== Problem: Startup ==  #1988

  Rainbow needs to be started before the first activity is started, so that it
  can be ready to go when Sugar starts launching activities.

Presently:

  The startup sequence looks something like:

  1. `/etc/inittab' specifies that the default runlevel is 5.
  2. Upon entering runlevel 5, the DBus system daemon is started with priority
     22.
  3. `/etc/inittab' specifies that `/sbin/olpc-dm' should be started on
     runlevel 5.
  4. `/sbin/olpc-dm' does some basic configuration of the terminal, language
     environment, and the session (e.g. does a PAM login); then it logs in as
     user `olpc' and runs `startx'.
  5. `startx' runs `xinit' which reads through /home/olpc/.xinitrc, does some
     further graphical configuration, and then exec's `sugar'.
  6. `sugar' is a script that uses `dbus-launch' to start the DBus session daemon.
  7. The session daemon, once started, runs its argument `sugar-shell' with
     appropriate environment variables.
  8. `sugar-shell' brings up Sugar.

    NB: Blizzard suggests that the current startup is hackish and should be
        changed.
          * In particular, we should use something less vulnerable to
            modification than .xinitrc to start up.
          * Also, he said that rc-scripts are going away to be replaced by a
            monolithic start script.

Plan:

  In the first iteration, we will simply launch Rainbow via rc-script at a
  higher priority than `messagebus'. Since `sugar-shell' presently records its
  DBus session bus's address to a `session.info' file, we will simply read this
  file to acquire access to the main DBus session bus.

Followup:

  Ivan suggests that we use `init' to start and respawn Rainbow.

    "Creating an Unkillable Process", Silas Brown, Linux Gazette #139, 2007
    http://linuxgazette.net/139/brownss.html

  gives a nice overview of how to do this in a hardened way.

  In the longer term, we may want to make the session bus talk to Rainbow in
  some other fashion, for example by enclosing `sugar-shell' in a security stub
  or by bringing the session bus up entirely separately from X.