Subject: Re: bug#30066: 'get-bytevector-some' returns only 1 byte from unbuffered ports

From: Mark H Weaver <[email-masked]>
Date: Thu, 11 Jan 2018 14:55:07 -0500

Hi Ludovic,

[email-masked] (Ludovic Courtès) writes:

> Andy Wingo <[email-masked]> skribis:
>
>> I suggest taking the buffered bytes from the read buffer, if any.  Then
>> if the port is unbuffered, make a bytevector and call scm_i_read_bytes;
>> otherwise do the scm_fill_input path that's there already.
>>
>> One more thing, if the port goes EOF, you need to
>> scm_port_buffer_set_has_eof_p.
>
> I think the attached patch addresses these issues.  WDYT?

[...]

> diff --git a/libguile/r6rs-ports.c b/libguile/r6rs-ports.c
> index e944c7aab..a3d638ca0 100644
> --- a/libguile/r6rs-ports.c
> +++ b/libguile/r6rs-ports.c
> @@ -1,4 +1,4 @@
> -/* Copyright (C) 2009, 2010, 2011, 2013-2015 Free Software Foundation, Inc.
> +/* Copyright (C) 2009, 2010, 2011, 2013-2015, 2018 Free Software Foundation, Inc.
>   *
>   * This library is free software; you can redistribute it and/or
>   * modify it under the terms of the GNU Lesser General Public License
> @@ -481,9 +481,9 @@ SCM_DEFINE (scm_get_bytevector_some, "get-bytevector-some", 1, 0, 0,
>              "position to point just past these bytes.")
>  #define FUNC_NAME s_scm_get_bytevector_some
>  {
> -  SCM buf;
> +  SCM buf, bv;
>    size_t cur, avail;
> -  SCM bv;
> +  const size_t max_buffer_size = 4096;
>  
>    SCM_VALIDATE_BINARY_INPUT_PORT (1, port);
>  
> @@ -494,9 +494,31 @@ SCM_DEFINE (scm_get_bytevector_some, "get-bytevector-some", 1, 0, 0,
>        return SCM_EOF_VAL;
>      }
>  
> -  bv = scm_c_make_bytevector (avail);
> -  scm_port_buffer_take (buf, (scm_t_uint8 *) SCM_BYTEVECTOR_CONTENTS (bv),
> -                        avail, cur, avail);
> +  if (SCM_UNBUFFEREDP (port) && (avail < max_buffer_size))
> +    {
> +      /* PORT is unbuffered.  Read as much as possible from PORT.  */
> +      size_t read;
> +
> +      bv = scm_c_make_bytevector (max_buffer_size);
> +      scm_port_buffer_take (buf, (scm_t_uint8 *) SCM_BYTEVECTOR_CONTENTS (bv),
> +                            avail, cur, avail);
> +
> +      read = scm_i_read_bytes (port, bv, avail,
> +                               SCM_BYTEVECTOR_LENGTH (bv) - avail);

Here's the R6RS specification for 'get-bytevector-some':

  "Reads from BINARY-INPUT-PORT, blocking as necessary, until bytes are
   available from BINARY-INPUT-PORT or until an end of file is reached.
   If bytes become available, 'get-bytevector-some' returns a freshly
   allocated bytevector containing the initial available bytes (at least
   one), and it updates BINARY-INPUT-PORT to point just past these
   bytes.  If no input bytes are seen before an end of file is reached,
   the end-of-file object is returned."

By my reading of this, we should block only if necessary to ensure that
we return at least one byte (or EOF).  In other words, if we can return
at least one byte (or EOF), then we must not block, which means that we
must not initiate another 'read'.

Out of curiosity, is there a reason why you're using an unbuffered port
in your use case?

       Mark