Remove custom fields in tls.Config

[Lekensteyn]

quic-go uses qtls which requires a match between its data structures and the standard library to match. This includes tls.Config, tls.ConnectionState, tls.ClientHelloInfos, and some others. See https://github.com/quic-go/qtls-go1-20/blob/master/unsafe.go

It would be nice if we can reduce custom fields unless there is no other way. This can be done through contexts.

In commit 01665a5 ("crypto/tls: add CFControl parameter to Config"), we add a tls.Config#CFControl field to propagate information from the TLS handshake (tls.Config#GetConfigForClient) to a HTTP request handler (http.Request#TLS.CFControl). To replace this with contexts:

1. Set http.Server#ConnContext to add an empty structure to the context of the connection.
2. In tls.Config#GetConfigForClient, extract the previous structure pointer from the context through tls.ClientHelloInfos#Context and initialize it. Note that while modifications to the context itself are not propagated, any changes to the context value (a pointer to a structure) will be preserved. This is a crucial detail.
3. In a HTTP handler, extract the structure pointer again from the context through http.Request#Context.

To demonstrate the above conversion:

type contextKey struct{ name string }

var controlContextKey = &contextKey{"connection-metadata"}

type ConnState struct {
	ExampleValue []byte
}

func main() {
	srv := http.Server{
		ConnContext: func(ctx context.Context, c net.Conn) context.Context {
			// 1. allocate context value in the connection context
			return context.WithValue(ctx, controlContextKey, &ConnState{})
		},
	}
	cert, err := tls.LoadX509KeyPair("example-cert.pem", "example-key.pem")
	if err != nil {
		log.Fatal(err)
	}
	tlsConfig := &tls.Config{
		GetConfigForClient: func(chi *tls.ClientHelloInfo) (*tls.Config, error) {
			// 2. initialize state in context value
			cs := chi.Context().Value(controlContextKey).(*ConnState)
			cs.ExampleValue = []byte("hello world\n")
			return &tls.Config{Certificates: []tls.Certificate{cert}}, nil
		},
	}

	http.HandleFunc("/", func(rw http.ResponseWriter, req *http.Request) {
		// 3. extract value from context
		cs := req.Context().Value(controlContextKey).(*ConnState)
		rw.Write(cs.ExampleValue)
	})

	// Setup HTTPS server
	netListener, err := net.Listen("tcp", "localhost:4433")
	if err != nil {
		log.Fatal(err)
	}
	ln := tls.NewListener(netListener, tlsConfig)
	if err := srv.Serve(ln); err != http.ErrServerClosed {
		log.Fatal(err)
	}
}

I'll work on changes to the cf branch to remove the CFControl field and investigate how other fields can similarly be removed.

[bwesterb]

Great idea.

[Lekensteyn]

tls.Config#CFControl was removed and tls.Config#CFEventHandler was moved into tls.CFEventHandlerContextKey via PR #147