AW: keep-alive and vary in 304 responses

[Plüm , Rüdiger , Vodafone Group]

C2 General

> -----Ursprüngliche Nachricht-----
> Von: Stefan Eissing <stefan.eiss...@greenbytes.de>
> Gesendet: Mittwoch, 10. April 2019 12:10
> An: dev@httpd.apache.org
> Betreff: Re: keep-alive and vary in 304 responses
> 
> 
> > Am 09.04.2019 um 18:48 schrieb Roy T. Fielding <field...@gbiv.com>:
> >
> >>
> >> 2. Validation responses lose the "Vary" header from the unconditional
> response. This happens on resources where mod_deflate is active.
> >>  The 200 response without any "if-modified-since" etc. carries "Vary:
> Accept-Encoding" as it should.
> >>  The 304 response with "if-modified-since" has no "Vary" header.
> >>  The code in mod_deflate looks as if it tries to add it in any case,
> where is it lost?
> >
> > That's one of many bugs related to design problems with mod_deflate.
> Basically,
> > it chooses to insert itself into the response handling after the other
> checks are done,
> > which means a 304 doesn't even know yet whether the response would use
> gzip.
> > The solution (which I never found time to implement) is to replace
> dynamic gzip with
> > a transfer encoding generator, and/or to rewrite the mod_deflate CE
> encoder to act more
> > like mod_cache.
> 
> Thinking out loud here:
> 
> Acting more like mod_cache would then split the deflate output filter
> into a resource and a protocol type one, I assume?

IMHO the "secret" of mod_cache is that its filter is also inserted via the
insert_error_filter hook and not just via the insert_filter hook (like 
mod_expires and mod_header do).
With mod_deflate this will be more complex as mod_deflate does not add itself.
It is either added by the core via SetOuputFilter or via mod_filter and they
all only consider insert_filter and not insert_error_filter. We probably need
a possibility to tell filters to be added in insert_error_filter.

Regards

Rüdiger