nil as unspecified, test for invalid channels

third_party/terraform/resources/resource_container_cluster.go.erb

@@ -735,9 +735,11 @@ func resourceContainerCluster() *schema.Resource {
 					Schema: map[string]*schema.Schema{
 						"channel": {
 							Type:             schema.TypeString,
+							Default:          "UNSPECIFIED",
 							Optional:         true,
 							ForceNew:         true,
 							ValidateFunc:     validation.StringInSlice([]string{"UNSPECIFIED", "RAPID", "REGULAR", "STABLE"}, false),
+							DiffSuppressFunc: emptyOrDefaultStringSuppress("UNSPECIFIED"),
 						},
 					},
 				},
@@ -2607,6 +2609,11 @@ func flattenReleaseChannel(c *containerBeta.ReleaseChannel) []map[string]interfa
 		result = append(result, map[string]interface{}{
 			"channel": c.Channel,
 		})
+	} else {
+		// Explicitly set the release channel to the default.
+		result = append(result, map[string]interface{}{
+			"channel": "UNSPECIFIED",
+		})
 	}
 	return result
 }

third_party/terraform/tests/resource_container_cluster_test.go.erb

@@ -410,6 +410,27 @@ func TestAccContainerCluster_withReleaseChannelEnabled(t *testing.T) {
 	})
 }
 
+func TestAccContainerCluster_withReleaseChannelDefault(t *testing.T) {
+	t.Parallel()
+	clusterName := fmt.Sprintf("cluster-test-%s", acctest.RandString(10))
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckContainerClusterDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccContainerCluster_withReleaseChannelDefault(clusterName),
+			},
+			{
+				ResourceName:        "google_container_cluster.with_default_release_channel",
+				ImportStateIdPrefix: "us-central1-a/",
+				ImportState:         true,
+				ImportStateVerify:   true,
+			},
+		},
+	})
+}
+
 func TestAccContainerCluster_withInvalidReleaseChannel(t *testing.T) {
 	t.Parallel()
 	clusterName := fmt.Sprintf("cluster-test-%s", acctest.RandString(10))
@@ -2049,6 +2070,16 @@ resource "google_container_cluster" "with_release_channel" {
   }
 }`, clusterName, channel)
 }
+
+func testAccContainerCluster_withReleaseChannelDefault(clusterName string) string {
+	return fmt.Sprintf(`
+resource "google_container_cluster" "with_default_release_channel" {
+	name               = "%s"
+	location           = "us-central1-a"
+	initial_node_count = 1
+	release_channel {}
+}`, clusterName)
+}
 <% end -%>
 
 func testAccContainerCluster_removeNetworkPolicy(clusterName string) string {