CONNECTION_REFUSED errors are not sent out from the server

[marten-seemann]

The server intends to send out CONNECTION_REFUSED errors to connection that are in the process of handshaking when it is closed:

quic-go/server.go

Lines 690 to 712 in b3eb375

	func (s *baseServer) handleNewConn(conn quicConn) {
	connCtx := conn.Context()
	if s.acceptEarlyConns {
	// wait until the early connection is ready, the handshake fails, or the server is closed
	select {
	case <-s.errorChan:
	conn.destroy(&qerr.TransportError{ErrorCode: ConnectionRefused})
	return
	case <-conn.earlyConnReady():
	case <-connCtx.Done():
	return
	}
	} else {
	// wait until the handshake is complete (or fails)
	select {
	case <-s.errorChan:
	conn.destroy(&qerr.TransportError{ErrorCode: ConnectionRefused})
	return
	case <-conn.HandshakeComplete():
	case <-connCtx.Done():
	return
	}
	}

Unfortunately, this doesn't work: destroy destroys the connection without sending out a CONNECTION_CLOSE frame. We didn't have an integration test for this, so this went undetected for the longest time. Fortunately, this is a pretty rare situation, so it probably doesn't matter too much in production.

However, with #3549, we'll need to send CONNECTION_REFUSED in a lot more situations, so we might as well get things in order before tackling that issue.