[PATCH 2 of 2] Cache: send conditional requests only for cached 200 OK responses

[Maxim Dounin]

Hello!

On Wed, Nov 19, 2014 at 01:52:34PM -0800, Piotr Sikora wrote:

> Hey Maxim,
> 
> > And what's wrong with it?
> > I don't see why this should be status-specific.
> 
> Because, according to the RFC, 304 Not Modified can be only emitted
> for 200 OK responses, so we know that revalidation of other status
> codes cannot happen, at least not when talking with well-behaving
> server, so sending conditional requests for such cached responses is
> pointless.

For example, it can be usable with 206 responses as well (and this 
is perfectly allowed by the RFC).

> Maybe I should elaborate a bit more on the problem I'm trying to
> workaround, which is also the underlying reason for the fix I made in
> fd283aa92e04.
> 
> Prior to that commit, when nginx cached 404 Not Found response without
> Last-Modified header, i.e. with such minimal headers:
> 
>     HTTP/1.1 404 Not Found
>     Date: Wed, 19 Nov 2014 21:12:47 GMT
> 
> it would always try to revalidate it with bogus If-Modified-Since header:
> 
>     GET /notfound HTTP/1.1
>     Host: www.example.com
>     If-Modified-Since: Thu, 01 Jan 1970 00:00:00 GMT
> 
> for which upstream server would usually respond with full 404 Not
> Found response (not 304 Not Modified), unless the file appeared on the
> upstream server (and would have resulted in 200 OK for unconditional
> requests), then it would respond with 304 Not Modified (thanks to the
> broken If-Modified-Since test on their side), revalidating 404 Not
> Found based on the 200 OK response.
> 
> The problem mostly disappeared after fd283aa92e04, but it can still
> happen, e.g. when cached 404 Not Found response with Last-Modified
> header will be revalidated (and hence cached forever) by 304 Not
> Modified sent instead of 200 OK response with Last-Modified header
> older than the one in cached response.
> 
> Hopefully that explains the problem enough... Let me know if you still
> have any doubt as to why it is important to change the current
> behavior to send conditional requests only for cached 200 OK
> responses.

I still think this approach is wrong, the behaviour shouldn't 
depend on the status code.

-- 
Maxim Dounin
http://nginx.org/