verify DiffSuppresFunc behavior

Terraform used to provide empty diffs to the provider when calculating
`ignore_changes`, which would cause some DiffSuppressFunc to fail, as
can be seen in #18209.

Verify that this is no longer the case in 0.12

builtin/providers/test/resource_diff_suppress.go

@@ -1,17 +1,18 @@
 package test
 
 import (
+	"fmt"
+	"math/rand"
 	"strings"
 
 	"github.com/hashicorp/terraform/helper/schema"
 )
 
 func testResourceDiffSuppress() *schema.Resource {
 	diffSuppress := func(k, old, new string, d *schema.ResourceData) bool {
-		if strings.Contains(new, "replace") {
+		if old == "" || strings.Contains(new, "replace") {
 			return false
 		}
-
 		return true
 	}
 
@@ -55,14 +56,34 @@ func testResourceDiffSuppress() *schema.Resource {
 				ForceNew:         true,
 				DiffSuppressFunc: diffSuppress,
 			},
+
+			"node_pool": {
+				Type:     schema.TypeList,
+				Optional: true,
+				Computed: true,
+				ForceNew: true,
+				Elem: &schema.Resource{
+					Schema: map[string]*schema.Schema{
+						"name": &schema.Schema{
+							Type:     schema.TypeString,
+							Optional: true,
+							Computed: true,
+							ForceNew: true,
+						},
+					},
+				},
+			},
 		},
 	}
 }
 
 func testResourceDiffSuppressCreate(d *schema.ResourceData, meta interface{}) error {
-	d.SetId("testId")
+	d.Set("network", "modified")
+	d.Set("subnetwork", "modified")
 
-	return testResourceRead(d, meta)
+	id := fmt.Sprintf("%x", rand.Int63())
+	d.SetId(id)
+	return nil
 }
 
 func testResourceDiffSuppressRead(d *schema.ResourceData, meta interface{}) error {

builtin/providers/test/resource_diff_suppress_test.go

@@ -1,10 +1,14 @@
 package test
 
 import (
+	"errors"
 	"strings"
 	"testing"
 
+	"github.com/hashicorp/terraform/addrs"
+
 	"github.com/hashicorp/terraform/helper/resource"
+	"github.com/hashicorp/terraform/terraform"
 )
 
 func TestResourceDiffSuppress_create(t *testing.T) {
@@ -45,3 +49,78 @@ resource "test_resource_diff_suppress" "foo" {
 		},
 	})
 }
+
+func TestResourceDiffSuppress_updateIgnoreChanges(t *testing.T) {
+	// None of these steps should replace the instance
+	id := ""
+	checkFunc := func(s *terraform.State) error {
+		root := s.ModuleByPath(addrs.RootModuleInstance)
+		res := root.Resources["test_resource_diff_suppress.foo"]
+		if id != "" && res.Primary.ID != id {
+			return errors.New("expected no resource replacement")
+		}
+		id = res.Primary.ID
+		return nil
+	}
+
+	resource.UnitTest(t, resource.TestCase{
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckResourceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource_diff_suppress" "foo" {
+	val_to_upper = "foo"
+
+	network    = "foo"
+	subnetwork = "foo"
+
+	node_pool {
+	  name = "default-pool"
+	}
+	lifecycle {
+		ignore_changes = ["node_pool"]
+	}
+}
+				`),
+				Check: checkFunc,
+			},
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource_diff_suppress" "foo" {
+	val_to_upper = "foo"
+
+	network    = "ignored"
+	subnetwork = "ignored"
+
+	node_pool {
+		name = "default-pool"
+	}
+	lifecycle {
+		ignore_changes = ["node_pool"]
+	}
+}
+				`),
+				Check: checkFunc,
+			},
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource_diff_suppress" "foo" {
+	val_to_upper = "foo"
+
+	network    = "ignored"
+	subnetwork = "ignored"
+
+	node_pool {
+		name = "ignored"
+	}
+	lifecycle {
+		ignore_changes = ["node_pool"]
+	}
+}
+			`),
+				Check: checkFunc,
+			},
+		},
+	})
+}