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From Justin Erenkrantz <jerenkra...@ebuilt.com>
Subject Re: FW: problem with AddOutputFilterByType directive
Date Wed, 20 Feb 2002 01:12:16 GMT
On Tue, Feb 19, 2002 at 03:27:04PM -0800, Ryan Bloom wrote:
> We have a function, ap_pass_brigade(), which is called by every content
> generator, by definition.  Just put a hook into that function.

And have that hook called every time data is sent through a
filter (also output filters call ap_pass_brigade)?  We can only make
this determination once (when we make this decision we have to be
sure it is right).  But, there also becomes a point that when we
positively know the answer, we may be too late to insert arbitrary
filters.

I understand why you guys are proposing this solution, but I think
you're missing my point.  Given our current architecture, we have
no way of guaranteeing the content-type until most filters have
been run.

You can't guarantee that the content-type is correct when
ap_pass_brigade() is called (first time or many times).  You have
no idea when the content-type will be set.  Any of the filters are
free to change the content-type as they execute themselves.
Consider the following case:

A JSP file that has:
<% response.setContentType("text/plain") %>
...jsp code...

At what point do we run this hook that checks content-type?  The
first time ap_pass_brigade() is called, it may have text/html set.
That may be because mod_mime saw .jsp and said, "Okay, text/html
for .jsp."  However, the JSP engine (via an Apache 2.0 filter) says,
"nah, this should be text/plain."  If we base it on the first call to
ap_pass_brigade(), we add "text/html"'s filters.  If we base it on
the second call to ap_pass_brigade, you now need "text/plain"'s
filters.  Say, we have JSP page that produces PHP code (hey, it's
valid in our architecture), and the PHP script now changes it to
"application/x-ogg".  So, the content-type is now something else.
It can arbitrarily flip-flop as ap_pass_brigade() is called and the
filter tree is walked.

However, in our current implementation, once we reach the HTTP_HEADER
state, the content-type must be defined.  How about we do it as a
filter that ran then?  Possibly even in ap_http_header_filter.  So,
let's say we do it right before ap_http_header_filter - we're
*guaranteed* to know content_type by then, right?  Add the filter as
requested by AddOutputFilterByType.

Is there a violation of our filter ordering semantics by running this
filter out-of-order?  We'll be running this filter during HTTP_HEADER.
Assume you have two filters - one is explicitly at a higher-priority
to ensure that one is always run *after* another.  Now, say that the
lower-priority filter is added by this directive (but not the other) -
we're violating the priorities.  I think that's bad.  So, perhaps, we
could restrict AddOutputFilterByType to only >= HTTP_HEADER filters.
If it is less than that, we could produce a config-time error.  That
seems something that might work.  Thoughts?  I think it may make
us too restrictive with this directive though.  -- justin


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