Re: Transparent Content Negotiation and Caching

Everything you say is right, as usual.

> * The HTTP parser sets cache_control.flags |= CACHE_VARY when the
>   response contains a Vary header (http_parse.c:1206). I couldn't see
>   polipo checking the _value_ of the Vary header anywhere, so it clearly
>   doesn't cache different variation of the response to fulfil subsequent
>   requests based on the new request's Accept-* headers.

That's right.  It will revalidate every instance with Vary.  (This
triggers a bug in Apache's mod_gzip, which we work around by checking
the size and MIME type in addition to ETag and Last-Modified during
revalidation.)

> * There are additional CACHE_VARY checks in validateEntry (diskcache.c)

That one's easy -- ignore on-disk data that has Vary but no ETag.  If
you don't, you could be merging data from different instances (the one
in memory and the one on disk).

>   and httpTweakCacheability (http.c) that I haven't read through.

That one's probably redundant, but it cannot harm.

-- Juliusz

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