Fix read fsm bug

Author: theodorsm

conn.go

@@ -287,7 +287,7 @@ func (c *Conn) HandshakeContext(ctx context.Context) error {
 	c.closeLock.Unlock()
 
 	if c.handshakeConfig13 != nil {
-		initialFlight := flight13_0
+		initialFlight := flight13_1
 		initialFSMState := handshakePreparing13
 
 		if err := c.handshake13(ctx, c.handshakeConfig13, initialFlight, initialFSMState); err != nil {
@@ -803,13 +803,22 @@ func (c *Conn) readAndBuffer(ctx context.Context) error { //nolint:cyclop
 			done:         make(chan struct{}),
 			isRetransmit: isRetransmit,
 		}
-		select {
-		case c.handshakeRecv <- s:
-			// If the other party may retransmit the flight,
-			// we should respond even if it not a new message.
-			<-s.done
-		case <-c.fsm.Done():
-		case <-c.fsm13.Done():
+		if c.fsm13 != nil {
+			select {
+			case c.handshakeRecv <- s:
+				// If the other party may retransmit the flight,
+				// we should respond even if it not a new message.
+				<-s.done
+			case <-c.fsm13.Done():
+			}
+		} else {
+			select {
+			case c.handshakeRecv <- s:
+				// If the other party may retransmit the flight,
+				// we should respond even if it not a new message.
+				<-s.done
+			case <-c.fsm.Done():
+			}
 		}
 	}
 

flight_13.go

@@ -6,6 +6,7 @@ package dtls
 type flightVal13 uint8
 
 /*
+// [RFC9147 Section-5.7]
 
 Full DTLS Handshake (with Cookie Exchange):