fix(core): toJsonString() does not deal correctly with list tokens

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

@@ -233,7 +233,15 @@ function tokenAwareStringify(root: any, space: number, ctx: IResolveContext) {
         //
         // Therefore, if we encounter lists we need to defer to a custom resource to handle
         // them properly at deploy time.
-        pushIntrinsic(CfnUtils.stringify(Stack.of(ctx.scope), `CdkJsonStringify${stringifyCounter++}`, intrinsic));
+        const stack = Stack.of(ctx.scope);
+
+        // Because this will be called twice (once during `prepare`, once during `resolve`),
+        // we need to make sure to be idempotent, so use a cache.
+        const stringifyResponse = stringifyCache.obtain(stack, JSON.stringify(intrinsic), () =>
+          CfnUtils.stringify(stack, `CdkJsonStringify${stringifyCounter++}`, intrinsic),
+        );
+
+        pushIntrinsic(stringifyResponse);
         return;
 
       case ResolutionTypeHint.NUMBER:
@@ -418,4 +426,28 @@ function quoteString(s: string) {
   return s.substring(1, s.length - 1);
 }
 
-let stringifyCounter = 1;
+let stringifyCounter = 1;
+
+/**
+ * A cache scoped to object instances, that's maintained externally to the object instances
+ */
+class ScopedCache<O extends object, K, V> {
+  private cache = new WeakMap<O, Map<K, V>>();
+
+  public obtain(object: O, key: K, init: () => V): V {
+    let kvMap = this.cache.get(object);
+    if (!kvMap) {
+      kvMap = new Map();
+      this.cache.set(object, kvMap);
+    }
+
+    let ret = kvMap.get(key);
+    if (ret === undefined) {
+      ret = init();
+      kvMap.set(key, ret);
+    }
+    return ret;
+  }
+}
+
+const stringifyCache = new ScopedCache<Stack, string, string>();

packages/@aws-cdk/core/test/cloudformation-json.test.ts

@@ -1,4 +1,4 @@
-import { App, Aws, CfnOutput, Fn, IPostProcessor, IResolvable, IResolveContext, Lazy, Stack, Token } from '../lib';
+import { App, Aws, CfnOutput, CfnResource, Fn, IPostProcessor, IResolvable, IResolveContext, Lazy, Stack, Token } from '../lib';
 import { Intrinsic } from '../lib/private/intrinsic';
 import { evaluateCFN } from './evaluate-cfn';
 
@@ -102,10 +102,22 @@ describe('tokens that return literals', () => {
     const someList = Token.asList(new Intrinsic({ Ref: 'Thing' }));
 
     // WHEN
-    expect(stack.resolve(stack.toJsonString({ someList }))).toEqual({
+    new CfnResource(stack, 'Resource', {
+      type: 'AWS::Banana',
+      properties: {
+        someJson: stack.toJsonString({ someList }),
+      },
+    });
+
+    const asm = app.synth();
+    const template = asm.getStackByName(stack.stackName).template;
+    const stringifyLogicalId = Object.keys(template.Resources).filter(id => id.startsWith('CdkJsonStringify'))[0];
+    expect(stringifyLogicalId).toBeDefined();
+
+    expect(template.Resources.Resource.Properties.someJson).toEqual({
       'Fn::Join': ['', [
         '{"someList":',
-        { 'Fn::GetAtt': [expect.stringContaining('CdkJsonStringify'), 'Value'] },
+        { 'Fn::GetAtt': [stringifyLogicalId, 'Value'] },
         '}',
       ]],
     });