[systemd-devel] Calling sd_bus_reply_method_return outside method handler

[Jean Valjean]

Thank you for your explanation, Lenart. I tried different approach.
Everything is running on a single thread. I have a global
`sd_bus_message* test` variable. I do the following

int method_handler(sd_bus_message* m, void* /*userdata*/,
sd_bus_error* /*error*/) {
  test = sd_bus_message_ref(m);
  return 0;
}

And create an object with

static const sd_bus_vtable vtable[] = {
  SD_BUS_VTABLE_START(0),
  SD_BUS_PROPERTY("Description", "s", property_handler, 0,
SD_BUS_VTABLE_PROPERTY_CONST),
  SD_BUS_METHOD("Do",  "s", "s", method_handler, SD_BUS_VTABLE_UNPRIVILEGED),
  SD_BUS_VTABLE_END
};

r = sd_bus_add_object_vtable(
        bus_,
        nullptr,
        path,
        interface,
        vtable,
        nullptr
);

Connect to bus, request service name and start waiting for incoming messages
sd_bus_open_user(&bus_);
sd_bus_request_name(bus_, service_name_, 0);

  while(true){
     sd_bus_process(bus_, NULL);

    if (r > 0) {
      continue;
    }

    if (test) {
      int r = sd_bus_message_get_errno(test);

      if(!r) {
        const char* reply = "reply";
        r  = sd_bus_reply_method_return(test, "s", reply);
      }else {
        std::cout << "Error\n";
      }

      sd_bus_message_unref(test);
      test = nullptr;
    }
    r = sd_bus_wait(bus_, 1e+6);
  }

sd_bus_reply_method_return(test, "s", reply) doesn't do anything if
called outside method_handler.
I think that the call that is responsible for invoking method_handler
sends an error back to the client when method_handler returns.
Hence the connection to client is actually closed when I call
sd_bus_reply_method_return(test, "s", reply).
In that case I suppose sd_bus_message_get_errno should return an error
but it doesn't.

I'm using mailinglist for the first time so I'm not sure it that OK to
post code here. Hope it will show correctly in your email client.


On Tue, 15 Jan 2019 at 15:05, Lennart Poettering <mzerqung at 0pointer.de> wrote:
>
> On Mo, 14.01.19 21:07, Jean Valjean (valjean.jean1802 at gmail.com) wrote:
>
> > I want to send a reply to a method call outside method_handler. Is it
> > possible?
> > In the method_handler I increase reference count of sd_bus_message and send
> > it to the other thread. From there I want to call
> > sd_bus_reply_method_return to send a reply but get Unknown method or
> > interface error when I invoke method with bussctl --user call. If I call
> > the return method from method_handler everything works as expected.
>
> Note that sd-bus is not thread-safe on its own. It's threads-aware
> however: if you are careful to lock around all invocations of sd_bus
> using a single connection (and its associated messages) is generally
> fine, but it's up to you to carefully lock.
>
> So yeah, it's certainly OK to reply to an incoming bus call msgs
> outside of the stack method handler stack frame, but if that means
> threads, then make sure you know what you are doing.
>
> Lennart
>
> --
> Lennart Poettering, Red Hat