rproxy as an Apache module
Martin Pool

The medium-term plan is that this code will become a module or two
within Apache 2.x.  These modules will do two distinct things: firstly
be a proxy within the server framework provided by Apache, and
secondly act as an in-process encoder for content generated by the
server.  Probably there will be two different modules to do this with
some common code.

Because we're planning to move into Apache there are some nice
features we can count on Apache to provide for us.  It's almost
certainly not worth implementing these from scratch:

 * clever logging

 * error recovery and cleanup

 * full HTTP parsing/generation

 * forking/reuse/keep-alive of connections

It should be pretty simple to glue rproxy in as a proxy.  We might
have to pay some attention to

 * keep-alive connections

Perhaps a good way to do this is to glue in rproxy as an extension to
the existing mod_proxy: after all, a lot of the intelligence about how
to proxy requests is already there.  But if we do that, then perhaps
we should just go ahead and add it in a completely general way.

From the point of view of integration, keeping the cache and decoding
is more complicated, though the encoding part of the library contains
the more sophisticated algorithm.

To do hsync-encoding of locally-generated content

 * We need reasonably full IO-layers in APR, so that we can catch
   output wherever it's coming from.  This is at least not well tested
   in Apache 2.0, but it can be fixed.

 * We need to settle the push/pull issue: if Apache expects to call us
   when it has output we have to modify the library to allow content
   to be pushed in, rather than having it call back when it wants
   input.  I'm not going to worry about this for now.

There may be some difficult issues when glueing these together

 * memory allocation: I'm somewhat inclined to just make sure the
   library doesn't leak and then to glue it into Apache as-is.  This
   seems to be a reasonable approach for third-party libraries.  If
   we can't achieve that e.g. when