fixed permadiff when subnet is optioanl

.changelog/5369.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+compute: fixed a permadiff on `subnetwork` when it is optional on `google_compute_network_endpoint_group`
+```

google-beta/common_diff_suppress.go

@@ -200,3 +200,13 @@ func compareIpAddressOrSelfLinkOrResourceName(_, old, new string, _ *schema.Reso
 func alwaysDiffSuppress(_, _, _ string, _ *schema.ResourceData) bool {
 	return true
 }
+
+// Use this method when subnet is optioanl and auto_create_subnetworks = true
+// API sometimes choose a subnet so the diff needs to be ignored
+func compareOptionalSubnet(_, old, new string, _ *schema.ResourceData) bool {
+	if isEmptyValue(reflect.ValueOf(new)) {
+		return true
+	}
+	// otherwise compare as self links
+	return compareSelfLinkOrResourceName("", old, new, nil)
+}

google-beta/resource_compute_network_endpoint_group.go

@@ -88,7 +88,7 @@ you create the resource.`,
 				Type:             schema.TypeString,
 				Optional:         true,
 				ForceNew:         true,
-				DiffSuppressFunc: compareSelfLinkOrResourceName,
+				DiffSuppressFunc: compareOptionalSubnet,
 				Description:      `Optional subnetwork to which all network endpoints in the NEG belong.`,
 			},
 			"zone": {

google-beta/resource_compute_network_endpoint_group_test.go

@@ -0,0 +1,48 @@
+package google
+
+import (
+	"testing"
+
+	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/resource"
+)
+
+func TestAccComputeNetworkEndpointGroup_networkEndpointGroup(t *testing.T) {
+	t.Parallel()
+
+	context := map[string]interface{}{
+		"random_suffix": randString(t, 10),
+	}
+
+	vcrTest(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckComputeNetworkEndpointGroupDestroyProducer(t),
+		Steps: []resource.TestStep{
+			{
+				Config: testAccComputeNetworkEndpointGroup_networkEndpointGroup(context),
+			},
+			{
+				ResourceName:            "google_compute_network_endpoint_group.neg",
+				ImportState:             true,
+				ImportStateVerify:       true,
+				ImportStateVerifyIgnore: []string{"network", "subnetwork", "zone"},
+			},
+		},
+	})
+}
+
+func testAccComputeNetworkEndpointGroup_networkEndpointGroup(context map[string]interface{}) string {
+	return Nprintf(`
+resource "google_compute_network_endpoint_group" "neg" {
+  name         = "tf-test-my-lb-neg%{random_suffix}"
+  network      = google_compute_network.default.id
+  default_port = "90"
+  zone         = "us-central1-a"
+}
+
+resource "google_compute_network" "default" {
+  name                    = "tf-test-neg-network%{random_suffix}"
+  auto_create_subnetworks = true
+}
+`, context)
+}

google-beta/resource_gke_hub_feature_membership_test.go

@@ -5,7 +5,7 @@ import (
 	"fmt"
 	"testing"
 
-	dcl "github.com/GoogleCloudPlatform/declarative-resource-client-library/dcl"
+	"github.com/GoogleCloudPlatform/declarative-resource-client-library/dcl"
 	gkehub "github.com/GoogleCloudPlatform/declarative-resource-client-library/services/google/gkehub/beta"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/resource"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/terraform"