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Subject: Re: Change the content-length header for other filters
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References: <c9ee1adf-05d7-9fb5-4dfd-cc0409afd61c@zks.uni-leipzig.de>
From: Sorin Manolache <sorinm@gmail.com>
Message-ID: <9b73c2a1-4fda-0d6d-37a2-6125521cec54@gmail.com>
Date: Wed, 21 Dec 2016 22:52:04 +0100
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On 2016-12-21 22:10, André Rothe wrote:
> Hi,
>
> I have a filter, which changes the content length of a POST request.
> There are some key-value-pairs of the request, which the filter removes
> before other filters process the request.
>
> But after my filter completes the request processing, I'll get:
>
> Sending error response: The request contained fewer content data than
> specified by the content-length header
>
> I have tried to change the header key "Content-Length" and set the
> new value like:
>
> apr_table_set(f->r->headers_in, "Content-Length",
>     apr_psprintf(f->r->pool, "%ld", len));
>
> but it has no effect outside of my filter. The incoming request has a
> content length of 1107 bytes. I modify the bucket brigade and it
> contains at the end of my filter code only 1074 bytes (which is also
> stored into "len").
>
> What can I do to send the new content length along the filter chain?
>
> Thank you
> André

Hello,

Could you please give us more details about how the body of the post 
request is read? Is it read in a third-party handler? Is is read by a 
standard apache module such as mod_proxy? If it's a third-party handler, 
do you happen to have the code?

Why I'm asking: because it may happen that the reader (i.e. the code 
that triggers the chain of input filters) reads first the Content-Length 
header and then attempts to read N bytes, where N is the value of the 
Content-Length filter. In this case, it is no use to set Content-Length 
in your filter because anyway the reader has read the value of the 
Content-Length header before your filter had the opportunity to change it.

A well-behaved reader should read until it finds an EOS bucket in the 
retrieved brigade. It should not rely on Content-Length. A trivial 
example why it should not use Content-Length is request body 
compression. A reader would get the brigade filtered by the INFLATE 
filter of mod_deflate, which contains many more bytes than indicated by 
Content-Length as this header contains the size of the compressed body.

Best regards,
Sorin