Raspberry, meet elnode

28 Feb 2013 • emacs • pi

So, I was curious about elnode. And I wanted to play more with my Raspberry Pi. It only seemed logical to combine the two.

You know, because I can.

(I promised @nicferrier a writeup on this project, so here goes.)

Installing Emacs 24

The Emacs that comes with the Raspian distribution is version 23.4. Unfortunately, elnode requires Emacs 24, so I had to compile my own. I dreaded doing this, but it actually turned out pretty straightforward.

You need to apt-get install some build dependencies like libncurses-dev (and probably build-essentials if you don’t already have them), then you’re ready to go:

 > wget http://ftp.gnu.org/pub/gnu/emacs/emacs-24.2.tar.gz
 > tar xvfz emacs-24.2.tar.gz
 > cd emacs-24.2
 > ./configure
 > make 
 > sudo make install

This would be a good opportunity to make a sandwich, or learn some sanskrit, as building everything will take quite a while.

Running Emacs

By default the elnode webserver runs on Emacs startup, so the idea is to fire up emacs and just leave it. One way of doing that would be to ssh into the Pi and run emacs inside a tmux session (I tend to use tmux for all remote access anyway). This makes the emacs process persistent, and when I next log in I can just resume the session.

However, with this approach I’d still need to login in and start manually every time the server restarts, so the setup is kind of fragile. Especially since I have builders running round the house cutting the main power every now and then.

So instead I use a simple init script to run emacs in daemon mode (using the aptly named --daemon switch). This way I can still connect to the running emacs process using emacsclient, but I don’t have to bother with starting it manually anymore.

The only caveat is that since I want start-stop-daemon to fork off a new process to run emacs as the pi user (running a web-connected emacs as root seems like a really bad idea), I can’t rely on it to generate a correct PID file for me. Instead, we’ll have to do that from inside emacs (see next section).

/etc/init.d/elnode

#!/bin/sh

set -e

# Must be a valid filename
NAME=elnode
PIDFILE=/home/pi/elnode/elnode.pid
UID=1000
#This is the command to be run, give the full pathname
DAEMON=/usr/local/bin/emacs
DAEMON_OPTS="--daemon --load /home/pi/.emacs.d/init.el --load /home/pi/elnode/web.el"

export PATH="${PATH:+$PATH:}/usr/sbin:/sbin"

case "$1" in
    start)
        echo -n "Starting daemon: "$NAME
        start-stop-daemon --start --quiet --pidfile $PIDFILE --chuid $UID --exec $DAEMON -- $DAEMON_OPTS
        echo "."
	    ;;
    stop)
        echo -n "Stopping daemon: "$NAME
        start-stop-daemon --stop --quiet --oknodo --pidfile $PIDFILE
        echo "."
	    ;;
    restart)
        echo -n "Restarting daemon: "$NAME
        start-stop-daemon --stop --quiet --oknodo --retry 30 --pidfile $PIDFILE
        start-stop-daemon --start --quiet --pidfile $PIDFILE --chuid $UID --exec $DAEMON -- $DAEMON_OPTS
	    echo "."
	    ;;

    *)
	    echo "Usage: "$1" {start|stop|restart}"
	    exit 1
esac

exit 0

When I start the deamon I get some weird error messages from elnode, but as far as I can tell it actually runs fine.

pi@raspberrypi ~ $ sudo /etc/init.d/elnode start                                                                                                                  
Restarting daemon: elnode("/usr/local/bin/emacs" "--load" "/home/pi/.emacs.d/init.el" "--load" "/home/pi/elnode/web.el")
Starting Emacs daemon.
deleting server process
elnode-error: elnode--sentinel 'deleted.' for process  *elnode-webserver-proc* with buffer *elnode-webserver*
found the server process - NOT deleting
elnode-error: Elnode server stopped
.

Running elnode

Elnode is available from Marmalade, which makes it very easy to install from inside emacs. I also grapped the xmlgen library to help with HTML generation.

This is my little Hello World-script. There’s nothing remarkable about it, really. The first section generates a PID file to be used by the init script to check if the process is running. I stop the default server, as it binds to “localhost”, and I want to bind to the actual server name (raspberrypi.local in my case) in order to serve remote requests. You can see it in action here.

Did I say there was nothing remarkable about this script? Yeah, apart from the fact that it’s running a webserver inside a 25 year old text editor on a computer that’s about as large as a deck of cards.

/home/pi/elnode/web.el

;;; -*- lexical-binding: t -*-

;; write the pid file
(with-temp-file "/home/pi/elnode/elnode.pid" (insert (format "%d" (emacs-pid))))

(require 'cl)

;; stop the default server
(elnode-stop 8000)

(defvar request-count 0)

(defun hello-handler (httpconn)
  "Hello, world" 
  (elnode-http-start httpconn 200 '("Content-Type" . "text/html"))
  (elnode-http-return httpconn
		      (xmlgen 
		       `(html
			 (head
			  (title "Hello, elnode!")
			  (style "body { margin:100px auto auto auto; width: 400px; }"))
			 (body
			  (p "Hello, I am a small "
			     (a :href "https://github.com/nicferrier/elnode" "webserver")
			     " running inside Emacs 24 on a "
			     (a :href "http://www.raspberrypi.org" "Raspberry Pi")
			     ".")
			  (p "The local time is "
			     ,(format-time-string "%H:%M")
			     " and I have served a total of "
			     ,(incf request-count)
			     " requests."))))))

(defun root-handler (httpconn)
  (elnode-hostpath-dispatcher httpconn '((".*/favicon.ico" . elnode-send-404)
					 (".*" . hello-handler))))

(elnode-start 'root-handler :port 8000 :host "raspberrypi.local")

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