[tor-dev] Comments on librarifying an Extended ORPort client?

[Nick Mathewson]

On Wed, Jan 23, 2013 at 7:02 PM, George Kadianakis <desnacked at riseup.net> wrote:
> Hi Nick,

Hi, George!

 [...]
> Unfortunately, I don't know how to do such a thing without influencing
> a certain networking programming style to the transport author. For
> example, if I coded the Extended ORPort client in Twisted, the
> transport author would probably have to use Twisted for the rest of
> her communication with the Extended ORPort. Similarly, if I coded the
> Extended ORPort client using the low-level 'socket' module, a
> transport author who wanted to use Twisted would be in trouble.
>
> As a matter of fact, I'm not sure how to do this task without coding
> separate different Extended ORPort clients for each networking library
> (one for Twisted, one for 'socket', etc.). But this sounds like too
> much work.

So generally speaking, if you want to implement a protocol in a
transport-neutral way, you're into architecturally tricky territory.
One option is to have a transport abstraction that different
transports can implement -- but in practice at most one library's set
of transports will start out implementing your transport abstraction,
so you're not in luck there.  Another option is to make a
protocol-handling object that receives bytes and tells you when you
can write bytes, and then write code to plug it onto different kinds
of transport -- but neither is quite so clean and simple as writing
the protocol with exactly one transport in mind.

In practice, I think that you don't need to worry about an arbitrary
number of transports -- in general, asynchonous ones tend to converge
to interfaces like twisted or libevent, and synchonous ones tend to
look like socket. For flexibility here, you might want to carefully
restrict which methods of the underlying transport you call--possibly
via a tidy little wrapper -- so that it's easy to tell what methods
you're really relying on.

(Of course, none of these are necessarily going to be great ideas in
practice; if they don't work, I'd suggest something else.)

yrs,
-- 
Nick