From: Andy Wingo <wingo@pobox.com>
To: Chris Vine <chris@cvine.freeserve.co.uk>
Cc: guile-user <guile-user@gnu.org>, guile-devel <guile-devel@gnu.org>
Subject: Re: anyone define port types?
Date: Sun, 19 Jun 2016 19:48:03 +0200 [thread overview]
Message-ID: <8737o9kr3g.fsf@pobox.com> (raw)
In-Reply-To: <20160619163327.36246706@dell.homenet> (Chris Vine's message of "Sun, 19 Jun 2016 16:33:27 +0100")
On Sun 19 Jun 2016 17:33, Chris Vine <chris@cvine.freeserve.co.uk> writes:
> The answer I have adopted when reading from TCP sockets is to extract
> individual bytes only from the port into a bytevector using R6RS's
> get-u8 procedure and (if the port is textual rather than binary) to
> reconstruct characters from that using bytevector->string at, say, a
> line end[1]. An EAGAIN/EWOULDBLOCK exception is then just treated as
> an invitation to return to the prompt, and read state is retained in
> the bytevector.
Yep, makes sense, though it will be really slow of course. And, you'd
have to make people rewrite their code to use your I/O primitives
instead of using read-line from (ice-9 rdelim); a bit of a drag, but
OK.
>> Why not just (install-suspendable-ports!) and
>>
>> (parameterize ((current-read-waiter my-read-waiter)) ...)
>>
>> etc? It is entirely possible with Guile 2.1.3 to build an
>> asynchronous coroutine-style concurrent system in user-space using
>> these primitives. See the wip-ethread branch for an example
>> implementation.
>
> I would want to continue using an external event loop implemented with
> poll() or select() and delimited continuations.
Yes yes a million times yes! That's what I want too. To that end, the
current-read-waiter parameter's value is a function which takes the port
being waited on, and when it returns the operation will retry. In
wip-ethreads I suspend to a prompt, add the port's fd to an epoll set,
and when the fd becomes readable again we resume the continuation,
resulting in a return from the current-read-waiter.
> I don't think I have got to grips with how to do that with read-waiter,
> because the read-waiter comprises in effect another loop (in which the
> main event loop with its own prompts would have to run) until the read
> request has been satisfied. I would need to think about it. Since
> ethreads use a poll()/epoll() loop, presumably you think it is
> straightforward enough to integrate the two, even if at present I don't.
Here is the code. First, a helper:
;; The AFTER-SUSPEND thunk allows the user to suspend the current
;; thread, saving its state, and then perform some other nonlocal
;; control flow.
;;
(define* (suspend #:optional (after-suspend (lambda (ctx thread) #f)))
((abort-to-prompt (econtext-prompt-tag (current-econtext))
after-suspend)))
Assume there is some current-econtext parameter with a "context" which
holds the prompt tag associated with the scheduler. As you see when the
continuation resumes, it resumes by calling a thunk, allowing exceptions
to be thrown from the context of the suspend.
Here's the main loop function, which you could replace with the GLib
main loop or whatever:
(define* (run #:optional (ctx (ensure-current-econtext))
#:key (install-suspendable-ports? #t))
(when install-suspendable-ports? (install-suspendable-ports!))
(parameterize ((current-econtext ctx)
(current-read-waiter wait-for-readable)
(current-write-waiter wait-for-writable))
(let lp ()
(run-ethread ctx (next-thread ctx))
(lp))))
Cool. Now the wait functions:
(define (wait-for-readable port)
(wait-for-events port (port-read-wait-fd port) (logior EPOLLIN EPOLLRDHUP)))
(define (wait-for-writable port)
(wait-for-events port (port-write-wait-fd port) EPOLLOUT))
Now the wait-for-events function:
(define (wait-for-events port fd events)
(handle-events
port
events
(suspend
(lambda (ctx thread)
...))))
Well that's a mess, but the thing to know is that the `suspend' will
abort to the relevant prompt, and then invoke the thunk that's its
argument. Here's `handle-events' and we are done:
(define (handle-events port events revents)
(unless (zero? (logand revents EPOLLERR))
(error "error reading from port" port)))
But I guess that error could be "reading or writing"; oh well.
