path: root/2022/captions/emacsconf-2022-dbus--the-wheels-on-dbus--ian-eure--main.vtt

WEBVTT captioned by sachac

NOTE What is D-Bus?

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Welcome to my EmacsConf 2022 talk, The Wheels on D-Bus.

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In this talk, we'll cover what D-Bus is,

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why you might want to use it, and how to use it with Emacs.

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D-Bus is fundamentally based on passing messages

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in between processes, using the bus as a mediator.

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On top of this is built an RPC system with method invocation

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that has argument lists and return values,

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like you might find in any programming language.

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These are commonly used for verb-type actions

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like "restart my computer."

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You can also associate a collection of attributes

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with objects on the bus, and these are called properties.

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The properties can be read-only, write-only, or read-write.

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Signals are a way of notifying participants on the bus

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of updated state, and are the basis

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for building dynamic user interfaces

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that react to changes in the system.

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It has a static and strong type system,

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so if you send a message with the wrong type signature,

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it simply gets rejected instead of going through

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to the remote service.

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It also manages service life cycles,

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so you're not running services at all times.

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They can be started and stopped by D-Bus on demand.

NOTE Why D-Bus?

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D-Bus has two major use cases.

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The first is acting as a lower-level substrate

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for higher-level programs,

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like a graphical desktop environment.

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For example, if you want to manage your network connectivity

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from your graphical environment,

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instead of having to build all of that from the ground up,

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you can rely on the D-Bus service to do that

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and only build the graphical component of it.

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This gives you consistency between desktop environments

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and reduces code duplication.

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Another application is automating desktop programs.

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If your program offers a D-Bus service,

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then it can be remote-controlled,

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and if all of your programs offer D-Bus,

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you can control your entire desktop.

NOTE The D-Bus Model

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Let's look at the abstractions that D-Bus provides.

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The top level object is called a bus,

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and it's like a partition

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that messages get exchanged inside of.

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Messages don't cross buses.

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Inside of a bus are services.

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Services are normally identified in reverse FQDN order,

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so org.foobar.FooService.

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Each service provides some set of features

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related to a particular area of functionality.

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Inside of each service are objects.

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Objects use a path notation,

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and usually follow the same reverse FQDN format

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as the service identifier.

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Each object has one or more interfaces.

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An interface is like a facet that you can use

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to interact with an object,

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and inside of the interface are properties, methods,

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and signals, which we covered before.

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Properties are attributes that can be read or written.

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Methods are verbs that you can call to invoke an action,

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and a signal is something that's used to move state

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in between a service and another participant on the bus.

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There can be any number of interfaces on an object,

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any number of objects in a service,

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and any number of services on a bus,

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and any number of buses on a system.

NOTE Well-known Busses

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There are two well-known busses,

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and these roughly map to those two use cases

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I mentioned before.

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The system bus is for interfacing with hardware

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and operating-system-level concerns

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like disks, networks, and so forth.

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The session bus is tied to a user login,

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and is more in the desktop automation use case.

NOTE Common interfaces

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There are some common interfaces you'll find

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if you go exploring D-Bus.

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The Introspectable interface is the basis

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of a lot of the reflection features.

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It has a single method called introspect

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that returns the XML interface description

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of whatever you call it on.

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Peer is used for lower level connectivity,

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for example, pinging a service to see if it's running.

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And the Properties interface is the basis

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of the read-write properties,

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which are secretly method calls under the cover.

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Just about every object you interact with on D-Bus

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will support all three of these interfaces.

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Additionally, ObjectManager is used for services

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that manage collections of objects.

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For example, the disk service has an object

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for each disk that's attached,

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and the object manager allows you

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to enumerate all of those.

NOTE Emacs Native D-Bus

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Emacs supports D-Bus natively since version 23.1.

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It's a combination of native bindings

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with a C library and dbus.el.

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While there are some ports of D-Bus

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to non-Linux operating systems,

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it's probably only available on Linux

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and almost certainly only usable on Linux.

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If you want to interact with D-Bus from Emacs,

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it's fairly straightforward.

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There's a collection of functions like `dbus-get-property'

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or `dbus-call-method', et cetera,

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and they almost all take this same set

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of four arguments at the beginning:

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bus, service, path, and interface.

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In this case, it takes a single additional property,

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which is the one to read.

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And what we're calling is the hostname1 service,

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which gives you just a little bit of information

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about the system, like its hostname or its chassis.

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And in this case, you can see I'm running

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this presentation off my laptop.

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The problem with this and what I don't like about it

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is that all of these identifiers are very verbose

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and very repetitive.

