gbn: complete handshake if client has completed it

This fixes a bug where the server would not complete the handshake if
the server restarted the handshake after sending it's SYN, which the
client then received and then completed the handshake.
This could previously happen if the server timedout when waiting for the
clients SYN response, or if the client's SYNACK was lost due to packet
loss.

gbn/gbn_server.go

@@ -84,6 +84,7 @@ func (g *GoBackNConn) serverHandshake() error { // nolint:gocyclo
 	var n uint8
 	var resent bool
 
+handshakeLoop:
 	for {
 		g.log.Debugf("Waiting for client SYN")
 		select {
@@ -111,6 +112,24 @@ func (g *GoBackNConn) serverHandshake() error { // nolint:gocyclo
 
 		switch msg.(type) {
 		case *PacketSYN:
+
+		case *PacketSYNACK, *PacketData:
+			// If we receive a SYNACK or DATA packet after we have
+			// restarted the handshake, we can be sure that the
+			// client has received our SYN and has completed the
+			// handshake. We can therefore complete the handshake
+			// ourselves.
+			if resent {
+				g.log.Tracef("Received %T after restarting "+
+					"handshake", msg)
+				g.timeoutManager.Received(msg)
+
+				break handshakeLoop
+			}
+
+			g.log.Tracef("Expected SYN, got %T", msg)
+
+			continue
 		default:
 			g.log.Tracef("Expected SYN, got %T", msg)
 			continue
@@ -169,6 +188,8 @@ func (g *GoBackNConn) serverHandshake() error { // nolint:gocyclo
 
 		switch msg.(type) {
 		case *PacketSYNACK:
+			g.log.Debugf("Received SYNACK")
+
 			// Notify the timeout manager we've received the SYNACK
 			// response from the counterparty.
 			g.timeoutManager.Received(msg)
@@ -185,8 +206,6 @@ func (g *GoBackNConn) serverHandshake() error { // nolint:gocyclo
 		break
 	}
 
-	g.log.Debugf("Received SYNACK")
-
 	// Set all variables that are dependent on the value of N that we get
 	// from the client
 	g.setN(n)