Allow updating guest accelerator count to 0 on compute_instance (#15410)

Author: ScottSuarez

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

@@ -3376,12 +3376,23 @@ func suppressEmptyGuestAcceleratorDiff(_ context.Context, d *schema.ResourceDiff
 		return nil
 	}
 
-	firstAccel, ok := new[0].(map[string]interface{})
+	// Check if old had a non-zero count
+	if len(old) > 0 {
+		if apiAccel, ok := old[0].(map[string]interface{}); ok {
+			if oldCount, ok := apiAccel["count"].(int); ok && oldCount != 0 {
+				// Old count wasn't 0, so don't clear the diff
+				return nil
+			}
+		}
+	}
+
+	// Check new accelerator configuration
+	configAccel, ok := new[0].(map[string]interface{})
 	if !ok {
 		return fmt.Errorf("Unable to type assert guest accelerator")
 	}
 
-	if firstAccel["count"].(int) == 0 {
+	if configAccel["count"].(int) == 0 {
 		if err := d.Clear("guest_accelerator"); err != nil {
 			return err
 		}

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

@@ -2504,10 +2504,16 @@ func TestAccComputeInstance_guestAccelerator(t *testing.T) {
 					testAccCheckComputeInstanceHasGuestAccelerator(&instance, "nvidia-tesla-t4", 1),
 				),
 			},
+			{
+				Config:             testAccComputeInstance_guestAccelerator(instanceName, 0),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckComputeInstanceExists(t, "google_compute_instance.foobar", &instance),
+					testAccCheckComputeInstanceHasGuestAccelerator(&instance, "nvidia-tesla-t4", 0),
+				),
+			},
 			computeInstanceImportStep("us-east1-d", instanceName, []string{"metadata.baz", "metadata.foo"}),
 		},
 	})
-
 }
 
 func TestAccComputeInstance_guestAcceleratorSkip(t *testing.T) {
@@ -5841,6 +5847,9 @@ func testAccCheckComputeInstanceHasMultiNic(instance *compute.Instance) resource
 func testAccCheckComputeInstanceHasGuestAccelerator(instance *compute.Instance, acceleratorType string, acceleratorCount int64) resource.TestCheckFunc {
 	return func(s *terraform.State) error {
 		if len(instance.GuestAccelerators) != 1 {
+			if int(acceleratorCount) == 0 && len(instance.GuestAccelerators) == 0 {
+				return nil
+			}
 			return fmt.Errorf("Expected only one guest accelerator")
 		}