Fix dashboard port conflict message and allow starting on random port (#1189)

* fix dashboard start

Signed-off-by: Pravin Pushkar <ppushkar@microsoft.com>

* added comments

Signed-off-by: Pravin Pushkar <ppushkar@microsoft.com>

---------

Signed-off-by: Pravin Pushkar <ppushkar@microsoft.com>

cmd/dashboard.go

@@ -65,6 +65,9 @@ dapr dashboard
 # Start dashboard locally in a specified port 
 dapr dashboard -p 9999
 
+# Start dashboard locally on a random port which is free.
+dapr dashboard -p 0
+
 # Port forward to dashboard in Kubernetes 
 dapr dashboard -k 
 
@@ -99,6 +102,12 @@ dapr dashboard -k -p 0
 			os.Exit(1)
 		}
 
+		if err := utils.CheckIfPortAvailable(dashboardLocalPort); err != nil {
+			print.FailureStatusEvent(os.Stderr, "Please select a different port with %q flag: %s", "-p", err)
+			print.InfoStatusEvent(os.Stdout, "You can also use port 0 to select a random free port.")
+			os.Exit(1)
+		}
+
 		if kubernetesMode {
 			config, client, err := kubernetes.GetKubeConfigClient()
 			if err != nil {

pkg/standalone/dashboard.go

@@ -18,10 +18,19 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strconv"
+
+	"github.com/phayes/freeport"
 )
 
 // NewDashboardCmd creates the command to run dashboard.
 func NewDashboardCmd(inputInstallPath string, port int) (*exec.Cmd, error) {
+	if port == 0 {
+		freePort, err := freeport.GetFreePort()
+		if err != nil {
+			return nil, err
+		}
+		port = freePort
+	}
 	dashboardPath, err := lookupBinaryFilePath(inputInstallPath, "dashboard")
 	if err != nil {
 		return nil, err

pkg/standalone/dashboard_test.go

@@ -28,4 +28,13 @@ func TestDashboardRun(t *testing.T) {
 		assert.Equal(t, cmd.Args[1], "--port")
 		assert.Equal(t, cmd.Args[2], "9090")
 	})
+
+	t.Run("start dashboard on random free port", func(t *testing.T) {
+		cmd, err := NewDashboardCmd("", 0)
+
+		assert.NoError(t, err)
+		assert.Contains(t, cmd.Args[0], "dashboard")
+		assert.Equal(t, cmd.Args[1], "--port")
+		assert.NotEqual(t, cmd.Args[2], "0")
+	})
 }

utils/utils.go

@@ -266,6 +266,16 @@ func IsAddressLegal(address string) bool {
 	return isLegal
 }
 
+// CheckIfPortAvailable returns an error if the port is not available else returns nil.
+func CheckIfPortAvailable(port int) error {
+	ln, err := net.Listen("tcp", fmt.Sprintf(":%d", port))
+	if err != nil {
+		return err
+	}
+	ln.Close()
+	return nil
+}
+
 // GetEnv get value from environment variable.
 func GetEnv(envName string, defaultValue string) string {
 	if val, ok := os.LookupEnv(envName); ok {