skip guest accelerators if count is 0. (#866)

* skip guest accelerators if count is 0.

Instances in instance groups in google will fail to provision, despite
requesting 0 GPUs. This came up for me when trying to provision
a similar instance group in all available regions, but only asking for
GPU's in those that support them by parameterizing the `count` and
setting it to 0.

This might be a violation of some terraform principles. For example,
testing locally with this change `terraform` did not recognize that
indeed my infra needed to be re-deployed (from it's pov, I assume it
believes this because inputs hadn't changed). Additionally, there may be
valid reasons for creating an instance template with 0 gpu's that can be
tuned upwards.

* Add guest accelerator skip test for instances.

* do not leave empty pointers to guest accelerators.

* attempt to clear guest accelerator diff

* conditionally customize diff for guest accels

google/resource_compute_instance.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"github.com/hashicorp/errwrap"
+	"github.com/hashicorp/terraform/helper/customdiff"
 	"github.com/hashicorp/terraform/helper/schema"
 	"github.com/hashicorp/terraform/helper/validation"
 	"github.com/mitchellh/hashstructure"
@@ -496,6 +497,7 @@ func resourceComputeInstance() *schema.Resource {
 			"guest_accelerator": &schema.Schema{
 				Type:     schema.TypeList,
 				Optional: true,
+				Computed: true,
 				ForceNew: true,
 				Elem: &schema.Resource{
 					Schema: map[string]*schema.Schema{
@@ -556,6 +558,14 @@ func resourceComputeInstance() *schema.Resource {
 				Deprecated: "Use timeouts block instead.",
 			},
 		},
+		CustomizeDiff: customdiff.All(
+			customdiff.If(
+				func(d *schema.ResourceDiff, meta interface{}) bool {
+					return d.HasChange("guest_accelerator")
+				},
+				suppressEmptyGuestAcceleratorDiff,
+			),
+		),
 	}
 }
 
@@ -1216,22 +1226,67 @@ func expandInstanceGuestAccelerators(d TerraformResourceData, config *Config) ([
 		return nil, nil
 	}
 	accels := configs.([]interface{})
-	guestAccelerators := make([]*computeBeta.AcceleratorConfig, len(accels))
-	for i, raw := range accels {
+	guestAccelerators := make([]*computeBeta.AcceleratorConfig, 0, len(accels))
+	for _, raw := range accels {
 		data := raw.(map[string]interface{})
+		if data["count"].(int) == 0 {
+			continue
+		}
 		at, err := ParseAcceleratorFieldValue(data["type"].(string), d, config)
 		if err != nil {
 			return nil, fmt.Errorf("cannot parse accelerator type: %v", err)
 		}
-		guestAccelerators[i] = &computeBeta.AcceleratorConfig{
+		guestAccelerators = append(guestAccelerators, &computeBeta.AcceleratorConfig{
 			AcceleratorCount: int64(data["count"].(int)),
 			AcceleratorType:  at.RelativeLink(),
-		}
+		})
 	}
 
 	return guestAccelerators, nil
 }
 
+// suppressEmptyGuestAcceleratorDiff is used to work around perpetual diff
+// issues when a count of `0` guest accelerators is desired. This may occur when
+// guest_accelerator support is controlled via a module variable. E.g.:
+//
+// 		guest_accelerators {
+//      	count = "${var.enable_gpu ? var.gpu_count : 0}"
+//          ...
+// 		}
+// After reconciling the desired and actual state, we would otherwise see a
+// perpetual resembling:
+// 		[] != [{"count":0, "type": "nvidia-tesla-k80"}]
+func suppressEmptyGuestAcceleratorDiff(d *schema.ResourceDiff, meta interface{}) error {
+	oldi, newi := d.GetChange("guest_accelerator")
+
+	old, ok := oldi.([]interface{})
+	if !ok {
+		return fmt.Errorf("Expected old guest accelerator diff to be a slice")
+	}
+
+	new, ok := newi.([]interface{})
+	if !ok {
+		return fmt.Errorf("Expected new guest accelerator diff to be a slice")
+	}
+
+	if len(old) != 0 && len(new) != 1 {
+		return nil
+	}
+
+	firstAccel, ok := new[0].(map[string]interface{})
+	if !ok {
+		return fmt.Errorf("Unable to type assert guest accelerator")
+	}
+
+	if firstAccel["count"].(int) == 0 {
+		if err := d.Clear("guest_accelerator"); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
 func resourceComputeInstanceDelete(d *schema.ResourceData, meta interface{}) error {
 	config := meta.(*Config)
 

