fix(sfn): stop replacing JsonPath.DISCARD with null (aws#24717)

Follow-up to aws#24593. The `renderJsonPath` function is subsituting a literal `null` for `JsonPath.DISCARD`, which results in the key being dropped if the value is sent across a language boundary, which effectively changes semantics.

The `JsonPath.DISCARD` value is a `Token` that ultimately resolves to `null`, and it must be preserved as such so that it is safe to exchange across languages.

Thanks to @beck3905 for reporting & diagnosing this.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-stepfunctions/lib/states/state.ts

@@ -1,7 +1,7 @@
 import { Token } from '@aws-cdk/core';
 import { IConstruct, Construct, Node } from 'constructs';
 import { Condition } from '../condition';
-import { FieldUtils, JsonPath } from '../fields';
+import { FieldUtils } from '../fields';
 import { StateGraph } from '../state-graph';
 import { CatchProps, Errors, IChainable, INextable, RetryProps } from '../types';
 
@@ -578,7 +578,6 @@ export function renderList<T>(xs: T[], mapFn: (x: T) => any, sortFn?: (a: T, b:
  */
 export function renderJsonPath(jsonPath?: string): undefined | null | string {
   if (jsonPath === undefined) { return undefined; }
-  if (jsonPath === JsonPath.DISCARD) { return null; }
 
   if (!Token.isUnresolved(jsonPath) && !jsonPath.startsWith('$')) {
     throw new Error(`Expected JSON path to start with '$', got: ${jsonPath}`);

packages/@aws-cdk/aws-stepfunctions/test/state.test.ts

@@ -1,26 +1,33 @@
+import * as assert from '@aws-cdk/assertions';
 import * as cdk from '@aws-cdk/core';
 import { FakeTask } from './fake-task';
-import { renderGraph } from './private/render-util';
-import { JsonPath } from '../lib';
+import { JsonPath, StateMachine } from '../lib';
 
 test('JsonPath.DISCARD can be used to discard a state\'s output', () => {
-  const stack = new cdk.Stack();
-
+  // GIVEN
+  const app = new cdk.App();
+  const stack = new cdk.Stack(app, 'TestStack');
   const task = new FakeTask(stack, 'my-state', {
     inputPath: JsonPath.DISCARD,
     outputPath: JsonPath.DISCARD,
     resultPath: JsonPath.DISCARD,
   });
+  new StateMachine(stack, 'state-machine', {
+    definition: task,
+  });
+
+  // WHEN
+  const definitionString = new assert.Capture();
+  assert.Template.fromStack(stack).hasResourceProperties('AWS::StepFunctions::StateMachine', {
+    DefinitionString: definitionString,
+  });
+
+  // THEN
+  const definition = JSON.parse(definitionString.asString());
 
-  expect(renderGraph(task)).toEqual({
-    StartAt: 'my-state',
+  expect(definition).toMatchObject({
     States: {
       'my-state': {
-        End: true,
-        Type: 'Task',
-        Resource: expect.any(String),
-        Parameters: expect.any(Object),
-        // The important bits:
         InputPath: null,
         OutputPath: null,
         ResultPath: null,

packages/@aws-cdk/core/lib/private/cloudformation-lang.ts

@@ -452,4 +452,4 @@ class ScopedCache<O extends object, K, V> {
   }
 }
 
-const stringifyCache = new ScopedCache<Stack, string, string>();
+const stringifyCache = new ScopedCache<Stack, string, string>();

packages/@aws-cdk/core/lib/private/resolve.ts

@@ -192,6 +192,14 @@ export function resolve(obj: any, options: IResolveOptions): any {
     return arr;
   }
 
+  //
+  // literal null -- from JsonNull resolution, preserved as-is (semantically meaningful)
+  //
+
+  if (obj === null) {
+    return obj;
+  }
+
   //
   // tokens - invoke 'resolve' and continue to resolve recursively
   //

packages/@aws-cdk/core/lib/token.ts

@@ -4,7 +4,7 @@ import { unresolved } from './private/encoding';
 import { Intrinsic } from './private/intrinsic';
 import { resolve } from './private/resolve';
 import { TokenMap } from './private/token-map';
-import { IResolvable, ITokenResolver } from './resolvable';
+import { IResolvable, ITokenResolver, IResolveContext } from './resolvable';
 import { TokenizedStringFragments } from './string-fragments';
 
 /**
@@ -236,12 +236,18 @@ export class Tokenization {
  * An object which serializes to the JSON `null` literal, and which can safely
  * be passed across languages where `undefined` and `null` are not different.
  */
-export class JsonNull {
+export class JsonNull implements IResolvable {
   /** The canonical instance of `JsonNull`. */
   public static readonly INSTANCE = new JsonNull();
 
+  public readonly creationStack: string[] = [];
+
   private constructor() { }
 
+  public resolve(_ctx: IResolveContext): any {
+    return null;
+  }
+
   /** Obtains the JSON representation of this object (`null`) */
   public toJSON(): any {
     return null;