DBus and PolicyKit from Common Lisp

Intro

Regardless of your position on DBus, sometimes you might need to interact with it. Common Lisp currently has at least two libraries ready for you, one of them is in Quicklisp, https://github.com/death/dbus/.

Setup

Have it loaded and create a package to use it, then change into it.

;; (asdf:load-system '#:dbus)
;; or
;; (ql:quickload "dbus)

(defpackage #:example
  (:use #:cl #:dbus))

(in-package #:example)

For reference, I'm going to refer to a (very old) polkit example in Python. For reference I'm reproducing it here (it still works in current Python 3 without any changes except the print):

import dbus

bus = dbus.SystemBus()
proxy = bus.get_object('org.freedesktop.PolicyKit1', '/org/freedesktop/PolicyKit1/Authority')
authority = dbus.Interface(proxy, dbus_interface='org.freedesktop.PolicyKit1.Authority')

system_bus_name = bus.get_unique_name()

subject = ('system-bus-name', {'name' : system_bus_name})
action_id = 'org.freedesktop.policykit.exec'
details = {}
flags = 1            # AllowUserInteraction flag
cancellation_id = '' # No cancellation id

result = authority.CheckAuthorization(subject, action_id, details, flags, cancellation_id)

print(result)

So, how does this look in Common Lisp? Mostly the same, except that at least at the moment you have to specify the variant type explicitly! This was also the reason to document the example, it's quite hard to understand what's wrong if there's a mistake, including the socket connection just dying on you and other fun stuff.

(with-open-bus (bus (system-server-addresses))
  (with-introspected-object (authority bus
                                       "/org/freedesktop/PolicyKit1/Authority"
                                       "org.freedesktop.PolicyKit1")
    (let* ((subject `("system-bus-name" (("name" ((:string) ,(bus-name bus))))))
           (action-id "org.freedesktop.policykit.exec")
           (details ())
           (flags 1)
           (cancellation-id "")
           (result
                (authority "org.freedesktop.PolicyKit1.Authority" "CheckAuthorization"
                           subject action-id details flags cancellation-id)))
      (format T "~A~%" result))))

Note the encoding of the dictionary: The type of the whole argument is specified as (sa{sv}), a structure of a string and a dictionary of strings to variants - we're spelling out the variant type here, compared to what's automatically done by the Python library.