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

["Ludovic Courtès" <ludo@gnu.org>]

Hi Andreas,

And welcome back!  :-)

Andreas Rottmann <mail@r0tty.org> skribis:

> Andreas Rottmann <mail@r0tty.org> 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?

Thanks,
Ludo’.