Client session caching support is minimal

[ctz]

At the moment client-side session caching only respects SSL_CTX_sess_set_cache_size, but not the cache mode, callbacks, SSL_get_session etc.

Meaningfully implementing SSL_get_session would mean rustls client sessions would need to become serialisable -- they currently are not.

[ctz]

I think this issue is mostly about client-side session caching, so probably shouldn't have much bearing on nginx.

I wonder what happens if we accept the SSL_CTX_set_tlsext_ticket_key_cb but never call the callback?

To get this fully working we'd need to implement the rustls ProducesTickets trait in terms of calling the callback, then using the returned cipher and mac keys to implement the RFC5077 "recommended" construction which AIUI the openssl API is tightly coupled to.

[cpu]

I will pull the relevant comments into a new issue.

#30