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

With ee66c793f8 (fast-import: add mode to sign commits with invalid
signatures, 2026-03-12), git-fast-import(1) learned to verify commit
signatures during import and replace signatures that fail verification
with a newly generated one. Extend the same behavior to signed tag
objects by introducing a 'sign-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 5e89829..783e0e7 100644
--- a/builtin/fast-import.c
+++ b/builtin/fast-import.c
@@ -191,6 +191,7 @@ static const char *global_prefix;
 static enum sign_mode signed_tag_mode = SIGN_VERBATIM;
 static enum sign_mode signed_commit_mode = SIGN_VERBATIM;
 static const char *signed_commit_keyid;
+static const char *signed_tag_keyid;
 
 /* Memory pools */
 static struct mem_pool fi_mem_pool = {
@@ -3110,6 +3111,19 @@ static void handle_tag_signature_if_invalid(struct strbuf *buf,
 
 	strbuf_setlen(msg, sig_offset);
 
+	if (signed_tag_mode == SIGN_SIGN_IF_INVALID) {
+		strbuf_attach(&payload, sigc.payload, sigc.payload_len,
+			      sigc.payload_len + 1);
+		sigc.payload = NULL;
+		strbuf_reset(&signature);
+
+		if (sign_buffer(&payload, &signature, signed_tag_keyid,
+				SIGN_BUFFER_USE_DEFAULT_KEY))
+			die(_("failed to sign tag object"));
+
+		strbuf_addbuf(msg, &signature);
+	}
+
 out:
 	signature_check_clear(&sigc);
 	strbuf_release(&signature);
@@ -3142,6 +3156,7 @@ 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_SIGN_IF_INVALID:
 	case SIGN_STRIP_IF_INVALID:
 		handle_tag_signature_if_invalid(buf, msg, sig_offset);
 		break;
@@ -3153,9 +3168,6 @@ static void handle_tag_signature(struct strbuf *buf, struct strbuf *msg, const c
 	case SIGN_ABORT_IF_INVALID:
 		die(_("'abort-if-invalid' is not a valid mode for "
 		      "git fast-import with --signed-tags=<mode>"));
-	case SIGN_SIGN_IF_INVALID:
-		die(_("'sign-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);
@@ -3749,7 +3761,7 @@ static int parse_one_option(const char *option)
 		if (parse_sign_mode(option, &signed_commit_mode, &signed_commit_keyid))
 			usagef(_("unknown --signed-commits mode '%s'"), option);
 	} else if (skip_prefix(option, "signed-tags=", &option)) {
-		if (parse_sign_mode(option, &signed_tag_mode, NULL))
+		if (parse_sign_mode(option, &signed_tag_mode, &signed_tag_keyid))
 			usagef(_("unknown --signed-tags mode '%s'"), option);
 	} else if (!strcmp(option, "quiet")) {
 		show_stats = 0;

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 fd43b0b..bb4c800 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
+for mode in strip-if-invalid sign-if-invalid
 do
 	test_expect_success GPG "import tag with no signature with --signed-tags=$mode" '
 		test_when_finished rm -rf import &&
@@ -117,7 +117,15 @@
 		IMPORTED=$(git -C import rev-parse --verify refs/tags/openpgp-signed) &&
 		test $OPENPGP_SIGNED != $IMPORTED &&
 		git -C import cat-file tag "$IMPORTED" >actual &&
-		test_grep ! -E "^-----BEGIN PGP SIGNATURE-----" actual &&
+
+		if test "$mode" = strip-if-invalid
+		then
+			test_grep ! -E "^-----BEGIN PGP SIGNATURE-----" actual
+		else
+			test_grep -E "^-----BEGIN PGP SIGNATURE-----" actual &&
+			git -C import verify-tag "$IMPORTED"
+		fi &&
+
 		test_must_be_empty log
 	'
 
@@ -150,4 +158,33 @@
 	'
 done
 
+test_expect_success GPGSSH 'sign invalid tag with explicit keyid' '
+	test_when_finished rm -rf import &&
+	git init import &&
+
+	git fast-export --signed-tags=verbatim ssh-signed >output &&
+
+	# Change the tag message, which invalidates the signature. The tag
+	# message length should not change though, otherwise the corresponding
+	# `data <length>` command would have to be changed too.
+	sed "s/SSH signed tag/SSH forged tag/" output >modified &&
+
+	# Configure the target repository with an invalid default signing key.
+	test_config -C import user.signingkey "not-a-real-key-id" &&
+	test_config -C import gpg.format ssh &&
+	test_config -C import gpg.ssh.allowedSignersFile "${GPGSSH_ALLOWED_SIGNERS}" &&
+	test_must_fail git -C import fast-import --quiet \
+		--signed-tags=sign-if-invalid <modified >/dev/null 2>&1 &&
+
+	# Import using explicitly provided signing key.
+	git -C import fast-import --quiet \
+		--signed-tags=sign-if-invalid="${GPGSSH_KEY_PRIMARY}" <modified &&
+
+	IMPORTED=$(git -C import rev-parse --verify refs/tags/ssh-signed) &&
+	test $SSH_SIGNED != $IMPORTED &&
+	git -C import cat-file tag "$IMPORTED" >actual &&
+	test_grep -E "^-----BEGIN SSH SIGNATURE-----" actual &&
+	git -C import verify-tag "$IMPORTED"
+'
+
 test_done