In case anyone else runs into this issue and is worried about their code
blocking forever, here's a version that will time out. The only change is
the use of sync/timeout and value-evt.
https://docs.racket-lang.org/value-evt/index.html
(require value-evt)
(define bstr (make-shared-bytes 509 5))
(define rx-pipe-size 16777216)
(define-values (rx-in rx-out) (make-pipe rx-pipe-size))
(define (room-in-rx-pipe? bstr)
(define avail (- rx-pipe-size (pipe-content-length rx-out)))
(displayln (format "avail: ~a" avail))
(<= (bytes-length bstr) avail))
(displayln (format "space available? ~a " (room-in-rx-pipe? bstr)))
(displayln (format "pipe content length: ~a" (pipe-content-length rx-out)))
; if write-bytes takes more than 0.5 seconds then it will be interrupted
and num-bytes-written will end up being #f
; wrapping the write-bytes expressing in a thunk is important because we
don't want it to be evaluated before the sync
(define num-bytes-written (sync/timeout 0.5 (value-evt (thunk (write-bytes
bstr rx-out)))))
(unless (eq? num-bytes-written (bytes-length bstr))
(displayln (format "rx buffer overflow. pipe content length: ~a, written
~a, expected ~a"
(pipe-content-length rx-out) num-bytes-written
(bytes-length bstr))))
(displayln "done")
-------- output
avail: 16777216
space available? #t
pipe content length: 0
done
test.r
On Wed, Jun 16, 2021 at 4:13 PM David Storrs <[email protected]> wrote:
> Got it. Thanks.
>
> On Wed, Jun 16, 2021 at 3:45 PM Matthew Flatt <[email protected]> wrote:
>
>> At Wed, 16 Jun 2021 14:25:40 -0400, George Neuner wrote:
>> > It looks like the problem
>> > is that "flush" is not defined ...
>>
>> Yes, "returns without blocking after writing as many bytes as it can
>> immediately flush" is vague, and more or less intentionally so. The
>> intent it really "writes as much as is convenient, with the guarantee
>> that anything written is completely flushed". Maybe the documentation
>> should be revised to say something more like that.
>>
>> There's not intended to be a guarantee that as much is written as could
>> possibly make sense by `write-bytes-avail`. Implementation issues may
>> make writing additional bytes inconvenient enough that it doesn't
>> happen, for example, even if more is always written on the next
>> `write-bytes-avail*` call. Also, ports are meant to be used in
>> concurrent settings where the notion "as much as possible" is prone to
>> race conditions.
>>
>> The Racket BC and CS pipe implementations find different things
>> convenient, in this sense, and that's why they behave differently in
>> the example. (That is, it's not really about the Racket version, but CS
>> versus BC.)
>>
>> So, the intent is that you use `write-bytes` when you want to wait
>> until all bytes are written (maybe because you know that will be soon
>> enough, for whatever reason). But when using `write-bytes-avail`, be
>> prepared for a fraction of the bytes to be written, for whatever
>> reason.
>>
>
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