functional: add a new test TestUnitStartReplace

TestUnitStartReplace() tests whether a command "fleetctl start
--replace hello.service" works or not.

functional/unit_action_test.go

@@ -326,6 +326,82 @@ func TestUnitLoadReplace(t *testing.T) {
 	}
 }
 
+// TestUnitStartReplace() tests whether a command "fleetctl start --replace
+// hello.service" works or not.
+func TestUnitStartReplace(t *testing.T) {
+	cluster, err := platform.NewNspawnCluster("smoke")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cluster.Destroy()
+
+	m, err := cluster.CreateMember()
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = cluster.WaitForNMachines(m, 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// start a unit and assert it shows up
+	if _, _, err := cluster.Fleetctl(m, "start", fxtHelloService); err != nil {
+		t.Fatalf("Unable to start fleet unit: %v", err)
+	}
+	stdout, _, err := cluster.Fleetctl(m, "list-units", "--no-legend")
+	if err != nil {
+		t.Fatalf("Failed to run list-units: %v", err)
+	}
+	units := strings.Split(strings.TrimSpace(stdout), "\n")
+	if len(units) != 1 {
+		t.Fatalf("Did not find 1 unit in cluster: \n%s", stdout)
+	}
+
+	// replace the unit and assert it shows up
+	if _, _, err := cluster.Fleetctl(m, "start", fxtHelloService); err != nil {
+		t.Fatalf("Unable to start fleet unit: %v", err)
+	}
+	err = genNewFleetService(tmpHelloService, fxtHelloService, "sleep 2", "sleep 1")
+	if err != nil {
+		t.Fatalf("Failed to generate a temp fleet service: %v", err)
+	}
+	if _, _, err := cluster.Fleetctl(m, "start", "--replace", tmpHelloService); err != nil {
+		t.Fatalf("Unable to replace fleet unit: %v", err)
+	}
+	stdout, _, err = cluster.Fleetctl(m, "list-units", "--no-legend")
+	if err != nil {
+		t.Fatalf("Failed to run list-units: %v", err)
+	}
+	units = strings.Split(strings.TrimSpace(stdout), "\n")
+	if len(units) != 1 {
+		t.Fatalf("Did not find 1 unit in cluster: \n%s", stdout)
+	}
+	os.Remove(tmpHelloService)
+
+	// destroy the unit and ensure it disappears from the unit list.
+	// It could take a little time until the unit gets destroyed.
+	maxAttempts := 3
+	for {
+		if _, _, err := cluster.Fleetctl(m, "destroy", fxtHelloService); err != nil {
+			t.Fatalf("Failed to destroy unit: %v", err)
+		}
+		_, err = cluster.WaitForNActiveUnits(m, 0)
+		if err == nil {
+			break
+		} else {
+			maxAttempts--
+		}
+	}
+
+	stdout, _, err = cluster.Fleetctl(m, "list-units", "--no-legend")
+	if err != nil {
+		t.Fatalf("Failed to run list-units: %v", err)
+	}
+	if strings.TrimSpace(stdout) != "" {
+		t.Fatalf("Did not find 0 units in cluster: \n%s", stdout)
+	}
+}
+
 func TestUnitSSHActions(t *testing.T) {
 	cluster, err := platform.NewNspawnCluster("smoke")
 	if err != nil {