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And if you end up calling these a lot,

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it gets old really quickly.

NOTE Debase

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So I wrote a wrapper called Debase,

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which is convenience on top of the built-in functions.

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Most of the stock functions have Debase versions

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just by replacing "dbus" with "debase".

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And let's look how that works.

NOTE Debase: Objects

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The fundamental idea of Debase is that you can bind together

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all of those arguments into a single object

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that represents the endpoint.

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This is an EIEIO class, and it takes keyword arguments,

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so there's never any chance

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of mixing up which thing is what.

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So this sets the endpoint to that object,

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calls `debase-get-property' on it,

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and you can see it works exactly the same.

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The thing that's really nice about this, though,

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is it knows that so many of these arguments

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are very similar that it can compute most of them

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if you don't provide them all.

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So if you just say service, it will assume

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that you want the same object that matches

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and the same interface that matches,

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and it works just the same.

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I find this very, very convenient.

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You can also reuse the object

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instead of having to repeat every argument

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with every function call,

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which is a really great improvement in ergonomics.

NOTE Debase: Retarget objects

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Because so many objects have multiple interfaces,

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you often find yourself needing to look

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at a different aspect of that object.

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This is supported with the built-in EIEIO `clone' method,

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which takes an object

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and a set of keyword arguments to replace.

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So in this case, we can see we're calling

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the Properties method,

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but everything else on that endpoint is the same.

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And then we're gonna call the method GetAll

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on that Properties interface,

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and it's going to return all the properties

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of the org.freedesktop.hostname1 interface

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inside of that object.

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And if we run that, we can see there's the hostname

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and some other information about the laptop

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that I'm running this on.

NOTE Debase: Object binding

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Debase also supports object binding.

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This creates a lexical context in which the Debase object

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is the implicit target of any D-Bus function.

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This is really convenient if you need

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to fetch multiple properties or otherwise interact

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with the same endpoint in multiple different ways.

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And you can see I'm still on a laptop

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and it's still named meson.

NOTE Debase: Raw binding

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You can also, if you don't want to use the object,

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you can provide the raw argument list.

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Under the covers, this is basically an `flet'

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where you're currying all of these functions

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so they start with those argument lists.

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And you can see I'm running on a Linux machine,

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which should not be surprising.

NOTE Debase: Codegen

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Debase also has an experimental code generation feature.

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It outputs EIEIO code with one class per D-Bus interface.

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This includes accessors for all of its properties

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with an in-process cache, so if you read one property,

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you don't have to go back to the bus to read it again.

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It also outputs generic functions and method implementations

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for the D-Bus interface methods.

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It includes name-mangling options,

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so you can control how everything is named.

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And you can generate the code either

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via introspecting a live system

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or providing an XML interface description,

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which is handy if you want to use it

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as part of a non-interactive build.

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I think this has a lot of promise,

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but it doesn't feel quite right yet,

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so any feedback or contributions are very welcome.

NOTE Debase: Codegen example

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Let's generate some Elisp code

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for that hostname1 service we were interacting with before.

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`debase-gen-class' is the generation class,

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and it says to create a class that matches this interface,

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named "hostname1", and then the rest of these arguments

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are the same ones to target the endpoint,

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just like with `debase-object',

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because it extends `debase-object'.

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`debase-gen-code' is a generic function

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that takes any `debase-gen' class.

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There are different classes for functions,

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properties, et cetera,

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and it creates all of the code for it.

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If we evaluate it, we can see the results

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look about like we would expect:

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creates a defclass named "hostname1",

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which extends `debase-object',

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has all of the slots and accessors defined,

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and then methods that define everything

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that you might want to do with it, including documentation.

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This is based on introspecting a running system,

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but as I mentioned,

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you can provide an XML interface description instead,

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if you like.

NOTE Debase: ObjectManager

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Debase also comes with `debase-objectmanager',

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which is convenience for the D-Bus ObjectManager interface.

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This is used in a lot of places in D-Bus,

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where an object manages other objects.

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For example, the NetworkManager object

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manages network hardware objects,

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and using the ObjectManager interface,

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you can enumerate all of the network hardware,

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and by subscribing to the signals,

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you can be notified when they change.

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`debase-objectmanager' keeps a local cache,

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and will fire a callback on any change.

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So it's the building block for that dynamic user interface,

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like you would see in a desktop system,

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but inside of Emacs.

NOTE Demo: Discomfort

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Let's do some demos.

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Discomfort is an interface I wrote for UDisks2,

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which is what manages all of the block device hardware.

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And again, it has that dynamic desktop-like interactivity,

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and mostly will just do what you mean.

