bus: automatically do a NOP reply when a NULL callback is specified for a method in a vtable

Also, allow specifiying NULL as signature in vtables equivalent to ""
for empty parameter lists.

TODO

@@ -182,9 +182,7 @@ Features:
   - merge busctl into systemctl or so?
   - synthesize sd_bus_message objects from kernel messages
   - properly implement name registry ioctls for kdbus
-  - get rid of object hash table, use decision tree everyhwere instead?
   - implement monitor logic
-  - object vtable logic
   - longer term:
     * priority queues
     * priority inheritance

src/libsystemd-bus/sd-bus.c

@@ -2100,7 +2100,15 @@ static int method_callbacks_run(
                 return 1;
         }
 
-        return c->vtable->method.handler(bus, m, u);
+        if (c->vtable->method.handler)
+                return c->vtable->method.handler(bus, m, u);
+
+        /* If the method callback is NULL, make this a successful NOP */
+        r = sd_bus_reply_method_return(bus, m, NULL);
+        if (r < 0)
+                return r;
+
+        return 1;
 }
 
 static int invoke_property_get(
@@ -3761,7 +3769,7 @@ static int add_object_vtable_internal(
                         if (!member_name_is_valid(v->method.member) ||
                             !signature_is_valid(v->method.signature, false) ||
                             !signature_is_valid(v->method.result, false) ||
-                            !v->method.handler ||
+                            !(v->method.handler || (isempty(v->method.signature) && isempty(v->method.result))) ||
                             v->flags & (SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE|SD_BUS_VTABLE_PROPERTY_INVALIDATE_ONLY)) {
                                 r = -EINVAL;
                                 goto fail;

src/libsystemd-bus/test-bus-objects.c

@@ -39,8 +39,6 @@
  *   Add in:
  *
  *   automatic properties for Set()
- *   automatic NULL method
- *   allow NULL as signatures in vtable
  *   node hierarchy updates during dispatching
  *   emit_interfaces_added/emit_interfaces_removed
  *
@@ -157,6 +155,7 @@ static const sd_bus_vtable vtable[] = {
         SD_BUS_WRITABLE_PROPERTY("Something", "s", get_handler, set_handler, 0, 0),
         SD_BUS_PROPERTY("AutomaticStringProperty", "s", NULL, offsetof(struct context, automatic_string_property), 0),
         SD_BUS_PROPERTY("AutomaticIntegerProperty", "u", NULL, offsetof(struct context, automatic_integer_property), 0),
+        SD_BUS_METHOD("NoOperation", "", "", NULL, 0),
         SD_BUS_VTABLE_END
 };
 
@@ -241,6 +240,9 @@ static int client(struct context *c) {
         assert_se(sd_bus_set_fd(bus, c->fds[1], c->fds[1]) >= 0);
         assert_se(sd_bus_start(bus) >= 0);
 
+        r = sd_bus_call_method(bus, "org.freedesktop.systemd.test", "/foo", "org.freedesktop.systemd.test", "NoOperation", &error, NULL, NULL);
+        assert_se(r >= 0);
+
         r = sd_bus_call_method(bus, "org.freedesktop.systemd.test", "/foo", "org.freedesktop.systemd.test", "AlterSomething", &error, &reply, "s", "hallo");
         assert_se(r >= 0);