The `$XDG_RUNTIME_DIR/bus` fallback
I've noticed that some implementations (including the on in this repository) use $XDG_RUNTIME_DIR/bus
as a fallback when $DBUS_SESSION_BUS_ADDRESS
is unset. I don't see this mentioned in the D-Bus specification, but having a well-known fallback like this is very convenient.
I think it might be a good idea to add a note on this fallback to the relevant section of the dbus spec and make it part of the standard.