network: disallow double connections to the same peer

Makes no sense and C# node does it too.

pkg/network/server.go

@@ -26,6 +26,7 @@ const (
 )
 
 var (
+	errAlreadyConnected = errors.New("already connected")
 	errIdenticalID      = errors.New("identical node id")
 	errInvalidHandshake = errors.New("invalid handshake")
 	errInvalidNetwork   = errors.New("invalid network")
@@ -272,6 +273,16 @@ func (s *Server) handleVersionCmd(p Peer, version *payload.Version) error {
 	if s.id == version.Nonce {
 		return errIdenticalID
 	}
+	peerAddr := p.PeerAddr().String()
+	s.lock.RLock()
+	for peer := range s.peers {
+		// Already connected, drop this connection.
+		if peer.Handshaked() && peer.PeerAddr().String() == peerAddr && peer.Version().Nonce == version.Nonce {
+			s.lock.RUnlock()
+			return errAlreadyConnected
+		}
+	}
+	s.lock.RUnlock()
 	return p.SendVersionAck(NewMessage(s.Net, CMDVerack, nil))
 }
 

pkg/network/server_test.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/CityOfZion/neo-go/pkg/network/payload"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestSendVersion(t *testing.T) {
@@ -57,14 +58,16 @@ func TestVerackAfterHandleVersionCmd(t *testing.T) {
 // invalid version and disconnects the peer.
 func TestServerNotSendsVerack(t *testing.T) {
 	var (
-		s = newTestServer()
-		p = newLocalPeer(t)
+		s  = newTestServer()
+		p  = newLocalPeer(t)
+		p2 = newLocalPeer(t)
 	)
 	s.id = 1
 	go s.run()
 
 	na, _ := net.ResolveTCPAddr("tcp", "0.0.0.0:3000")
 	p.netaddr = *na
+	p2.netaddr = *na
 	s.register <- p
 
 	// identical id's
@@ -72,6 +75,18 @@ func TestServerNotSendsVerack(t *testing.T) {
 	err := s.handleVersionCmd(p, version)
 	assert.NotNil(t, err)
 	assert.Equal(t, errIdenticalID, err)
+
+	// Different IDs, make handshake pass.
+	version.Nonce = 2
+	require.NoError(t, s.handleVersionCmd(p, version))
+	require.NoError(t, p.HandleVersionAck())
+	require.Equal(t, true, p.Handshaked())
+
+	// Second handshake from the same peer should fail.
+	s.register <- p2
+	err = s.handleVersionCmd(p2, version)
+	assert.NotNil(t, err)
+	require.Equal(t, errAlreadyConnected, err)
 }
 
 func TestRequestHeaders(t *testing.T) {