fix chat example.

* display actual port in instructions when starting server side.
* make debug mode false by default, as otherwise the same peer ID is
  generated every time because the source of randomness is constant.

chat/chat.go

@@ -129,39 +129,40 @@ func writeData(rw *bufio.ReadWriter) {
 }
 
 func main() {
-
 	sourcePort := flag.Int("sp", 0, "Source port number")
-	dest := flag.String("d", "", "Dest MultiAddr String")
-	help := flag.Bool("help", false, "Display Help")
-	debug := flag.Bool("debug", true, "Debug generated same node id on every execution.")
+	dest := flag.String("d", "", "Destination multiaddr string")
+	help := flag.Bool("help", false, "Display help")
+	debug := flag.Bool("debug", false, "Debug generates the same node ID on every execution")
 
 	flag.Parse()
 
 	if *help {
 		fmt.Printf("This program demonstrates a simple p2p chat application using libp2p\n\n")
-		fmt.Printf("Usage: Run './chat -sp <SOURCE_PORT>' where <SOURCE_PORT> can be any port number. Now run './chat -d <MULTIADDR>' where <MULTIADDR> is multiaddress of previous listener host.\n")
+		fmt.Println("Usage: Run './chat -sp <SOURCE_PORT>' where <SOURCE_PORT> can be any port number.")
+		fmt.Println("Now run './chat -d <MULTIADDR>' where <MULTIADDR> is multiaddress of previous listener host.")
 
 		os.Exit(0)
 	}
 
-	// If debug is enabled used constant random source else cryptographic randomness.
+	// If debug is enabled, use a constant random source to generate the peer ID. Only useful for debugging,
+	// off by default. Otherwise, it uses rand.Reader.
 	var r io.Reader
 	if *debug {
-		// Constant random source. This will always generate the same host ID on multiple execution.
-		// Don't do this in production code.
+		// Use the port number as the randomness source.
+		// This will always generate the same host ID on multiple executions, if the same port number is used.
+		// Never do this in production code.
 		r = mrand.New(mrand.NewSource(int64(*sourcePort)))
 	} else {
 		r = rand.Reader
 	}
 
-	// Creates a new RSA key pair for this host
+	// Creates a new RSA key pair for this host.
 	prvKey, _, err := crypto.GenerateKeyPairWithReader(crypto.RSA, 2048, r)
-
 	if err != nil {
 		panic(err)
 	}
 
-	// 0.0.0.0 will listen on any interface device
+	// 0.0.0.0 will listen on any interface device.
 	sourceMultiAddr, _ := multiaddr.NewMultiaddr(fmt.Sprintf("/ip4/0.0.0.0/tcp/%d", *sourcePort))
 
 	// libp2p.New constructs a new libp2p Host.
@@ -178,31 +179,44 @@ func main() {
 
 	if *dest == "" {
 		// Set a function as stream handler.
-		// This function  is called when a peer initiate a connection and starts a stream with this peer.
-		// Only applicable on the receiving side.
+		// This function is called when a peer connects, and starts a stream with this protocol.
+		// Only applies on the receiving side.
 		host.SetStreamHandler("/chat/1.0.0", handleStream)
 
-		fmt.Printf("Run './chat -d /ip4/127.0.0.1/tcp/%d/ipfs/%s' on another console.\n You can replace 127.0.0.1 with public IP as well.\n", *sourcePort, host.ID().Pretty())
+		// Let's get the actual TCP port from our listen multiaddr, in case we're using 0 (default; random available port).
+		var port string
+		for _, la := range host.Network().ListenAddresses() {
+			if p, err := la.ValueForProtocol(multiaddr.P_TCP); err == nil {
+				port = p
+				break
+			}
+		}
+
+		if port == "" {
+			panic("was not able to find actual local port")
+		}
+
+		fmt.Printf("Run './chat -d /ip4/127.0.0.1/tcp/%v/ipfs/%s' on another console.\n", port, host.ID().Pretty())
+		fmt.Println("You can replace 127.0.0.1 with public IP as well.")
 		fmt.Printf("\nWaiting for incoming connection\n\n")
+
 		// Hang forever
 		<-make(chan struct{})
-
 	} else {
+		fmt.Println("This node's multiaddresses:")
+		for _, la := range host.Addrs() {
+			fmt.Printf(" - %v\n", la)
+		}
+
+		fmt.Println()
 
-		// Add destination peer multiaddress in the peerstore.
+		// Add the destination's peer multiaddress in the peerstore.
 		// This will be used during connection and stream creation by libp2p.
 		peerID := addAddrToPeerstore(host, *dest)
 
-		fmt.Println("This node's multiaddress: ")
-		// IP will be 0.0.0.0 (listen on any interface) and port will be 0 (choose one for me).
-		// Although this node will not listen for any connection. It will just initiate a connect with
-		// one of its peer and use that stream to communicate.
-		fmt.Printf("%s/ipfs/%s\n", sourceMultiAddr, host.ID().Pretty())
-
-		// Start a stream with peer with peer Id: 'peerId'.
+		// Start a stream with the destination..
 		// Multiaddress of the destination peer is fetched from the peerstore using 'peerId'.
 		s, err := host.NewStream(context.Background(), peerID, "/chat/1.0.0")
-
 		if err != nil {
 			panic(err)
 		}
@@ -216,6 +230,5 @@ func main() {
 
 		// Hang forever.
 		select {}
-
 	}
 }