testing: Add option to run only a plan on a TestStep configuration

helper/resource/testing.go

@@ -151,6 +151,11 @@ type TestStep struct {
 	// test to pass.
 	ExpectError *regexp.Regexp
 
+	// PlanOnly can be set to only run `plan` with this configuration, and not
+	// actually apply it. This is useful for ensuring config changes result in
+	// no-op plans
+	PlanOnly bool
+
 	// PreventPostDestroyRefresh can be set to true for cases where data sources
 	// are tested alongside real resources
 	PreventPostDestroyRefresh bool

helper/resource/testing_config.go

@@ -53,6 +53,9 @@ func testStep(
 			"Error refreshing: %s", err)
 	}
 
+	// If this step is a PlanOnly step, skip over this first Plan and subsequent
+	// Apply, and use the follow up Plan that checks for perpetual diffs
+	if !step.PlanOnly {
 		// Plan!
 		if p, err := ctx.Plan(); err != nil {
 			return state, fmt.Errorf(
@@ -84,6 +87,7 @@ func testStep(
 				}
 			}
 		}
+	}
 
 	// Now, verify that Plan is now empty and we don't have a perpetual diff issue
 	// We do this with TWO plans. One without a refresh.

helper/resource/testing_test.go

@@ -120,6 +120,58 @@ func TestTest(t *testing.T) {
 	}
 }
 
+func TestTest_plan_only(t *testing.T) {
+	mp := testProvider()
+	mp.ApplyReturn = &terraform.InstanceState{
+		ID: "foo",
+	}
+
+	checkDestroy := false
+
+	checkDestroyFn := func(*terraform.State) error {
+		checkDestroy = true
+		return nil
+	}
+
+	mt := new(mockT)
+	Test(mt, TestCase{
+		Providers: map[string]terraform.ResourceProvider{
+			"test": mp,
+		},
+		CheckDestroy: checkDestroyFn,
+		Steps: []TestStep{
+			TestStep{
+				Config:             testConfigStr,
+				PlanOnly:           true,
+				ExpectNonEmptyPlan: false,
+			},
+		},
+	})
+
+	if !mt.failed() {
+		t.Fatal("test should've failed")
+	}
+
+	expected := `Step 0 error: After applying this step, the plan was not empty:
+
+DIFF:
+
+CREATE: test_instance.foo
+  foo: "" => "bar"
+
+STATE:
+
+<no state>`
+
+	if mt.failMessage() != expected {
+		t.Fatalf("Expected message: %s\n\ngot:\n\n%s", expected, mt.failMessage())
+	}
+
+	if !checkDestroy {
+		t.Fatal("didn't call check for destroy")
+	}
+}
+
 func TestTest_idRefresh(t *testing.T) {
 	// Refresh count should be 3:
 	//   1.) initial Ref/Plan/Apply