Use random port when no port specified with -http (#316)

* use random port when no port specified

* remove debugging print statement

internal/driver/webui.go

 }
 
 func serveWebInterface(hostport string, p *profile.Profile, o *plugin.Options, wantBrowser bool) error {
-	host, portStr, err := net.SplitHostPort(hostport)
-	if err != nil {
-		return fmt.Errorf("could not split http address: %v", err)
-	}
-	port, err := strconv.Atoi(portStr)
+	host, port, err := getHostAndPort(hostport)
 	if err != nil {
-		return fmt.Errorf("invalid port number: %v", err)
-	}
-	if host == "" {
-		host = "localhost"
+		return err
 	}
-
 	interactiveMode = true
 	ui := makeWebInterface(p, o)
 	for n, c := range pprofCommands {
 		server = defaultWebServer
 	}
 	args := &plugin.HTTPServerArgs{
-		Hostport: net.JoinHostPort(host, portStr),
+		Hostport: net.JoinHostPort(host, strconv.Itoa(port)),
 		Host:     host,
 		Port:     port,
 		Handlers: map[string]http.Handler{
 	return server(args)
 }
 
+func getHostAndPort(hostport string) (string, int, error) {
+	host, portStr, err := net.SplitHostPort(hostport)
+	if err != nil {
+		return "", 0, fmt.Errorf("could not split http address: %v", err)
+	}
+	if host == "" {
+		host = "localhost"
+	}
+	var port int
+	if portStr == "" {
+		ln, err := net.Listen("tcp", net.JoinHostPort(host, "0"))
+		if err != nil {
+			return "", 0, fmt.Errorf("could not generate random port: %v", err)
+		}
+		port = ln.Addr().(*net.TCPAddr).Port
+		err = ln.Close()
+		if err != nil {
+			return "", 0, fmt.Errorf("could not generate random port: %v", err)
+		}
+	} else {
+		port, err = strconv.Atoi(portStr)
+		if err != nil {
+			return "", 0, fmt.Errorf("invalid port number: %v", err)
+		}
+	}
+	return host, port, nil
+}
 func defaultWebServer(args *plugin.HTTPServerArgs) error {
 	ln, err := net.Listen("tcp", args.Hostport)
 	if err != nil {

internal/driver/webui_test.go

 	}
 }
 
+func TestGetHostAndPort(t *testing.T) {
+	if runtime.GOOS == "nacl" {
+		t.Skip("test assumes tcp available")
+	}
+
+	type testCase struct {
+		hostport       string
+		wantHost       string
+		wantPort       int
+		wantRandomPort bool
+	}
+
+	testCases := []testCase{
+		{":", "localhost", 0, true},
+		{":4681", "localhost", 4681, false},
+		{"localhost:4681", "localhost", 4681, false},
+	}
+	for _, tc := range testCases {
+		host, port, err := getHostAndPort(tc.hostport)
+		if err != nil {
+			t.Errorf("could not get host and port for %q: %v", tc.hostport, err)
+		}
+		if got, want := host, tc.wantHost; got != want {
+			t.Errorf("for %s, got host %s, want %s", tc.hostport, got, want)
+			continue
+		}
+		if !tc.wantRandomPort {
+			if got, want := port, tc.wantPort; got != want {
+				t.Errorf("for %s, got port %d, want %d", tc.hostport, got, want)
+				continue
+			}
+		}
+	}
+}
+
 func TestIsLocalHost(t *testing.T) {
 	for _, s := range []string{"localhost:10000", "[::1]:10000", "127.0.0.1:10000"} {
 		host, _, err := net.SplitHostPort(s)