fix listen addrs race

Copy the listen address list before returning it.

If you're wondering about the syntax:
https://github.com/go101/go101/wiki/How-to-perfectly-clone-a-slice%3F

swarm_addr.go

@@ -35,8 +35,8 @@ func (s *Swarm) InterfaceListenAddresses() ([]ma.Multiaddr, error) {
 	s.listeners.RUnlock() // RLock end
 
 	if ifaceListenAddres != nil && !isEOL {
-		// Cache is valid
-		return ifaceListenAddres, nil
+		// Cache is valid, clone the slice
+		return append(ifaceListenAddres[:0:0], ifaceListenAddres...), nil
 	}
 
 	// Cache is not valid
@@ -64,5 +64,5 @@ func (s *Swarm) InterfaceListenAddresses() ([]ma.Multiaddr, error) {
 
 	s.listeners.Unlock() // Lock end
 
-	return ifaceListenAddres, nil
+	return append(ifaceListenAddres[:0:0], ifaceListenAddres...), nil
 }

swarm_addr_test.go

@@ -35,3 +35,21 @@ func TestDialBadAddrs(t *testing.T) {
 	test(m("/ip6/fe80::100"))              // link local
 	test(m("/ip4/127.0.0.1/udp/1234/utp")) // utp
 }
+
+func TestAddrRace(t *testing.T) {
+	ctx := context.Background()
+	s := makeSwarms(ctx, t, 1)[0]
+
+	a1, err := s.InterfaceListenAddresses()
+	if err != nil {
+		t.Fatal(err)
+	}
+	a2, err := s.InterfaceListenAddresses()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(a1) > 0 && len(a2) > 0 && &a1[0] == &a2[0] {
+		t.Fatal("got the exact same address set twice; this could lead to data races")
+	}
+}