google/resource_compute_instance_template.go

@@ -502,15 +502,18 @@ func expandInstanceTemplateGuestAccelerators(d TerraformResourceData, config *Co
 		return nil
 	}
 	accels := configs.([]interface{})
-	guestAccelerators := make([]*computeBeta.AcceleratorConfig, len(accels))
-	for i, raw := range accels {
+	guestAccelerators := make([]*computeBeta.AcceleratorConfig, 0, len(accels))
+	for _, raw := range accels {
 		data := raw.(map[string]interface{})
-		guestAccelerators[i] = &computeBeta.AcceleratorConfig{
+		if data["count"].(int) == 0 {
+			continue
+		}
+		guestAccelerators = append(guestAccelerators, &computeBeta.AcceleratorConfig{
 			AcceleratorCount: int64(data["count"].(int)),
 			// We can't use ParseAcceleratorFieldValue here because an instance
 			// template does not have a zone we can use.
 			AcceleratorType: data["type"].(string),
-		}
+		})
 	}
 
 	return guestAccelerators

google/resource_compute_instance_template_test.go

@@ -351,7 +351,7 @@ func TestAccComputeInstanceTemplate_guestAccelerator(t *testing.T) {
 		CheckDestroy: testAccCheckComputeInstanceTemplateDestroy,
 		Steps: []resource.TestStep{
 			resource.TestStep{
-				Config: testAccComputeInstanceTemplate_guestAccelerator(acctest.RandString(10)),
+				Config: testAccComputeInstanceTemplate_guestAccelerator(acctest.RandString(10), 1),
 				Check: resource.ComposeTestCheckFunc(
 					testAccCheckComputeInstanceTemplateExists("google_compute_instance_template.foobar", &instanceTemplate),
 					testAccCheckComputeInstanceTemplateHasGuestAccelerator(&instanceTemplate, "nvidia-tesla-k80", 1),
@@ -367,6 +367,28 @@ func TestAccComputeInstanceTemplate_guestAccelerator(t *testing.T) {
 
 }
 
+func TestAccComputeInstanceTemplate_guestAcceleratorSkip(t *testing.T) {
+	t.Parallel()
+
+	var instanceTemplate compute.InstanceTemplate
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckComputeInstanceTemplateDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: testAccComputeInstanceTemplate_guestAccelerator(acctest.RandString(10), 0),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckComputeInstanceTemplateExists("google_compute_instance_template.foobar", &instanceTemplate),
+					testAccCheckComputeInstanceTemplateLacksGuestAccelerator(&instanceTemplate),
+				),
+			},
+		},
+	})
+
+}
+
 func TestAccComputeInstanceTemplate_minCpuPlatform(t *testing.T) {
 	t.Parallel()
 
@@ -664,6 +686,16 @@ func testAccCheckComputeInstanceTemplateHasGuestAccelerator(instanceTemplate *co
 	}
 }
 
+func testAccCheckComputeInstanceTemplateLacksGuestAccelerator(instanceTemplate *compute.InstanceTemplate) resource.TestCheckFunc {
+	return func(s *terraform.State) error {
+		if len(instanceTemplate.Properties.GuestAccelerators) > 0 {
+			return fmt.Errorf("Expected no guest accelerators")
+		}
+
+		return nil
+	}
+}
+
 func testAccCheckComputeInstanceTemplateHasMinCpuPlatform(instanceTemplate *compute.InstanceTemplate, minCpuPlatform string) resource.TestCheckFunc {
 	return func(s *terraform.State) error {
 		if instanceTemplate.Properties.MinCpuPlatform != minCpuPlatform {
@@ -1087,7 +1119,7 @@ resource "google_compute_instance_template" "foobar" {
 }`, i, i, i)
 }
 
