warn when we encounter a useless transport

swarm_transport.go

@@ -70,6 +70,10 @@ func (s *Swarm) TransportForListening(a ma.Multiaddr) transport.Transport {
 func (s *Swarm) AddTransport(t transport.Transport) error {
 	protocols := t.Protocols()
 
+	if len(protocols) == 0 {
+		return fmt.Errorf("useless transport handles no protocols: %T", t)
+	}
+
 	s.transports.Lock()
 	defer s.transports.Unlock()
 	var registered []string

transport_test.go

@@ -0,0 +1,47 @@
+package swarm_test
+
+import (
+	"context"
+	"testing"
+
+	swarmt "github.com/libp2p/go-libp2p-swarm/testing"
+
+	peer "github.com/libp2p/go-libp2p-peer"
+	transport "github.com/libp2p/go-libp2p-transport"
+	ma "github.com/multiformats/go-multiaddr"
+)
+
+type dummyTransport struct {
+	protocols []int
+	proxy     bool
+}
+
+func (dt *dummyTransport) Dial(ctx context.Context, raddr ma.Multiaddr, p peer.ID) (transport.Conn, error) {
+	panic("unimplemented")
+}
+
+func (dt *dummyTransport) CanDial(addr ma.Multiaddr) bool {
+	panic("unimplemented")
+}
+
+func (dt *dummyTransport) Listen(laddr ma.Multiaddr) (transport.Listener, error) {
+	panic("unimplemented")
+}
+
+func (dt *dummyTransport) Proxy() bool {
+	return dt.proxy
+}
+
+func (dt *dummyTransport) Protocols() []int {
+	return dt.protocols
+}
+
+func TestUselessTransport(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	swarm := swarmt.GenSwarm(t, ctx)
+	err := swarm.AddTransport(new(dummyTransport))
+	if err == nil {
+		t.Fatal("adding a transport that supports no protocols should have failed")
+	}
+}