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Date: Tue, 11 Nov 2003 12:39:16 -0800
From: Justin Erenkrantz <justin@erenkrantz.com>
To: dev@httpd.apache.org
Subject: Re: should input filter return the exact amount of bytes asked  for?
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--On Tuesday, November 11, 2003 11:24 AM -0600 "William A. Rowe, Jr." 
<wrowe@apache.org> wrote:

>> More bytes = Not OK.  (Theoretically possible though with bad filters.)
>
> Wrong.  This is OK across the board, please consider;

Uh, no.  We changed the filter semantics some time ago to stop this insanity. 
It was inefficient to call AP_MODE_READBYTES and have it return more than 
asked for.  Check out the CVS log for util_filter.h, specifically around 
revision 1.62.

> module requests 1000 arbitrary bytes;
>
>   codepage module requests 1000
>
>     http reads one 'chunk' available, 8000 bytes
>     and will return that page
>
>   codepage can translate 7998 bytes and comes to
>   a screeching halt for a 3 byte sequence.  returns
>   our Now Translated 4000 bytes
>
> module sees a 4000 byte heap bucket.
>
> What can you do?  Instead of treating that bucket as a singleton
> when you want 1000 bytes, consume the first 1000 bytes from that
> bucket (or the brigade.)

No.  That means you have 3k more bytes you have to consume that you didn't ask 
for.  The filter wouldn't return it again.  Writing code that used input 
filters and having to deal with that it could get more than asked for was just 
confusing and led to lots of error-prone code.

If it asks for 1k in AP_MODE_READBYTES, it gets at most 1k.  Anything else is 
broken.  (util_filter.h AP_MODE_READBYTES says as much, but that's not fair, 
because I wrote that comment.)

> Please review the archives for this discussion (the brigades on the
> apr list, the filter api on httpd.)  This was a very long thread, but the
> net result of filters is that you get what is available/handy, not any
> specific number of bytes.

That *was* indeed the position at one time, but when I redid the input filters 
(which was about rewrite #14 of input filters), we corrected this because it 
was causing lots of problems to return more than asked for - this is when we 
added the mode argument to ap_get_brigade.  mod_ssl's input filtering code was 
just broken under that old API.

And, the big boys even reviewed the code and semantic changes before it went 
in.  So, it was definitely RTC.  ;-)  -- justin