allow for inspection of previously-set callbacks

[pyca]

allow for inspection of previously-set callbacks

[amluto]

Why on earth should two different libraries be trying to coordinate their info_callbacks on the same Context using pyOpenSSL as an intermediary?

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In Twisted, the application hands us a Context object and that's basically the whole interface. Now we have features that Twisted wants to provide directly, but mangling the application-provided Context would break an application relying on that callback staying set.

The typical Python idiom for this sort of thing is for the set_..._callback function to return the previously-set callback for specifically this reason.

[glyph]

It's worth noting that OpenSSL recognizes the need for this as well, and does have SSL_CTX_get_info_callback and SSL_get_info_callback; they're just not exposed by pyOpenSSL.