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

@@ -53,34 +53,38 @@ func testStep(
 			"Error refreshing: %s", err)
 	}
 
-	// Plan!
-	if p, err := ctx.Plan(); err != nil {
-		return state, fmt.Errorf(
-			"Error planning: %s", err)
-	} else {
-		log.Printf("[WARN] Test: Step plan: %s", p)
-	}
-
-	// We need to keep a copy of the state prior to destroying
-	// such that destroy steps can verify their behaviour in the check
-	// function
-	stateBeforeApplication := state.DeepCopy()
-
-	// Apply!
-	state, err = ctx.Apply()
-	if err != nil {
-		return state, fmt.Errorf("Error applying: %s", err)
-	}
-
-	// Check! Excitement!
-	if step.Check != nil {
-		if step.Destroy {
-			if err := step.Check(stateBeforeApplication); err != nil {
-				return state, fmt.Errorf("Check failed: %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(
+				"Error planning: %s", err)
 		} else {
-			if err := step.Check(state); err != nil {
-				return state, fmt.Errorf("Check failed: %s", err)
+			log.Printf("[WARN] Test: Step plan: %s", p)
+		}
+
+		// We need to keep a copy of the state prior to destroying
+		// such that destroy steps can verify their behaviour in the check
+		// function
+		stateBeforeApplication := state.DeepCopy()
+
+		// Apply!
+		state, err = ctx.Apply()
+		if err != nil {
+			return state, fmt.Errorf("Error applying: %s", err)
+		}
+
+		// Check! Excitement!
+		if step.Check != nil {
+			if step.Destroy {
+				if err := step.Check(stateBeforeApplication); err != nil {
+					return state, fmt.Errorf("Check failed: %s", err)
+				}
+			} else {
+				if err := step.Check(state); err != nil {
+					return state, fmt.Errorf("Check failed: %s", err)
+				}
 			}
 		}
 	}

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