See
http://git.savannah.gnu.org/cgit/guile.git/tree/module/ice-9/ethreads.scm?h=wip-ethreads&id=253dc1a7114b89351a3aa330caf173b98c5a65dd,
it's not long but it takes some time to read. I think I can fairly ask
that of you though, given your interest in this area :)
> On a side issue, I am still trying to understand the point of causing
> guile-2.2's read of a non-blocking C port to block. The whole point of
> making a descriptor non-blocking is that that shouldn't happen, and
> there are circumstances where pealing individual bytes off a
> non-blocking port as they become available is what you want to do. It
> makes guile's select wrapper unusable with TCP sockets on linux. I
> understand that suspendable-ports work differently, but that is another
> matter.
I would start by saying that many things are possible including adding
the ability to enable the error-throwing behavior. I think your use
case is a great one and we should support it well. I think though that
perhaps you didn't see how the waiter parameters could apply to your use
case; are things clearer now? I think that once you allow yourself to
use port buffers, things will be much easier and much more efficient as
well. But, I could be misunderstanding too.
Guile's I/O in the past was very oriented towards ASCII. 2.0 went a
long way towards good textual I/O with other encodings, and with 2.2 we
will have good textual non-blocking I/O. It could be that by focusing
on this, I have neglected some binary non-blocking I/O use cases, though
I think that get-bytevector-some, having enabled suspendable ports,
serves many of these purposes very well. But if there is a need for an
additional binary I/O primitive, let's figure out what it is and
implement it.
Andy
next prev parent reply other threads:[~2016-06-19 17:48 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-28 19:04 anyone define port types? Andy Wingo
2016-03-28 23:53 ` Matt Wette
2016-04-05 14:06 ` Mark H Weaver
2016-04-05 23:55 ` Matt Wette
2016-06-11 16:50 ` Andy Wingo
2016-03-29 7:58 ` tomas
2016-03-29 8:52 ` Nala Ginrut
2016-03-30 6:29 ` Panicz Maciej Godek
2016-03-30 11:18 ` Jan Nieuwenhuizen
2016-03-30 17:17 ` Panicz Maciej Godek
2016-03-30 17:53 ` Marko Rauhamaa
2016-03-30 19:02 ` Panicz Maciej Godek
2016-03-30 19:57 ` Marko Rauhamaa
2016-03-31 16:11 ` Barry Fishman
2016-03-31 19:28 ` Marko Rauhamaa
2016-03-30 19:43 ` Jan Wedekind
2016-03-30 20:07 ` Marko Rauhamaa
2016-03-30 21:01 ` Jan Wedekind
2016-03-30 22:44 ` Marko Rauhamaa
2016-03-31 20:42 ` Jan Wedekind
2016-03-31 22:28 ` Marko Rauhamaa
2016-06-11 16:53 ` Andy Wingo
2016-04-01 14:38 ` Ludovic Courtès
2016-06-11 16:57 ` Andy Wingo
2016-04-14 14:08 ` Ludovic Courtès
2016-06-11 17:02 ` Andy Wingo
2016-06-12 8:25 ` Chris Vine
2016-06-19 9:13 ` Andy Wingo
2016-06-19 9:55 ` Marko Rauhamaa
2016-06-19 15:27 ` Andy Wingo
2016-06-19 15:33 ` Chris Vine
2016-06-19 17:48 ` Andy Wingo [this message]
2016-06-19 20:09 ` Chris Vine
2016-06-20 3:38 ` William ML Leslie
2016-06-20 6:45 ` Chris Vine
2016-06-20 7:34 ` Andy Wingo
2016-06-20 9:01 ` Chris Vine
2016-06-22 22:44 ` Chris Vine
2016-06-23 7:36 ` Andy Wingo
2016-06-23 8:56 ` Andy Wingo
2016-06-23 9:24 ` Chris Vine
2016-06-23 9:50 ` Marko Rauhamaa
2016-06-23 10:43 ` Andy Wingo
2016-06-23 11:49 ` William ML Leslie
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