poatmyp ("point at my pdf") is a simple, one-file server that lets
people share a display of a pdf (and not more) in a web browser.  With
javascript and websockets on in a user's browser, they will have a
shared pointer.  Without javascript, people will need to manually reload
when a new slide should be shown, and they won't have a shared pointer.

Running it for fun::

  python3 poatmyp.py -a

Then point a browser to http://localhost:8900 or http://your-ip:8900,
upload a pdf and share the link you're being redirected to (this may
fail if the browsers are configured to deny websockets to http pages).

Dependencies: python3-pil, python3-pdfrw, mupdf-tools

Proper installation:

You'll need a reverse proxy to do the https termination, and you'll have
to come up with a start script.

Here's an nginx snippet that lets you run poatmyp within another service
at the location /p ("pdf" or "presentation"):

        rewrite ^/p$ /p/ permanent;
        location /p {
                rewrite ^/p(.*)$ $1 break;
                proxy_pass http://localhost:8900;
        }

        location ~ ^/p/[^/]*/websocket$ {
                rewrite ^/p(.*)$ $1 break;
                proxy_pass http://localhost:8900;
                proxy_http_version 1.1;
                proxy_set_header Upgrade $http_upgrade;
                proxy_set_header Connection "upgrade";
                proxy_set_header Host $host;
        }


As to a start script... you'll want to run poatmyp as a minimally
privileged user, as rendering PDFs is something likely to be
exploitable now and then (though mupdf appears to be a lot less stinky
than poppler).  If you can sandbox it, even better.  Here's a baseline
initscript (a systemd unit is quite a bit simpler); of course, you'll
have to adapt the script location::

  ### BEGIN INIT INFO
  # Provides:          poatmyp
  # Required-Start:    $local_fs $network
  # Required-Stop:     $local_fs
  # Default-Start:     2 3 4 5
  # Default-Stop:      0 1 6
  # Short-Description: poatmyp shared pdf daemon
  # Description:       A piece of twisted listening on 8900.
  ### END INIT INFO

  SCRIPT_LOCATION=/home/msdemlei/gavo/poatmyp/poatmyp.py
  DAEMONOPTS="--pidfile /run/poatmyp.pid --oknodo --exec /usr/bin/python3"

  do_start() {
		  start-stop-daemon --start $DAEMONOPTS \
			  --make-pidfile --background --chuid nobody \
			  -- $SCRIPT_LOCATION
  }

  do_stop() {
		  start-stop-daemon --stop $DAEMONOPTS \
			  --remove-pidfile \
			  -- $SCRIPT_LOCATION
  }


  case "$1" in
	  start)
		  do_start
		  ;;
	  stop)
		  do_stop
		  ;;
	  restart)
		  do_stop
		  do_start
		  ;;
	  *)
		  echo "USAGE: $0 start|stop|restart"
		  ;;
  esac

  exit 0