-func testAccComputeInstanceTemplate_guestAccelerator(i string) string {
+func testAccComputeInstanceTemplate_guestAccelerator(i string, count uint8) string {
 	return fmt.Sprintf(`
 resource "google_compute_instance_template" "foobar" {
 	name = "instance-test-%s"
@@ -1110,10 +1142,10 @@ resource "google_compute_instance_template" "foobar" {
 	}
 
 	guest_accelerator {
-		count = 1
+		count = %d
 		type = "nvidia-tesla-k80"
 	}
-}`, i)
+}`, i, count)
 }
 
 func testAccComputeInstanceTemplate_minCpuPlatform(i string) string {

google/resource_compute_instance_test.go

@@ -801,7 +801,7 @@ func TestAccComputeInstance_guestAccelerator(t *testing.T) {
 		CheckDestroy: testAccCheckComputeInstanceDestroy,
 		Steps: []resource.TestStep{
 			resource.TestStep{
-				Config: testAccComputeInstance_guestAccelerator(instanceName),
+				Config: testAccComputeInstance_guestAccelerator(instanceName, 1),
 				Check: resource.ComposeTestCheckFunc(
 					testAccCheckComputeInstanceExists("google_compute_instance.foobar", &instance),
 					testAccCheckComputeInstanceHasGuestAccelerator(&instance, "nvidia-tesla-k80", 1),
@@ -817,6 +817,29 @@ func TestAccComputeInstance_guestAccelerator(t *testing.T) {
 
 }
 
+func TestAccComputeInstance_guestAcceleratorSkip(t *testing.T) {
+	t.Parallel()
+
+	var instance compute.Instance
+	instanceName := fmt.Sprintf("terraform-test-%s", acctest.RandString(10))
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckComputeInstanceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: testAccComputeInstance_guestAccelerator(instanceName, 0),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckComputeInstanceExists("google_compute_instance.foobar", &instance),
+					testAccCheckComputeInstanceLacksGuestAccelerator(&instance),
+				),
+			},
+		},
+	})
+
+}
+
 func TestAccComputeInstance_minCpuPlatform(t *testing.T) {
 	t.Parallel()
 
@@ -1298,6 +1321,16 @@ func testAccCheckComputeInstanceHasGuestAccelerator(instance *compute.Instance,
 	}
 }
 
+func testAccCheckComputeInstanceLacksGuestAccelerator(instance *compute.Instance) resource.TestCheckFunc {
+	return func(s *terraform.State) error {
+		if len(instance.GuestAccelerators) > 0 {
+			return fmt.Errorf("Expected no guest accelerators")
+		}
+
+		return nil
+	}
+}
+
 func testAccCheckComputeInstanceHasMinCpuPlatform(instance *compute.Instance, minCpuPlatform string) resource.TestCheckFunc {
 	return func(s *terraform.State) error {
 		if instance.MinCpuPlatform != minCpuPlatform {
@@ -2282,7 +2315,7 @@ resource "google_compute_subnetwork" "inst-test-subnetwork" {
 `, instance, network, subnetwork)
 }
 
-func testAccComputeInstance_guestAccelerator(instance string) string {
+func testAccComputeInstance_guestAccelerator(instance string, count uint8) string {
 	return fmt.Sprintf(`
 resource "google_compute_instance" "foobar" {
   name = "%s"
@@ -2305,10 +2338,10 @@ resource "google_compute_instance" "foobar" {
   }
 
   guest_accelerator {
-    count = 1
+    count = %d
     type = "nvidia-tesla-k80"
   }
-}`, instance)
+}`, instance, count)
 }
 
 func testAccComputeInstance_minCpuPlatform(instance string) string {