Send Trait for Connection

[sgessa]

I'm trying to use this library from a multi-threaded context.
Is there any particular reason the Connection struct can't implement Send?

[ghedo]

One problem is that the TLS handshake state (which is a raw pointer to a BoringSSL SSL object) is not thread-safe, which makes the whole Connection not thread-safe as well. It might be possible to make it thread-safe by wrapping the TLS state in Arc<Mutex<...> but that would need to be tested to make sure it doesn't break any assumptions made by BoringSSL.

[puckipedia]

I believe impling Send would be safe on both Context and Handshake, as that just means allowing the respective structs to move between threads, which is okayed by the ssl.h comments:

// An |SSL| object represents a single TLS or DTLS connection. Although the
// shared |SSL_CTX| is thread-safe, an |SSL| is not thread-safe and may only be
// used on one thread at a time.

[ctiller]

One would also need to deal with application data on streams - currently this is a Box<dyn std::any::Any> and it would need to be Box<dyn Send + std::any::Any>, which would break the current ffi interface, since it's taking a *mut c_void for application data, which is not Send.