terraform: test case for if diff returns nil

terraform/terraform.go

@@ -154,7 +154,7 @@ func (t *Terraform) diffWalkFn(
 		}
 
 		// If there were no diff items, return right away
-		if len(diff.Attributes) == 0 {
+		if diff == nil || len(diff.Attributes) == 0 {
 			return nil
 		}
 

terraform/terraform_test.go

@@ -202,6 +202,18 @@ func TestTerraformDiff(t *testing.T) {
 	}
 }
 
+func TestTerraformDiff_nil(t *testing.T) {
+	tf := testTerraform(t, "diff-nil")
+
+	diff, err := tf.Diff(nil)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	if len(diff.Resources) != 0 {
+		t.Fatalf("bad: %#v", diff.Resources)
+	}
+}
+
 func TestTerraformDiff_computed(t *testing.T) {
 	tf := testTerraform(t, "diff-computed")
 
@@ -269,6 +281,10 @@ func testProviderFunc(n string, rs []string) ResourceProviderFactory {
 					continue
 				}
 
+				if k == "nil" {
+					return nil, nil
+				}
+
 				if k == "compute" {
 					diff.Attributes[v.(string)] = &ResourceAttrDiff{
 						Old:         "",

terraform/test-fixtures/diff-nil/main.tf

@@ -0,0 +1,3 @@
+resource "aws_instance" "foo" {
+    nil = "1"
+}