Re: non-blocking reads/writes and event loops

[Andrew Cagney <ac131313@cygnus.com>]

Todd Whitesel wrote:
> 
> > > Agreed. If the API designers are dumb enough to trust threads absolutely,
> > > and don't give us an option, then well, that's life.
> >
> > For what it is worth, here is the ``It is hard'' reason number two:
> ...
> > Just making certain that your eyes are open :-)
> 
> No doubts about that. That's why I said "long term goal"...

Unfortunatly, unless feasible intermediate steps are identifed that show
how we can get from the eixsting GDB to this ``long term goal'' it will
just remain a long term goal :-(

To that end, I can see several strategies.  Interestingly at least one
discards the assumption that the event loop should _not_ be re-entrant.

o	invert the targets first
		
	This would be implemented
	using something like:

	(gdb) ->do-command
	  -> target_xfer_memory()
	    -> target->request_data()
	    while need more data, run inner event loop
	       <- target-supply-data()
	    <- return data

	That is, the GDB core would continue
	to assume that things are blocking
	but the targets would internally
	be implemented as non-blocking.

	Care would be needed that the internal
	event loop didn't re-call the CLI et.al.


o	invert the expression evaluator first

	In this case, legacy targets would
	be wrapped so that:

	(gdb) ->do-command
	   -> target_memory_request()
	     -> target_xfer_memory ()
	     schedule event to supply
	     returned data to expression evaluator
	-> supply_data to expression evaluator


(wonder if this makes any sense).  Of course there is option three, both
of the above.

For what its worth, for some reason I have preference for the first
option - not sure why, perhaphs it is simply that I'm more familar with
targets and back-ends.

As I noted above, one of the original design decisions for the event
loop that it not be re-entrant.  The above questions that decision. 
Another decision was that GDB's core assume no threads.  Should that too
be questioned?

enjoy,
	Andrew