fast-import: add '--signed-commits=<mode>' option

A '--signed-commits=<mode>' option is already available when using
`git fast-export` to decide what should be done at export time about
commit signatures. At import time though, there is no option, or
other way, in `git fast-import` to decide about commit signatures.

To remediate that, let's add a '--signed-commits=<mode>' option to
`git fast-import` too.

For now the supported <mode>s are the same as those supported by
`git fast-export`.

The code responsible for consuming a signature is refactored into
the import_one_signature() and discard_one_signature() functions,
which makes it easier to follow the logic and add new modes in the
future.

In the 'strip' and 'warn-strip' modes, we deliberately use
discard_one_signature() to discard the signature without parsing it.
This ensures that even malformed signatures, which would cause the
parser to fail, can be successfully stripped from a commit.

Signed-off-by: Christian Couder <[e-mail]>
Signed-off-by: Junio C Hamano <[e-mail]>

Documentation/git-fast-import.adoc

···
 	remote-helpers that use the `import` capability, as they are
 	already trusted to run their own code.
 
+--signed-commits=(verbatim|warn-verbatim|warn-strip|strip|abort)::
+	Specify how to handle signed commits.  Behaves in the same way
+	as the same option in linkgit:git-fast-export[1], except that
+	default is 'verbatim' (instead of 'abort').
+
 Options for Frontends
 ~~~~~~~~~~~~~~~~~~~~~
 

builtin/fast-import.c

···
 static const char **global_argv;
 static const char *global_prefix;
 
+static enum sign_mode signed_commit_mode = SIGN_VERBATIM;
+
 /* Memory pools */
 static struct mem_pool fi_mem_pool = {
 	.block_alloc = 2*1024*1024 - sizeof(struct mp_block),
···
 	parse_data(&sig->data, 0, NULL);
 }
 
+static void discard_one_signature(void)
+{
+	struct strbuf data = STRBUF_INIT;
+
+	read_next_command();
+	parse_data(&data, 0, NULL);
+	strbuf_release(&data);
+}
+
 static void add_gpgsig_to_commit(struct strbuf *commit_data,
 				 const char *header,
 				 struct signature_data *sig)
···
 	}
 }
 
+static void import_one_signature(struct signature_data *sig_sha1,
+				 struct signature_data *sig_sha256,
+				 const char *v)
+{
+	struct signature_data sig = { NULL, NULL, STRBUF_INIT };
+
+	parse_one_signature(&sig, v);
+
+	if (!strcmp(sig.hash_algo, "sha1"))
+		store_signature(sig_sha1, &sig, "SHA-1");
+	else if (!strcmp(sig.hash_algo, "sha256"))
+		store_signature(sig_sha256, &sig, "SHA-256");
+	else
+		die(_("parse_one_signature() returned unknown hash algo"));
+}
+
 static void parse_new_commit(const char *arg)
 {
 	static struct strbuf msg = STRBUF_INIT;
···
 	if (!committer)
 		die("Expected committer but didn't get one");
 
-	/* Process signatures (up to 2: one "sha1" and one "sha256") */
 	while (skip_prefix(command_buf.buf, "gpgsig ", &v)) {
-		struct signature_data sig = { NULL, NULL, STRBUF_INIT };
+		switch (signed_commit_mode) {
 
-		parse_one_signature(&sig, v);
+		/* First, modes that don't need the signature to be parsed */
+		case SIGN_ABORT:
+			die("encountered signed commit; use "
+			    "--signed-commits=<mode> to handle it");
+		case SIGN_WARN_STRIP:
+			warning(_("stripping a commit signature"));
+			/* fallthru */
+		case SIGN_STRIP:
+			discard_one_signature();
+			break;
 
-		if (!strcmp(sig.hash_algo, "sha1"))
-			store_signature(&sig_sha1, &sig, "SHA-1");
-		else if (!strcmp(sig.hash_algo, "sha256"))
-			store_signature(&sig_sha256, &sig, "SHA-256");
-		else
-			BUG("parse_one_signature() returned unknown hash algo");
+		/* Second, modes that parse the signature */
+		case SIGN_WARN_VERBATIM:
+			warning(_("importing a commit signature verbatim"));
+			/* fallthru */
+		case SIGN_VERBATIM:
+			import_one_signature(&sig_sha1, &sig_sha256, v);
+			break;
 
+		/* Third, BUG */
+		default:
+			BUG("invalid signed_commit_mode value %d", signed_commit_mode);
+		}
 		read_next_command();
 	}
 
···
 		option_active_branches(option);
 	} else if (skip_prefix(option, "export-pack-edges=", &option)) {
 		option_export_pack_edges(option);
+	} else if (skip_prefix(option, "signed-commits=", &option)) {
+		if (parse_sign_mode(option, &signed_commit_mode))
+			usagef(_("unknown --signed-commits mode '%s'"), option);
 	} else if (!strcmp(option, "quiet")) {
 		show_stats = 0;
 		quiet = 1;

t/meson.build

···
   't9302-fast-import-unpack-limit.sh',
