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

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

[Bruce Lee (李昀峻)]

Does busctl set-property can get its return value?
I use the below command, and I found it has no return value.
Dose set-property does not provide a return message like the get-property function?
If can get its return value from set-property, how to do it?

i.e.
busctl set-property xyz.openbmc_project.Hwmon-3687764522.Hwmon1 /xyz/openbmc_project/sensors/voltage/vbat xyz.openbmc_project.Sensor.Value Value d 22
or
in bmcweb the code as below:
i.e.
crow::connections::systemBus->async_method_call(
	[sensorAsyncResp](const boost::system::error_code ec) {
		if (ec)
        {
			BMCWEB_LOG_DEBUG
				<< "setOverrideValueStatus DBUS error: "<< ec;
			messages::internalError(sensorAsyncResp->res);
			return;
		}
	},
	item.second, item.first,
	"org.freedesktop.DBus.Properties", "Set",
	"xyz.openbmc_project.Sensor.Value", "Value",
	std::variant<double>(iterator->second.first));

in dbus-monitor --system I also not found its return value when I use set-property 
dbus-monitor message as below:
method call time=1618547183.313540 sender=:1.87 -> destination=xyz.openbmc_project.Hwmon-3687764522.Hwmon1 serial=2 path=/xyz/openbmc_project/sensors/voltage/vbat; interfa
   string "xyz.openbmc_project.Sensor.Value"                                                                                                                               
   string "Value"                                                                                                                                                          
   variant       double 22                                                                                                                                                 
method call time=1618547183.317152 sender=:1.45 -> destination=org.freedesktop.DBus serial=35 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=GetConnect
   string ":1.87"                                                                                                                                                          
method return time=1618547183.317937 sender=org.freedesktop.DBus -> destination=:1.45 serial=4294967295 reply_serial=35                                                    
   uint32 0                                                                                                                                                                
method return time=1618547183.330441 sender=:1.45 -> destination=:1.87 serial=36 reply_serial=2  

The set-property code as below: 
int setSensorValue(const double& newValue, double& oldValue)
    {
            
        if (!internalSet)
        {  
            oldValue = newValue;
            overriddenState = true;
            // check thresholds for external set
            value = newValue;
            checkThresholds();
        }
        else if (!overriddenState)
        {
            oldValue = newValue;
        }
        return 1;
    }

sensorInterface->register_property(
	"Value", value, [&](const double& newValue, double& oldValue) {
	return setSensorValue(newValue, oldValue);
});

Thanks,
Bruce.

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

[Patrick Williams]

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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.

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

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

[Bruce Lee (李昀峻)]

> -----Original Message-----
> From: Patrick Williams <patrick@stwcx.xyz>
> Sent: Friday, April 16, 2021 10:58 PM
> To: Bruce Lee (李昀峻) <Bruce_Lee@quantatw.com>
> Cc: openbmc@lists.ozlabs.org; edtanous@google.com; Nan Zhou
> <nanzhou@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