step_create_ssm_tunnel: Add port availability check for LocalPortNumber

builder/amazon/common/step_create_ssm_tunnel.go

@@ -109,13 +109,17 @@ func (s *StepCreateSSMTunnel) Cleanup(state multistep.StateBag) {
 // ConfigureLocalHostPort finds an available port on the localhost that can be used for the remote tunnel.
 // Defaults to using s.LocalPortNumber if it is set.
 func (s *StepCreateSSMTunnel) ConfigureLocalHostPort(ctx context.Context) error {
+	minPortNumber, maxPortNumber := 8000, 9000
+
 	if s.LocalPortNumber != 0 {
-		return nil
+		minPortNumber = s.LocalPortNumber
+		maxPortNumber = minPortNumber
 	}
+
 	// Find an available TCP port for our HTTP server
 	l, err := net.ListenRangeConfig{
-		Min:     8000,
-		Max:     9000,
+		Min:     minPortNumber,
+		Max:     maxPortNumber,
 		Addr:    "0.0.0.0",
 		Network: "tcp",
 	}.Listen(ctx)

builder/amazon/common/step_create_ssm_tunnel_test.go

@@ -34,15 +34,27 @@ func TestStepCreateSSMTunnel_BuildTunnelInputForInstance(t *testing.T) {
 }
 
 func TestStepCreateSSMTunnel_ConfigureLocalHostPort(t *testing.T) {
-	tun := StepCreateSSMTunnel{}
-
-	ctx := context.TODO()
-	if err := tun.ConfigureLocalHostPort(ctx); err != nil {
-		t.Errorf("failed to configure a port on localhost")
+	tt := []struct {
+		Name      string
+		Step      StepCreateSSMTunnel
+		PortCheck func(int) bool
+	}{
+		{"WithLocalPortNumber", StepCreateSSMTunnel{LocalPortNumber: 9001}, func(port int) bool { return port == 9001 }},
+		{"WithNoLocalPortNumber", StepCreateSSMTunnel{}, func(port int) bool { return port >= 8000 && port <= 9000 }},
 	}
 
-	if tun.LocalPortNumber == 0 {
-		t.Errorf("failed to configure a port on localhost")
+	for _, tc := range tt {
+		tc := tc
+		t.Run(tc.Name, func(t *testing.T) {
+			step := tc.Step
+			if err := step.ConfigureLocalHostPort(context.TODO()); err != nil {
+				t.Errorf("failed to configure a port on localhost")
+			}
+
+			if !tc.PortCheck(step.LocalPortNumber) {
+				t.Errorf("failed to configure a port on localhost")
+			}
+		})
 	}
 
 }