Re: Comments on ‘wip-nio’

[Andy Wingo]

On Sun 25 Mar 2012 22:17, Neil Jerram <address@hidden> writes:

> Andy Wingo <address@hidden> writes:
>
>> How about, you write your code in such a way that it deals with eports.
>> You could surround your code in a prompt that if an eport operation
>> aborts because it would block, you save the continuation and ask your
>> main loop to call you again when the port is readable / writable.  When
>> it calls you again you resume the partial continuation.
>
> Thanks for the ideas.  I unfortunately have a bit of a cognitive deficit
> backlog before I can understand them fully.  I haven't yet understand
> prompts properly, and I see that ethreads are built on top of those.
> But I'll continue working at it!

Heh, sorry for the brevity!  The problem is that we don't explain them
very well.

Let's consider you have a d-bus callback with fd FD.  You want to do
write some bytes to that FD, then do something else.

You can:

  (define (call/resumable thunk)
    (let ((suspend-tag (make-prompt-tag "suspend")))
      (parameterize ((current-read-waiter
                      (lambda (fd) (abort-to-prompt suspend-tag fd))))
        (call-with-prompt suspend-tag
          thunk
          (lambda (k fd)
            (reschedule! fd (lamdbda () (call/resumable k))))))))

In the case of glib, you would implement reschedule! as some wrapper
that adds fd as a gsource to the current main loop.  When the gsource
fires, you would arrange to call the associated thunk -- in this case a
thunk that calls the captured continuation, within another
call/resumable block.

This assumes you are using (ice-9 eports) from wip-ethreads.  Please
take a look at that code.  It is a ports implementation, written in
Scheme, which uses non-blocking read and write primitives when it needs
to fill / flush its buffers.  If the I/O would block, it calls the
current-read-waiter / current-write-waiter.  Here we parameterize that
waiter to capture the continuation and reschedule.

Let me know if I can muddy^Hclarify things more for you ;-)

Cheers,

Andy
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