compute: Implement graceful switch for metadata_startup_script

This patch checks whether a graceful switch (without ForceNew)
is available between `metadata_startup_script` and `metadata.startup-script`.
Graceful switch can be executed in two situations:
1. When `metadata_startup_script` is created with the old
value of `metadata.startup-script`.
2. When `metadata_startup_script` is deleted and the old value remains
in `metadata.startup-script`

For all other changes in `metadata_startup_script`, function sets ForceNew.
Closes: hashicorp/terraform-provider-google#9459

Signed-off-by: Norbert Kamiński <[email protected]>

Author: Asiderr

mmv1/third_party/terraform/services/compute/resource_compute_instance.go.tmpl

@@ -816,7 +816,6 @@ func ResourceComputeInstance() *schema.Resource {
 			"metadata_startup_script": {
 				Type:        schema.TypeString,
 				Optional:    true,
-				ForceNew:    true,
 				Description: `Metadata startup scripts made available within the instance.`,
 			},
 
@@ -1316,6 +1315,9 @@ be from 0 to 999,999,999 inclusive.`,
 				},
 				suppressEmptyGuestAcceleratorDiff,
 			),
+			customdiff.ForceNewIf("metadata_startup_script", func(_ context.Context, d *schema.ResourceDiff, meta interface{}) bool {
+				return isGracefulMetadataStartupSwitch(d)
+			}),
 			validateSubnetworkProject,
 			forceNewIfNetworkIPNotUpdatable,
 			tpgresource.SetLabelsDiff,
@@ -2978,6 +2980,52 @@ func suppressEmptyGuestAcceleratorDiff(_ context.Context, d *schema.ResourceDiff
 	return nil
 }
 
+// Function checks whether a graceful switch (without ForceNew) is available
+// between `metadata_startup_script` and `metadata.startup-script`.
+// Graceful switch can be executed in two situations:
+// 1. When `metadata_startup_script` is created with the old value of
+// `metadata.startup-script`.
+// 2. When `metadata_startup_script` is deleted and the old value remains in
+// `metadata.startup-script`
+// For all other changes in `metadata_startup_script`, function sets ForceNew.
+func isGracefulMetadataStartupSwitch(d *schema.ResourceDiff) bool {
+	oldMd, newMd := d.GetChange("metadata")
+	oldMdMap := oldMd.(map[string]interface{})
+	newMdMap := newMd.(map[string]interface{})
+
+	//No new and old metadata
+	if len(oldMdMap) == 0 && len(newMdMap) == 0 {
+		return true
+	}
+
+	oldMds, newMds := d.GetChange("metadata_startup_script")
+	vMdOld, okOld := oldMdMap["startup-script"]
+	vMdNew, okNew := newMdMap["startup-script"]
+
+	// metadata_startup_script is created
+	if oldMds == "" {
+		if !okOld {
+			return true
+		} else if newMds == vMdOld {
+			return false
+		} else {
+			return true
+		}
+	}
+	// metadata_startup_script is deleted
+	if newMds == "" {
+		if !okNew {
+			return true
+		} else if oldMds == vMdNew {
+			return false
+		} else {
+			return true
+		}
+	}
+
+	return true
+}
+
 func resourceComputeInstanceDelete(d *schema.ResourceData, meta interface{}) error {
 	config := meta.(*transport_tpg.Config)
 	userAgent, err :=  tpgresource.GenerateUserAgentString(d, config.UserAgent)