is_method_error versus call() errors

[William Kennington]

I should have looked at the code prior to misremembering how sd_bus
handles method errors during sd_bus_call(). If you look at the code,
it does in fact return sd_bus_errors for method errors automaticaly.

https://github.com/systemd/systemd/blob/v237/src/libsystemd/sd-bus/sd-bus.c#L2130

You can ignore the previous patchset I sent adding the method_error
checks as they are superfluous. It looks like the only missing
convenience is the ignore errors variant of the call_noreply_noexcept.
On Tue, Sep 11, 2018 at 2:52 AM Thomaiyar, Richard Marian
<richard.marian.thomaiyar at linux.intel.com> wrote:
>
> In sdbus++ generated code we end up setting error_const for any issue
> (i.e. in the D-Bus daemon / server end).
>
> catch(sdbusplus::xyz::openbmc_project::Common::Error::InvalidArgument& e)
>      {
>          sd_bus_error_set_const(error, e.name(), e.description());
>          return -EINVAL;
>      }
>
> and in our sdbusplus (bus.hpp.in), the code is updated to throw the error
>
>      auto call(message::message& m, uint64_t timeout_us = 0)
>      {
>          sd_bus_error error = SD_BUS_ERROR_NULL;
>          sd_bus_message* reply = nullptr;
>          int r = _intf->sd_bus_call(_bus.get(), m.get(), timeout_us, &error,
>                                     &reply);
>          if (r < 0)
>          {
>              throw exception::SdBusError(&error, "sd_bus_call");
>          }
>
>          return message::message(reply, _intf, std::false_type());
>      }
>
> i.e. for any error thrown in the D-Bus daemon(server), the client
> application call() will just throw SdBusError, and not the message
> object. Hence there is no way to perform is_method_error() on the
> message right?
>
> Also, i believe even before throwing SdBusError(),
> sd_bus_message_unref(reply) has to be called, to make sure that messages
> are unref even for errors thrown from D-Bus daemon.
>
> Let me know if i misunderstand the logic in current code.
>
> Regards,
>
> Richard
>
>
> On 9/11/2018 3:19 AM, William Kennington wrote:
> > This is something that is a little confusing about the semantics of
> > sdbus. When using sd_bus_call(), it will only populate the returned
> > message pointer if the dbus rpc succeeds. In the case where something
> > fails at the dbus level, the sd_bus_call() method fails and the
> > message is never populated. If we don't check for errors on
> > sd_bus_call() but use sd_bus_message_is_method_error on the NULL
> > message, we get an -EINVAL and treat that as an error. This behavior
> > works fine if you only care that you had an error, but don't need to
> > interpret the error and perform special case handling depending on the
> > type. This is why I added the exceptions to sd_bus_call() so that we
> > can parse out specific errors like -EBUSY or -EINTR, but handle the
> > rest differently.
> >
> > sd_bus_message_is_method_error() is supposed to be used for checking
> > if the server we are talking to sent an error response, not if the
> > dbus layer had some kind of error. These errors are application
> > specific and can be parsed out of a valid message if they exist.
> > sd_bus_call() will still return success even if the server method sent
> > a METHOD_ERROR type of message. You can't technically rely on
> > sd_bus_message_read returning an error when reading from a
> > METHOD_ERROR type of message. Practically though if your METHOD_ERROR
> > message never have any content the read will fail since the message
> > won't contain the expected data types. Unfortunately this means we
> > will should keep all of the is_method_error checks and make sure c++
> > exception handling is in place for all of the daemons.On Mon, Sep 10,
> > 2018 at 9:18 AM Patrick Venture <venture at google.com> wrote:
> >> Given:
> >>
> >> _public_ int sd_bus_message_is_method_error(sd_bus_message *m, const
> >> char *name) {
> >>          assert_return(m, -EINVAL);
> >>          if (m->header->type != SD_BUS_MESSAGE_METHOD_ERROR)
> >>                  return 0;
> >>          if (name && (!m->error.name || !streq(m->error.name, name)))
> >>                  return 0;
> >>          return 1;
> >> }
> >>
> >> The failures from call() don't always set the reply (maybe don't ever,
> >> but I didn't read it closely).  Which means, is_method_error() can
> >> happen when the bus call itself was fine, so we need to check both IFF
> >> we're going to read the response.
> >>
> >> I'm working on eliminating the new crashes from phosphor-host-ipmid
> >> and other repositories, now that sdbusplus can except, which is not
> >> caught even though there's an error case for is_method_error()
> >> immediately after the call.
> >>
> >> In phosphor-pid-control, I dropped the is_method_error() check, but I
> >> think it's still valid before trying to read() a message -- so I will
> >> likely throw a patch adding that check.
> >>
> >> Patrick
>