Subject: bug#39610: R6RS `flush-output-port` not playing along with `transcoded-port`

From: Ludovic Courtès <[email-masked]>
Date: Mon, 23 Mar 2020 10:22:09 +0100

Hi,

Andreas Rottmann <[email-masked]> skribis:

>> Andreas Rottmann <[email-masked]> skribis:
>>
>>> Andreas Rottmann <[email-masked]> writes:
>>>
>>>> [...] I isolated the cause; the following snippet hangs on Guile 2.2
>>>> and 3.0, while it worked as expected on 2.0:
>>>>
>>>> ;; ------------------
>>>> (import (rnrs))
>>>>
>>>> (let* ((p (pipe))
>>>>        (in (car p))
>>>>        (out (transcoded-port (cdr p) (make-transcoder (utf-8-codec)))))
>>>>   (put-datum out "foo")
>>>>   (flush-output-port out)
>>>>   (display "Should have written to pipe by now, attempting reading a byte\n")
>>>>   (display "Got")
>>>>   (display (get-u8 in))
>>>>   (newline))
>>>> ;; -------------------
>>>>
>>>> It seems the underlying port is no longer flushed to the OS, so the
>>>> `get-u8` now hangs waiting for input, starting with Guile 2.2.
>>>>
>>> I'd like to add that this is indeed not passing data to the OS, as
>>> verified by strace. Also, I have now figured out the commit introducing
>>> the regression, namely 8399e7af5 ("Generic port facility provides
>>> buffering uniformly"); the commit before (e8eeeeb1d) still runs the
>>> above code to completion.
>>
>> Actually I think the code above behaves as expected.  ‘pipe’ returns
>> buffered ports by default.  When flushing the transcoded port,
>> ‘transcoded_port_write’ is called, but then bytes written to the pipe
>> are buffered.
>>
>> The fix is to add:
>>
>>   (setvbuf (cdr p) 'none)
>>
>> Does that make sense?
>>
> It makes sense, and I can confirm that it makes the boiled-down example
> I posted work.
>
> However, I'm not sure it is the expected behavior. Regardless of
> buffering modes used, I would expect a `flush-output-port` in a "port
> stack" (as produced by `transcoded-output-port`) to propagate all the
> way to the OS. It seems that was the case in Guile 2.0, as I'm pretty
> sure I observed the "breaking" behavior change with 8399e7af5 applied,
> and not with the commit preceding it.

Port types don’t have a “flush” operation, only “write”.  Thus, it’s
impossible to define a port type that would propagate flushes.
There are pros and cons I guess, but it seems like a reasonable choice
to me.

When implementing a “proxying” port type like ‘transcoded-port’ that
does its own buffering, it’s probably OK to say that it’s the proxy’s
responsibility to ensure there’s no double-buffering taking place.

> If the current behavior is indeed the intended one, we should make sure
> the docs somehow reflect this caveat, as I imagine it may surprise
> future Guile users which happen to use its R6RS support and pipes in
> combination.

Maybe we should not document this specific combination but rather the
more general issue?  I’m not sure how to do that.  Thoughts?

Thanks,
Ludo’.