lib: don't abort when D-Bus send_with_reply leaves a NULL pending call

lib/client_impl.c

@@ -260,7 +260,14 @@ static int make_request(DBusConnection *bus, int native, int use_pidfds, const c
 		dbus_message_iter_append_basic(&iter, type, &p);
 	}
 
-	dbus_connection_send_with_reply(bus, msg, &call, -1);
+	// send_with_reply can return TRUE with call == NULL if the connection is
+	// already disconnected, or if the message carries unix fds the transport
+	// cannot send. Blocking on a NULL pending call aborts in libdbus.
+	if (!dbus_connection_send_with_reply(bus, msg, &call, -1) || call == NULL) {
+		dbus_set_error_const(error, DBUS_ERROR_FAILED, "Could not send D-Bus message");
+		return -1;
+	}
+
 	dbus_connection_flush(bus);
 	dbus_message_unref(msg);
 	msg = NULL;

lib/meson.build

@@ -77,3 +77,17 @@ gamemode_dep = declare_dependency(
 libgamemodeauto_dep = declare_dependency(
   link_with: libgamemodeauto,
 )
+
+test_client_dead_bus = executable(
+  'test-client-dead-bus',
+  'test-client-dead-bus.c',
+  dependencies: [
+    dep_dbus,
+  ],
+  link_with: libgamemode,
+)
+
+test(
+  'client does not abort after the session bus fd is closed',
+  test_client_dead_bus,
+)

lib/test-client-dead-bus.c

@@ -0,0 +1,68 @@
+/*
+
+Copyright (c) 2017-2025, Feral Interactive and the GameMode contributors
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+ * Redistributions of source code must retain the above copyright notice,
+   this list of conditions and the following disclaimer.
+ * Redistributions in binary form must reproduce the above copyright
+   notice, this list of conditions and the following disclaimer in the
+   documentation and/or other materials provided with the distribution.
+ * Neither the name of Feral Interactive nor the names of its contributors
+   may be used to endorse or promote products derived from this software
+   without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGE.
+
+ */
+
+#include <dbus/dbus.h>
+#include <stdio.h>
+#include <unistd.h>
+
+extern int real_gamemode_request_end(void);
+
+int main(void)
+{
+	DBusError err;
+	DBusConnection *bus;
+	int fd = -1;
+
+	dbus_error_init(&err);
+	bus = dbus_bus_get(DBUS_BUS_SESSION, &err);
+	if (bus == NULL) {
+		fprintf(stderr, "skip: no session bus (%s)\n", err.message ? err.message : "unknown");
+		dbus_error_free(&err);
+		return 77;
+	}
+
+	dbus_connection_set_exit_on_disconnect(bus, FALSE);
+
+	if (!dbus_connection_get_socket(bus, &fd) || fd < 0) {
+		fprintf(stderr, "skip: session bus has no socket\n");
+		return 77;
+	}
+
+	// Reproduce gamescopereaper CloseAllFds: the socket is gone, but libdbus
+	// still believes the connection is up. UnregisterGame used to abort here.
+	if (close(fd) != 0) {
+		perror("close");
+		return 1;
+	}
+
+	(void)real_gamemode_request_end();
+	return 0;
+}