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This is definitely alpha state.

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It doesn't have all the features,

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but it's good enough that I use it daily.

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So here's Discomfort,

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and you can see it has a list of all your hardware,

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what type it is, and where it's mounted.

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I have a little USB extension cable here,

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and I'm gonna plug in a disc,

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just to show you how this works.

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You can see when I plug it in,

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just a moment later,

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it shows up in that list, automatically.

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I don't have to press any key,

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I don't have to refresh it, it's just there.

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If I unplug it, it's gone.

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Plug it back in,

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and there it is.

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And you can see it's an encrypted volume.

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So in order to do anything with this,

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I'm going to have to supply a password.

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Just pressing Enter goes into the "do what I mean" mode,

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and it asks for the password.

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In this case, I've chosen the very secure password

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of "password".

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I hit Enter, and it unlocks it, and it mounts it,

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and it opens `dired' looking at it.

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And here's a little README.

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Let's see what it says.

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"Hello, EmacsConf."

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So that's my demo of discomfort.

NOTE Demo: Remote eval

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In addition to acting as a client for D-Bus,

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Emacs can also offer services to other D-Bus clients.

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This is a really interesting opportunity

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because it allows many different programs

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to integrate with Emacs

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in ways that were previously very difficult.

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You can use this as an alternative to Emacs.

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The difference is D-Bus provides a full API,

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so instead of emacsclient being

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a sort of fire-and-forget system,

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you can actually get results back from the remote operation.

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So here's some code.

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Here's a `dbus-eval' function, which takes a string,

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reads it, and evaluates it,

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and returns whatever that value is.

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Then we have a `debase-bind' block

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that sets up an object on the session bus.

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Again, that's my user login bus.

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It offers this D-Bus service Emacs.

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This is a constant inside of the dbus.el package.

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And again, the path is a constant in there.

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And we're gonna create this interface,

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org.gnu.Emacs.Eval, and then register a method called Eval

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that calls that `dbus-eval' function.

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Pretty straightforward, only a handful of lines of code.

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To test this out, we're going to use the dbus-send utility.

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This is a command line program that interacts with D-Bus.

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We're going to tell it to wait for and print the reply,

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that the message should be sent to the session bus,

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that we're going to talk

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to the org.gnu.Emacs service on that bus,

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and the /org/gnu/Emacs object inside that service.

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On that object, we're gonna interact

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with the org.gnu.Emacs.Eval interface

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and call its Eval method.

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We're gonna call that method with a single string argument,

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which is indicated by the string prefix,

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and then a form to evaluate.

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I actually have to run this from a shell,

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because if I try using it in Org, it wedges.

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org-babel blocks waiting on completion,

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which blocks the D-Bus service from responding.

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I really wish Emacs was multi-threaded.

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But let's try it out.

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So if we run this, we can see that we get a return,

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and that's an unsigned integer of 32 bits

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with a value of 3.

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So like I was saying, this is really a two-way API

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where you can communicate back and forth

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between Emacs and another program.

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It's not just fire-and-forget.

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I think that's really cool.

NOTE Demo: Remote Org capture

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Let's try another demo.

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What about a remote org-capture?

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What if you could trigger an org-capture

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from any program on your desktop?

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I think that would be pretty cool.

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And we can see, there it is.

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All right, I think I've got that one covered.

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So I do want to say that remote eval is probably a bad idea

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from a security perspective,

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but the point of this is some quick and dirty demonstrations

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of what can happen and to get people's imaginations flowing,

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because I think this is something

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that offers a lot of promise for Emacs.

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I think having a full-blown Emacs desktop environment

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where it can do all the things that a GNOME

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or a KDE environment can do is very exciting.

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And if you want to have a traditional GUI with Emacs

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as a more integrated participant of it,

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its service mechanism offers a lot of ability to do that.

NOTE Future directions

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In the micro sense, I think there's a lot of improvements

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that can be made to either dbus.el or to dbase.

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The main one is handling of the type system.

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Lisp's dynamic type system doesn't mesh particularly well

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with the static strong type system that D-bus offers,

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and having some convenience to assist that

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would be very helpful.

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There's also some weird interfaces.

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For example, some things return identifiers

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as an array of integer code points instead of a string,

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and there should be a common way of handling that.

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I also think that the service support could be improved.

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Even though I gave the demo service,

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it's not really a great D-bus citizen

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because it doesn't offer that introspection mechanism,

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and so the actual methods are pretty much invisible

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to other participants,

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unless they already know that you're using Emacs.

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That's my talk.

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Thank you.

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You can find me on mastodon.social or on libera.chat.