grep "aborting due to invalid signature" log &&
+			return 0
+		fi &&
+
 		git -C import fast-import --quiet --signed-tags=$mode <modified >log 2>&1 &&
 
 		IMPORTED=$(git -C import rev-parse --verify refs/tags/openpgp-signed) &&