[PATCH 2/4] fast-import: add 'strip-if-invalid' mode to '--signed-tags=<mode>'

[Justin Tobler <jltobler@gmail.com>]

With c20f112e51 (fast-import: add 'strip-if-invalid' mode to
--signed-commits=<mode>, 2025-11-17), git-fast-import(1) learned to
verify commit signatures during import and strip signatures that fail
verification. Extend the same behavior to signed tag objects by
introducing a 'strip-if-invalid' mode for the '--signed-tags' option.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
---
 Documentation/git-fast-import.adoc |  7 ++-
 builtin/fast-import.c              | 40 +++++++++++++---
 t/t9306-fast-import-signed-tags.sh | 73 ++++++++++++++++++++++++++++++
 3 files changed, 110 insertions(+), 10 deletions(-)

diff --git a/Documentation/git-fast-import.adoc b/Documentation/git-fast-import.adoc
index 288f2b2a7e..d68bc52b7e 100644
--- a/Documentation/git-fast-import.adoc
+++ b/Documentation/git-fast-import.adoc
@@ -66,11 +66,10 @@ fast-import stream! This option is enabled automatically for
 remote-helpers that use the `import` capability, as they are
 already trusted to run their own code.
 
-`--signed-tags=(verbatim|warn-verbatim|warn-strip|strip|abort)`::
+`--signed-tags=<mode>`::
 	Specify how to handle signed tags. Behaves in the same way as
-	the `--signed-commits=<mode>` below, except that the
-	`strip-if-invalid` mode is not yet supported. Like for signed
-	commits, the default mode is `verbatim`.
+	the `--signed-commits=<mode>` below. Like for signed commits,
+	the default mode is `verbatim`.
 
 `--signed-commits=<mode>`::
 	Specify how to handle signed commits. The following <mode>s
diff --git a/builtin/fast-import.c b/builtin/fast-import.c
index 08ea27242d..5e89829aea 100644
--- a/builtin/fast-import.c
+++ b/builtin/fast-import.c
@@ -3089,7 +3089,34 @@ static void parse_new_commit(const char *arg)
 	b->last_commit = object_count_by_type[OBJ_COMMIT];
 }
 
-static void handle_tag_signature(struct strbuf *msg, const char *name)
+static void handle_tag_signature_if_invalid(struct strbuf *buf,
+					    struct strbuf *msg,
+					    size_t sig_offset)
+{
+	struct strbuf signature = STRBUF_INIT;
+	struct strbuf payload = STRBUF_INIT;
+	struct signature_check sigc = { 0 };
+
+	strbuf_addbuf(&payload, buf);
+	strbuf_addch(&payload, '\n');
+	strbuf_add(&payload, msg->buf, sig_offset);
+	strbuf_add(&signature, msg->buf + sig_offset, msg->len - sig_offset);
+
+	sigc.payload_type = SIGNATURE_PAYLOAD_TAG;
+	sigc.payload = strbuf_detach(&payload, &sigc.payload_len);
+
+	if (!check_signature(&sigc, signature.buf, signature.len))
+		goto out;
+
+	strbuf_setlen(msg, sig_offset);
+
+out:
+	signature_check_clear(&sigc);
+	strbuf_release(&signature);
+	strbuf_release(&payload);
+}
+
+static void handle_tag_signature(struct strbuf *buf, struct strbuf *msg, const char *name)
 {
 	size_t sig_offset = parse_signed_buffer(msg->buf, msg->len);
 
@@ -3115,6 +3142,9 @@ static void handle_tag_signature(struct strbuf *msg, const char *name)
 		/* Truncate the buffer to remove the signature */
 		strbuf_setlen(msg, sig_offset);
 		break;
+	case SIGN_STRIP_IF_INVALID:
+		handle_tag_signature_if_invalid(buf, msg, sig_offset);
+		break;
 
 	/* Third, aborting modes */
 	case SIGN_ABORT:
@@ -3123,9 +3153,6 @@ static void handle_tag_signature(struct strbuf *msg, const char *name)
 	case SIGN_ABORT_IF_INVALID:
 		die(_("'abort-if-invalid' is not a valid mode for "
 		      "git fast-import with --signed-tags=<mode>"));
-	case SIGN_STRIP_IF_INVALID:
-		die(_("'strip-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>"));
@@ -3198,8 +3225,6 @@ static void parse_new_tag(const char *arg)
 	/* tag payload/message */
 	parse_data(&msg, 0, NULL);
 
