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Date: Thu, 22 Dec 2016 15:05:24 +0100
Subject: Re: Change the content-length header for other filters
Message-ID: <650227071-7335@hogwarts.imise.uni-leipzig.de>
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From: Andre Rothe <andre.rothe@zks.uni-leipzig.de>
To: modules-dev@httpd.apache.org
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archived-at: Thu, 22 Dec 2016 14:05:39 -0000

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Hello,

I've got the source code of mod_WebObjects, which=20
is the destination module of the request. It registers a=20
handler to process the incoming requests like

ap_hook_handler(WebObjects_handler, NULL, NULL, APR_HOOK_MIDDLE);

As I understand the Apache request processing, in this case the=20
content generator is the mod_WebObjects (or the WebObjects=20
Application Server behind it) and my filter (input and output) are=20
placed by mod_ext_filter in the data processing axis.=20
Is it necessary to set mod_WebObjects as successor of mod_ext_filter
to define an order within the processing axis too?

Within the WebObjects_handler hook the request_rec will be read and=20
the headers_in table will be copied into an own structure with

retval =3D ap_setup_client_block(r, REQUEST_CHUNKED_ERROR);
apr_table_do(...);

There the content-length header will be used to allocate a buffer for=20
the content and the method reads the body content by

ap_get_client_block(...)

into that buffer. So it could be, that all of these things are finished in=20
mod_WebObjects during my filter code processes the new length=20
and ap_get_client_block() waits till body data are available (after my
filter code).=20

Can I stop the header processing in mod_WebObjects (before=20
apr_table_do()), maybe if I read the bucket brigade till EOS there?=20
It should only be possible after my filter returns APR_SUCCESS, right?

In mod_ext_filter I read the whole body before processing, because I=20
will get also an exception, if the request has two or more chunks. So I=20
read all chunks, copy it into one brigade (setaside), process it=20
in the external PHP scripts and send it further as one brigade.=20
This could generate performance issues, but at the moment,=20
the application is still very fast.=20

Thank you
Andr=C3=A9


 Von:   Nick Kew <niq@apache.org>=20
 An:   <modules-dev@httpd.apache.org>=20
 Gesendet:   12/22/2016 2:05 AM=20
 Betreff:   Re: Change the content-length header for other filters=20

On Wed, 2016-12-21 at 22:10 +0100, Andr=C3=A9 Rothe wrote:=20
=20
> But after my filter completes the request processing, I'll get:=20
> =20
> Sending error response: The request contained fewer content data than=20
> specified by the content-length header=20
=20
Sorin's reply is part of the story, and may or may not be useful=20
in your case.=20
=20
The basic issue you have to consider is that the headers arrive before=20
the body, and a handler that cares about Content-Length is likely=20
to have read it before your filter has reset it.=20
=20
The alternative - read and buffer the entire body before starting=20
to process it - becomes hopelessly inefficient for large requests.=20
=20
There's some discussion of the issue in the mod_proxy docs,=20
as mod_proxy has an option to support HTTP/1.0 backends that=20
need an explicit Content-Length.=20
=20
-- =20
Nick Kew=20
=20
=

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