fix(cancellablePromise): handle rejection when cancelling already-aborted promise (#4645)

Author: yaacovCR

src/execution/__tests__/cancellablePromise-test.ts

@@ -3,6 +3,8 @@ import { describe, it } from 'mocha';
 
 import { expectPromise } from '../../__testUtils__/expectPromise.js';
 
+import { promiseWithResolvers } from '../../jsutils/promiseWithResolvers.js';
+
 import { cancellablePromise } from '../cancellablePromise.js';
 
 describe('cancellablePromise', () => {
@@ -110,4 +112,34 @@ describe('cancellablePromise', () => {
 
     await expectPromise(withCancellation).toRejectWith('Cancelled!');
   });
+
+  it('handles later original rejections when already aborted', async () => {
+    const abortController = new AbortController();
+    abortController.abort(new Error('Cancelled!'));
+
+    const deferred = promiseWithResolvers<undefined>();
+
+    let unhandledRejection: unknown = null;
+    const unhandledRejectionListener = (reason: unknown) => {
+      unhandledRejection = reason;
+    };
+    // eslint-disable-next-line no-undef
+    process.on('unhandledRejection', unhandledRejectionListener);
+
+    try {
+      const withCancellation = cancellablePromise(
+        deferred.promise,
+        abortController.signal,
+      );
+      await expectPromise(withCancellation).toRejectWith('Cancelled!');
+
+      deferred.reject(new Error('Rejected later'));
+      await new Promise((resolve) => setTimeout(resolve, 20));
+    } finally {
+      // eslint-disable-next-line no-undef
+      process.removeListener('unhandledRejection', unhandledRejectionListener);
+    }
+
+    expect(unhandledRejection).to.equal(null);
+  });
 });

src/execution/cancellablePromise.ts

@@ -5,6 +5,9 @@ export function cancellablePromise<T>(
   abortSignal: AbortSignal,
 ): Promise<T> {
   if (abortSignal.aborted) {
+    // If cancellation has already happened, still drain the original promise to
+    // avoid unhandled rejections from work that settles later.
+    originalPromise.catch(() => undefined);
     // eslint-disable-next-line @typescript-eslint/prefer-promise-reject-errors
     return Promise.reject(abortSignal.reason);
   }