-	handle_tag_signature(&msg, t->name);
-
 	/* build the tag object */
 	strbuf_reset(&new_data);
 
@@ -3211,6 +3236,9 @@ static void parse_new_tag(const char *arg)
 	if (tagger)
 		strbuf_addf(&new_data,
 			    "tagger %s\n", tagger);
+
+	handle_tag_signature(&new_data, &msg, t->name);
+
 	strbuf_addch(&new_data, '\n');
 	strbuf_addbuf(&new_data, &msg);
 	free(tagger);
diff --git a/t/t9306-fast-import-signed-tags.sh b/t/t9306-fast-import-signed-tags.sh
index 363619e7d1..fd43b0b52a 100755
--- a/t/t9306-fast-import-signed-tags.sh
+++ b/t/t9306-fast-import-signed-tags.sh
@@ -77,4 +77,77 @@ test_expect_success GPGSSH 'import SSH signed tag with --signed-tags=strip' '
 	test_grep ! "SSH SIGNATURE" out
 '
 
+for mode in strip-if-invalid
+do
+	test_expect_success GPG "import tag with no signature with --signed-tags=$mode" '
+		test_when_finished rm -rf import &&
+		git init import &&
+
+		git fast-export --signed-tags=verbatim >output &&
+		git -C import fast-import --quiet --signed-tags=$mode <output >log 2>&1 &&
+		test_must_be_empty log
+	'
+
+	test_expect_success GPG "keep valid OpenPGP signature with --signed-tags=$mode" '
+		test_when_finished rm -rf import &&
+		git init import &&
+
+		git fast-export --signed-tags=verbatim openpgp-signed >output &&
+		git -C import fast-import --quiet --signed-tags=$mode <output >log 2>&1 &&
+		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 &&
+		test_must_be_empty log
+	'
+
+	test_expect_success GPG "handle signature invalidated by message change with --signed-tags=$mode" '
+		test_when_finished rm -rf import &&
+		git init import &&
+
+		git fast-export --signed-tags=verbatim openpgp-signed >output &&
+
+		# Change the tag message, which invalidates the signature. The tag
+		# message length should not change though, otherwise the corresponding
+		# `data <length>` command would have to be changed too.
+		sed "s/OpenPGP signed tag/OpenPGP forged tag/" output >modified &&
+
+		git -C import fast-import --quiet --signed-tags=$mode <modified >log 2>&1 &&
+
+		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 &&
+		test_must_be_empty log
+	'
+
+	test_expect_success GPGSM "keep valid X.509 signature with --signed-tags=$mode" '
+		test_when_finished rm -rf import &&
+		git init import &&
+
+		git fast-export --signed-tags=verbatim x509-signed >output &&
+		git -C import fast-import --quiet --signed-tags=$mode <output >log 2>&1 &&
+		IMPORTED=$(git -C import rev-parse --verify refs/tags/x509-signed) &&
+		test $X509_SIGNED = $IMPORTED &&
+		git -C import cat-file tag x509-signed >actual &&
+		test_grep -E "^-----BEGIN SIGNED MESSAGE-----" actual &&
+		test_must_be_empty log
+	'
+
+	test_expect_success GPGSSH "keep valid SSH signature with --signed-tags=$mode" '
+		test_when_finished rm -rf import &&
+		git init import &&
+
+		test_config -C import gpg.ssh.allowedSignersFile "${GPGSSH_ALLOWED_SIGNERS}" &&
+
+		git fast-export --signed-tags=verbatim ssh-signed >output &&
+		git -C import fast-import --quiet --signed-tags=$mode <output >log 2>&1 &&
+		IMPORTED=$(git -C import rev-parse --verify refs/tags/ssh-signed) &&
+		test $SSH_SIGNED = $IMPORTED &&
+		git -C import cat-file tag ssh-signed >actual &&
+		test_grep -E "^-----BEGIN SSH SIGNATURE-----" actual &&
+		test_must_be_empty log
+	'
+done
+
 test_done
-- 
2.53.0.381.g628a66ccf6