Add SSLSocket server handshake aliases

[samuel-williams-shopify]

TL;DR

Add OpenSSL::SSL::SSLSocket#start and #start_nonblock as aliases for #accept and #accept_nonblock.

Context

When OpenSSL::SSL::SSLServer#start_immediately is disabled, SSLServer#accept returns the accepted transport before performing the TLS handshake. The caller then invokes SSLSocket#accept on that already-accepted socket to perform the server-side handshake.

io-endpoint currently defines these aliases itself so accepted connections can be dispatched to a fiber or thread and the TLS handshake can be initiated with #start, without treating handshake initiation as another connection acceptance operation in generic server code.

Related to #760.

Changes

• Alias SSLSocket#start to SSLSocket#accept.
• Alias SSLSocket#start_nonblock to SSLSocket#accept_nonblock.
• Test that both aliases resolve to the existing methods.

The existing methods and behavior are unchanged.

Tophatting

• bundle exec rake compile
• bundle exec ruby -Itest -Ilib test/openssl/test_ssl.rb --name=test_server_handshake_aliases
• bundle exec rake test (628 tests, 4,606 assertions, 0 failures, 0 errors, 2 expected FIPS omissions)

[rhenium]

TLS is a client-initiated protocol, so if we had SSLSocket#start I'd expect it to mean #connect rather than #accept.

As I've written in #760, I don't really see problems with the current names #accept and #connect. What problem does adding aliases solve?

SSLSocket#{accept,connect} were taken from the corresponding OpenSSL C API SSL_accept() and SSL_connect(), so they are consistent in this regard at least.

[samuel-williams-shopify]

Thanks. The concrete problem is that accept cannot represent a post-accept negotiation capability in generic socket-handling code: every socket responds to accept, including sockets representing connections that have already been accepted.

io-endpoint accepts a connection, dispatches it to a fiber or thread, and then performs any required application-level negotiation:

if socket.respond_to?(:start)
  socket.start
end

The actual implementation is here: https://github.com/socketry/io-endpoint/blob/5e7c6aff6ba7b52c1a30f6d3a6e5a5aa86b41bf0/lib/io/endpoint/wrapper.rb#L217-L226

For an SSLSocket returned with SSLServer#start_immediately = false, that operation is the server-side TLS handshake. A plain socket requires no such step.

Therefore, io-endpoint currently adds SSLSocket#start itself as an alias for accept. The distinct name allows generic code to discover and invoke post-accept negotiation without checking for SSLSocket specifically or accidentally calling accept on an ordinary connected socket.

This PR does not replace or change accept and connect; it standardizes that distinct capability name for the server-side handshake.