Return value from D-Bus Interface.SignalPoll method

Fionn Cleary fionn.cleary at
Wed Mar 10 15:21:47 GMT 2021


As part of the new D-Bus additions I posted yesterday I have once again
noticed an oddity in the D-Bus introspection data, in particular the
SignalPoll method.  It’s listed as returning "a{sv}", that is, a dict of
string to variants, but if you look at what it actually returns on the
bus it’s a variant containing a "a{sv}", which is a variant, "v":

method call time=1615296463.836190 sender=:1.34 -> destination=fi.w1.wpa_supplicant1 serial=2 path=/fi/w1/wpa_supplicant1/Interfaces/0; interface=fi.w1.wpa_supplicant1.Interface; member=SignalPoll
method return time=1615296463.838788 sender=:1.25 -> destination=:1.34 serial=209 reply_serial=2
   variant       array [
         dict entry(
            string "rssi"
            variant                int32 -55
         dict entry(
            string "linkspeed"
            variant                int32 65
         dict entry(
            string "noise"
            variant                int32 -92
         dict entry(
            string "frequency"
            variant                uint32 2462
         dict entry(
            string "width"
            variant                string "40 MHz"

In the Qt code I have which does the SignalPoll I have to manually
update the introspected xml, changing the "a{sv}" to "v" before I can
successfully extract the return value from SignalPoll.  If we look at
the relevant code in dbus_new.c (wpas_dbus_handler_signal_poll) it opens
a variant container and then opens a dict inside that:

        if (!dbus_message_iter_open_container(&iter, DBUS_TYPE_VARIANT,
                                              "a{sv}", &variant_iter) ||
            !wpa_dbus_dict_open_write(&variant_iter, &iter_dict) ||

It would be cleaner to remove the outermost variant, but that would
break users of the existing API, so instead I would suggest changing the
SignalPoll return type to "v".  I presume I can’t be the only one who
has had to fight with this, but searching through the list archives for
"SignalPoll" gives me zero hits.

Any thoughts?  Changing the introspection data is a one-liner, just want
to see if anybody has any feedback before I post the patch.


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