fix metadata_startup_script

.changelog/6822.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+compute: fixed the error when `metadata` and `machine_type` are updated while `metadata_startup_script` was already provided on `google_compute_instance`
+```

google/metadata.go

@@ -142,8 +142,11 @@ func resourceInstanceMetadata(d TerraformResourceData) (*compute.Metadata, error
 	m := &compute.Metadata{}
 	mdMap := d.Get("metadata").(map[string]interface{})
 	if v, ok := d.GetOk("metadata_startup_script"); ok && v.(string) != "" {
-		if _, ok := mdMap["startup-script"]; ok {
-			return nil, errors.New("Cannot provide both metadata_startup_script and metadata.startup-script.")
+		if w, ok := mdMap["startup-script"]; ok {
+			// metadata.startup-script could be from metadata_startup_script in the first place
+			if v != w {
+				return nil, errors.New("Cannot provide both metadata_startup_script and metadata.startup-script.")
+			}
 		}
 		mdMap["startup-script"] = v
 	}

google/resource_compute_instance_test.go

@@ -2370,6 +2370,35 @@ func TestComputeInstance_networkIPCustomizedDiff(t *testing.T) {
 	}
 }
 
+func TestAccComputeInstance_metadataStartupScript_update(t *testing.T) {
+	t.Parallel()
+
+	var instance compute.Instance
+	var instanceName = fmt.Sprintf("tf-test-%s", randString(t, 10))
+
+	vcrTest(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckComputeInstanceDestroyProducer(t),
+		Steps: []resource.TestStep{
+			{
+				Config: testAccComputeInstance_metadataStartupScript(instanceName, "e2-medium", "abc"),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckComputeInstanceExists(
+						t, "google_compute_instance.foobar", &instance),
+				),
+			},
+			{
+				Config: testAccComputeInstance_metadataStartupScript(instanceName, "e2-standard-4", "xyz"),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckComputeInstanceExists(
+						t, "google_compute_instance.foobar", &instance),
+				),
+			},
+		},
+	})
+}
+
 func testAccCheckComputeInstanceUpdateMachineType(t *testing.T, n string) resource.TestCheckFunc {
 	return func(s *terraform.State) error {
 		rs, ok := s.RootModule().Resources[n]
@@ -6205,3 +6234,36 @@ resource "google_compute_instance" "foobar" {
 }
 `, instance)
 }
+
+func testAccComputeInstance_metadataStartupScript(instance, machineType, metadata string) string {
+	return fmt.Sprintf(`
+data "google_compute_image" "my_image" {
+  family  = "debian-11"
+  project = "debian-cloud"
+}
+
+resource "google_compute_instance" "foobar" {
+  name           = "%s"
+  machine_type   = "%s"
+  zone           = "us-central1-a"
+  can_ip_forward = false
+  tags           = ["foo", "bar"]
+
+  boot_disk {
+    initialize_params {
+      image = data.google_compute_image.my_image.self_link
+    }
+  }
+
+  network_interface {
+    network = "default"
+  }
+
+  metadata = {
+    foo = "%s"
+  }
+  metadata_startup_script = "echo hi > /test.txt"
+  allow_stopping_for_update = true
+}
+`, instance, machineType, metadata)
+}