synchronize gui's debug actions with the octave core thread (bug #41139)

[Daniel J Sebald]

Torsten,

I see you added a mutex-protected queue to make the debugger more 
robust.  I like the queue and making the debugger more robust has been a 
goal I worked on.  I would like to point out that with the queue you 
added this is getting very close to what I did with the prototype 
"background queue".  It's not too difficult to add such a thing. 
Foreground queue is the existing commands visible to the user (e.g., 
typing at the command line) and background queue would be commands not 
visible to the user (initiated by the GUI in response to debugger 
action, for example).  The idea is that with a generic background queue 
the GUI could send any command.

This construct:

      if (debug == "step")
        Fdbstep ();
      else if (debug == "cont")
        Fdbcont ();
      else if (debug == "quit")
        Fdbquit ();
      else
        Fdbstep (ovl (debug.toStdString ()));

would be much nicer, in my opinion, if done more directly as say:

  // There are no output arguments for dbstep.
  emit queue_background_octave_command ("dbstep ('out');", 0);

  // There are no output arguments for dbcont.
  emit queue_background_octave_command ("dbcont ();", 0);

  // There are no output arguments for dbquit.
  emit queue_background_octave_command ("dbquit ();", 0);

More sophisticated variations on the above would be:

  QString command;
  command = "dbstop (\"" + function_name + "\", ";
  command += QString ().setNum (line) + ");";

  // There is only one output argument for dbstop.
  emit queue_background_octave_command (command, 1);

It's much more direct and generic than queuing intermediate pseudo 
commands that are then processed to call callbacks.

My point is, things are getting really close (as a result of successive 
mods for solving existing problems) to a simple, generic way to 
communicate commands (and data) to/from each of the threads in the 
system.  If you are interested, there are some diff files attached to 
this patch:

http://savannah.gnu.org/patch/?8016

Dan