gpg-interface: check gpg signature creation status

When we create a signature, it may happen that gpg returns with
"success" but not with an actual detached signature on stdout.

Check for the correct signature creation status to catch these cases
better. Really, --status-fd parsing is the only way to check gpg status
reliably. We do the same for verify already.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gpg-interface.c

@@ -153,9 +153,11 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
 	struct child_process gpg = CHILD_PROCESS_INIT;
 	int ret;
 	size_t i, j, bottom;
+	struct strbuf gpg_status = STRBUF_INIT;
 
 	argv_array_pushl(&gpg.args,
 			 gpg_program,
+			 "--status-fd=2",
 			 "-bsau", signing_key,
 			 NULL);
 
@@ -167,10 +169,12 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
 	 */
 	sigchain_push(SIGPIPE, SIG_IGN);
 	ret = pipe_command(&gpg, buffer->buf, buffer->len,
-			   signature, 1024, NULL, 0);
+			   signature, 1024, &gpg_status, 0);
 	sigchain_pop(SIGPIPE);
 
-	if (ret || signature->len == bottom)
+	ret |= !strstr(gpg_status.buf, "\n[GNUPG:] SIG_CREATED ");
+	strbuf_release(&gpg_status);
+	if (ret)
 		return error(_("gpg failed to sign the data"));
 
 	/* Strip CR from the line endings, in case we are on Windows. */

t/t7004-tag.sh

@@ -1202,10 +1202,17 @@ test_expect_success GPG,RFC1991 \
 # try to sign with bad user.signingkey
 git config user.signingkey BobTheMouse
 test_expect_success GPG \
-	'git tag -s fails if gpg is misconfigured' \
+	'git tag -s fails if gpg is misconfigured (bad key)' \
 	'test_must_fail git tag -s -m tail tag-gpg-failure'
 git config --unset user.signingkey
 
+# try to produce invalid signature
+test_expect_success GPG \
+	'git tag -s fails if gpg is misconfigured (bad signature format)' \
+	'test_config gpg.program echo &&
+	 test_must_fail git tag -s -m tail tag-gpg-failure'
+
+
 # try to verify without gpg:
 
 rm -rf gpghome