gpg: do show gpg's error message upon failure

There are few things more frustrating when signing a commit fails than
reading a terse "error: gpg failed to sign the data" message followed by
the unsurprising "fatal: failed to write commit object" message.

In many cases where signing a commit or tag fails, `gpg` actually said
something helpful, on its stderr, and Git even consumed that, but then
keeps mum about it.

Teach Git to stop withholding that rather important information.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

gpg-interface.c

@@ -977,9 +977,13 @@ static int sign_buffer_gpg(struct strbuf *buffer, struct strbuf *signature,
 			break; /* found */
 	}
 	ret |= !cp;
+	if (ret) {
+		error(_("gpg failed to sign the data:\n%s"),
+		      gpg_status.len ? gpg_status.buf : "(no gpg output)");
+		strbuf_release(&gpg_status);
+		return -1;
+	}
 	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. */
 	remove_cr_after(signature, bottom);

t/t7510-signed-commit.sh

@@ -399,6 +399,10 @@ test_expect_success 'custom `gpg.program`' '
 
 	case "$1" in
 	-bsau)
+		test -z "$LET_GPG_PROGRAM_FAIL" || {
+			echo "zOMG signing failed!" >&2
+			exit 1
+		}
 		cat >sign.file
 		echo "[GNUPG:] SIG_CREATED $args" >&2
 		echo "-----BEGIN PGP MESSAGE-----"
@@ -420,7 +424,11 @@ test_expect_success 'custom `gpg.program`' '
 	git commit -S --allow-empty -m signed-commit &&
 	test_path_exists sign.file &&
 	git show --show-signature &&
-	test_path_exists verify.file
+	test_path_exists verify.file &&
+
+	test_must_fail env LET_GPG_PROGRAM_FAIL=1 \
+	git commit -S --allow-empty -m must-fail 2>err &&
+	grep zOMG err
 '
 
 test_done