[Twisted-Python] SNI callback with support for deferreds

[Jonathan Stoppani]

Hello,

A couple of days ago I asked on Stack Overflow about returning a deferred
from an SNI callback and have pyOpenSSL wait for it to fire before
continuing handling the request.

Thanks to some pointers by Gyph I've found a solution ("workaround") for my
problem, involving a fake TLSMemoryBIOProtocol to handle the client hello
until the SNI is received, firing the SNI callback, waiting for it to
callback and then re-feeding the resulting context to the real
TLSMemoryBIOProtocol.

The implementation of this solution is available at
https://gist.github.com/GaretJax/124c523a62ba48c9eec1, and I'd like to
contribute it back to Twisted, however, it has no unit tests and needs some
design decisions/validation.

I've opened a ticket to track it at
https://twistedmatrix.com/trac/ticket/8065. Real-life impediments
permitting, I'm willing to work on it and get the feature supported in
Twisted core.

Anyone willing to help me getting a proper patch?

Best,
Jonathan

P.S.: A big shout-out to Twisted for its excellent TLS support out of the
box. We got a straight A rating out of the box on ssl labs!
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