Add Support for Weak ETags

[Maxim Dounin]

Hello!

On Tue, Nov 12, 2013 at 12:24:54PM -0800, Piotr Sikora wrote:

> Hey Maxim,
> 
> > I don't think that limiting the scope to gzip and gunzip is
> > correct either.  From cache validation point of view, weak etags
> > are mostly identical to Last-Modified, and removing etags
> > completely should mostly follow ngx_http_clear_last_modified().
> 
> I strongly disagree. Modifying content (via addition/sub/ssi modules)
> can change enough to consider two pages completely different (at least
> as a general rule), so I don't think that they should retain any
> ETags.

The intended use case for addition filter is to use add 
headers/footers which don't change semantics.  I.e., week entity 
tags can be preserved.  Much like Last-Modified header currently.

The sub and ssi remove Last-Modified and all etags by default.  If 
needed though, they can be configured to preserve Last-Modified 
(and weak etags with a patch) with ssi_last_modified directive and 
friends:

http://nginx.org/r/ssi_last_modified

> I've got mixed feelings regarding xslt module. While I don't disagree
> as much, I think it's still safer to remove weak ETags there as well.

Much like with ssi, this is the default.  I don't think we should 
remove weak etags if we are explicitly configured to preserve 
Last-Modified though (http://nginx.org/r/xslt_last_modified).

-- 
Maxim Dounin
http://nginx.org/en/donation.html