[fix] whenResultIsFinished works on Promise of Array of Promises

.changeset/long-crabs-reply.md

@@ -0,0 +1,5 @@
+---
+"@apollo/server": patch
+---
+
+Improves timing of the `willResolveField` end hook on fields which return Promises resolving to Arrays. This makes the use of the `setCacheHint` method more reliable.

packages/server/src/__tests__/utils/schemaInstrumentation.test.ts

@@ -0,0 +1,37 @@
+import { describe, expect, it, jest } from '@jest/globals';
+import { whenResultIsFinished } from '../../utils/schemaInstrumentation';
+
+describe('whenResultIsFinished', () => {
+  it('passes result of Promise to the callback', async () => {
+    const expected = 1;
+    const result = Promise.resolve(expected);
+    const callback = jest.fn();
+    whenResultIsFinished(result, callback);
+    await new Promise((r) => setImmediate(r));
+    expect(callback).toBeCalledWith(null, expected);
+  });
+  it('passes result of Array of Promises to the callback', async () => {
+    const expected = 1;
+    const result = [Promise.resolve(expected)];
+    const callback = jest.fn();
+    whenResultIsFinished(result, callback);
+    await new Promise((r) => setImmediate(r));
+    expect(callback).toBeCalledWith(null, [expected]);
+  });
+  it('passes result which is not asynchronous directly to the callback', async () => {
+    const expected = 1;
+    const result = expected;
+    const callback = jest.fn();
+    whenResultIsFinished(result, callback);
+    await new Promise((r) => setImmediate(r));
+    expect(callback).toBeCalledWith(null, expected);
+  });
+  it('passes result of Promise of Array of Promises to the callback', async () => {
+    const expected = 1;
+    const result = Promise.resolve([Promise.resolve(expected)]);
+    const callback = jest.fn();
+    whenResultIsFinished(result, callback);
+    await new Promise((r) => setImmediate(r));
+    expect(callback).toBeCalledWith(null, [expected]);
+  });
+});

packages/server/src/utils/schemaInstrumentation.ts

@@ -106,14 +106,15 @@ function isPromise(x: any): boolean {
 
 // Given result (which may be a Promise or an array some of whose elements are
 // promises) Promises, set up 'callback' to be invoked when result is fully
-// resolved.
+// resolved. (Unfortunately, this does not perfectly handle every possible
+// return value shape, such as arrays of arrays of Promises.)
 export function whenResultIsFinished(
   result: any,
   callback: (err: Error | null, result?: any) => void,
 ) {
   if (isPromise(result)) {
     result.then(
-      (r: any) => callback(null, r),
+      (r: any) => whenResultIsFinished(r, callback),
       (err: Error) => callback(err),
     );
   } else if (Array.isArray(result)) {