   't9303-fast-import-compression.sh',
   't9304-fast-import-marks.sh',
+  't9305-fast-import-signatures.sh',
   't9350-fast-export.sh',
   't9351-fast-export-anonymize.sh',
   't9400-git-cvsserver-server.sh',

t/t9305-fast-import-signatures.sh

···
+#!/bin/sh
+
+test_description='git fast-import --signed-commits=<mode>'
+
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
+
+. ./test-lib.sh
+. "$TEST_DIRECTORY/lib-gpg.sh"
+
+test_expect_success 'set up unsigned initial commit and import repo' '
+	test_commit first &&
+	git init new
+'
+
+test_expect_success GPG 'set up OpenPGP signed commit' '
+	git checkout -b openpgp-signing main &&
+	echo "Content for OpenPGP signing." >file-sign &&
+	git add file-sign &&
+	git commit -S -m "OpenPGP signed commit" &&
+	OPENPGP_SIGNING=$(git rev-parse --verify openpgp-signing)
+'
+
+test_expect_success GPG 'import OpenPGP signature with --signed-commits=verbatim' '
+	git fast-export --signed-commits=verbatim openpgp-signing >output &&
+	git -C new fast-import --quiet --signed-commits=verbatim <output >log 2>&1 &&
+	IMPORTED=$(git -C new rev-parse --verify refs/heads/openpgp-signing) &&
+	test $OPENPGP_SIGNING = $IMPORTED &&
+	test_must_be_empty log
+'
+
+test_expect_success GPGSM 'set up X.509 signed commit' '
+	git checkout -b x509-signing main &&
+	test_config gpg.format x509 &&
+	test_config user.signingkey $GIT_COMMITTER_EMAIL &&
+	echo "Content for X.509 signing." >file-sign &&
+	git add file-sign &&
+	git commit -S -m "X.509 signed commit" &&
+	X509_SIGNING=$(git rev-parse HEAD)
+'
+
+test_expect_success GPGSM 'import X.509 signature fails with --signed-commits=abort' '
+	git fast-export --signed-commits=verbatim x509-signing >output &&
+	test_must_fail git -C new fast-import --quiet --signed-commits=abort <output
+'
+
+test_expect_success GPGSM 'import X.509 signature with --signed-commits=warn-verbatim' '
+	git -C new fast-import --quiet --signed-commits=warn-verbatim <output >log 2>&1 &&
+	IMPORTED=$(git -C new rev-parse --verify refs/heads/x509-signing) &&
+	test $X509_SIGNING = $IMPORTED &&
+	test_grep "importing a commit signature" log
+'
+
+test_expect_success GPGSSH 'set up SSH signed commit' '
+	git checkout -b ssh-signing main &&
+	test_config gpg.format ssh &&
+	test_config user.signingkey "${GPGSSH_KEY_PRIMARY}" &&
+	echo "Content for SSH signing." >file-sign &&
+	git add file-sign &&
+	git commit -S -m "SSH signed commit" &&
+	SSH_SIGNING=$(git rev-parse HEAD)
+'
+
+test_expect_success GPGSSH 'strip SSH signature with --signed-commits=strip' '
+	git fast-export --signed-commits=verbatim ssh-signing >output &&
+	git -C new fast-import --quiet --signed-commits=strip <output >log 2>&1 &&
+	IMPORTED=$(git -C new rev-parse --verify refs/heads/ssh-signing) &&
+	test $SSH_SIGNING != $IMPORTED &&
+	git -C new cat-file commit "$IMPORTED" >actual &&
+	test_grep ! -E "^gpgsig" actual &&
+	test_must_be_empty log
+'
+
+test_expect_success GPG 'setup a commit with dual OpenPGP signatures on its SHA-1 and SHA-256 formats' '
+	# Create a signed SHA-256 commit
+	git init --object-format=sha256 explicit-sha256 &&
+	git -C explicit-sha256 config extensions.compatObjectFormat sha1 &&
+	git -C explicit-sha256 checkout -b dual-signed &&
+	test_commit -C explicit-sha256 A &&
+	echo B >explicit-sha256/B &&
+	git -C explicit-sha256 add B &&
+	test_tick &&
+	git -C explicit-sha256 commit -S -m "signed" B &&
+	SHA256_B=$(git -C explicit-sha256 rev-parse dual-signed) &&
+
+	# Create the corresponding SHA-1 commit
+	SHA1_B=$(git -C explicit-sha256 rev-parse --output-object-format=sha1 dual-signed) &&
+
+	# Check that the resulting SHA-1 commit has both signatures
+	git -C explicit-sha256 cat-file -p $SHA1_B >out &&
+	test_grep -E "^gpgsig " out &&
+	test_grep -E "^gpgsig-sha256 " out
+'
+
+test_expect_success GPG 'strip both OpenPGP signatures with --signed-commits=warn-strip' '
+	git -C explicit-sha256 fast-export --signed-commits=verbatim dual-signed >output &&
+	test_grep -E "^gpgsig sha1 openpgp" output &&
+	test_grep -E "^gpgsig sha256 openpgp" output &&
+	git -C new fast-import --quiet --signed-commits=warn-strip <output >log 2>&1 &&
+	git -C new cat-file commit refs/heads/dual-signed >actual &&
+	test_grep ! -E "^gpgsig " actual &&
+	test_grep ! -E "^gpgsig-sha256 " actual &&
+	test_grep "stripping a commit signature" log >out &&
+	test_line_count = 2 out
+'
+
+test_done