How to get the return value using the command busctl set-property?

[Bruce Lee (李昀峻)]

> -----Original Message-----
> From: Patrick Williams <patrick at stwcx.xyz>
> Sent: Friday, April 16, 2021 10:58 PM
> To: Bruce Lee (李昀峻) <Bruce_Lee at quantatw.com>
> Cc: openbmc at lists.ozlabs.org; edtanous at google.com; Nan Zhou
> <nanzhou at google.com>
> Subject: Re: How to get the return value using the command busctl set-property?
> 
> On Fri, Apr 16, 2021 at 04:46:00AM +0000, Bruce Lee (李昀峻) wrote:
> > Does busctl set-property can get its return value?
> ...
> > If can get its return value from set-property, how to do it?
> 
> There isn't really a return value from a set-property call; there is only a possiblity
> of error.
> 
> If you look at 'man SD_BUS_PROPERTY' you'll see the function type for a
> property set is:
> 
>        typedef int (*sd_bus_property_set_t)(sd_bus *bus, const char *path,
>                                             const char *interface,
>                                             const char *property,
>                                             sd_bus_message *value,
>                                             void *userdata,
>                                             sd_bus_error *ret_error);
> 
> This is where the 'int' return you're seeing from these set-property handlers
> coming from.  The way systemd handles the return code is that any negative
> number becomes a negative errno style value that systemd turns into an
> appropriate error message back across the dbus.  There is a paragraph in the
> manpage with more details:
> 
>        If a callback was invoked to handle a request that expects a reply and
>        the callback returns a negative value, the value is interpreted as a
>        negative errno-style error code and sent back to the caller as a D-Bus
>        error as if sd_bus_reply_method_errno(3) was called. Additionally, all
>        callbacks take a sd_bus_error output parameter that can be used to
>        provide more detailed error information. If ret_error is set when the
>        callback finishes, the corresponding D-Bus error is sent back to the
>        caller as if sd_bus_reply_method_error(3) was called. Any error stored
>        in ret_error takes priority over any negative values returned by the
>        same callback when determining which error to send back to the caller.
> 
> The *best* way for a set-property handler to return an error is to use the
> sd_bus_reply_method_error or fill out the ret_error with sd_bus_error_set.
> Both the ASIO object_server.hpp and the
> sdbus++-generated server bindings catch excpetions thrown out of the
> set-property handlers and turn them into sd_bus_error_set calls.
> 

Thanks for your reply, it's very useful to me.

> Other than the negative value indicating a errno, the positive value has no
> meaning and does not do anything at a dbus level.
> 
> --
> Patrick Williams