non-blocking reads/writes and event loops

[Todd Whitesel]

> 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.

IMHO the simplest way is always to invert from the top down, so things are
just merged into the main event loop as you go.

> 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. 

Well that depends on how much of the event loop machinery is used by the
low-level inversion. If it calls things that assume a single global event
loop, then we have a problem.

If you have to invert a low-level algorithm early on, there are a couple
ways to do it:

  1. make the event loop machinery instantiable and use a new instance of it.
	THIS IS ARGUABLY NECESSARY FOR MULTIPLE TARGET CONNECTIONS ANYWAY.

  2. do not attempt to provide full non-blocking facilities yet, just show
	that the low-level code works correctly in inverted form -- its
	sub-event loop just calls it repeatedly, so it still blocks but at
	least uses the new code structure.

IMHO either is fine; which one you use depends on whether you have gotten
the instantiable event loop machinery working yet.

> Another decision was that GDB's core assume no threads.  Should that too
> be questioned?

I don't mind specific native target support assuming threads.

However using threads as a general method of avoiding the inversion of
GDB core code is a COP OUT.

-- 
Todd Whitesel
toddpw @ windriver.com