fix(mapAsyncIterable): always close source when mapped iterable is thrown (#4494)

Even when the underlying source does not implement a `.throw()` method.

Bug fix, but very low impact as `.throw()` should never need be used with our generator.

Author: yaacovCR

src/execution/__tests__/mapAsyncIterable-test.ts

@@ -264,6 +264,48 @@ describe('mapAsyncIterable', () => {
     await expectPromise(thrown).toRejectWith(message);
   });
 
+  it('close source when mapped iterable is thrown even when the underlying source does not implement a throw method', async () => {
+    const items = [1, 2, 3];
+    let returned = false;
+    const iterable: AsyncIterableIterator<number> = {
+      [Symbol.asyncIterator]() {
+        return this;
+      },
+      next() {
+        if (returned) {
+          return Promise.resolve({ done: true, value: undefined });
+        }
+        const value = items[0];
+        items.shift();
+        return Promise.resolve({
+          done: items.length === 0,
+          value,
+        });
+      },
+      return: () => {
+        returned = true;
+        return Promise.resolve({ done: true, value: undefined });
+      },
+    };
+
+    const doubles = mapAsyncIterable(iterable, (x) => x + x);
+
+    expect(await doubles.next()).to.deep.equal({ value: 2, done: false });
+    expect(await doubles.next()).to.deep.equal({ value: 4, done: false });
+
+    // Throw error
+    const message = 'allows throwing errors when mapping async iterable';
+    const thrown = doubles.throw(new Error(message));
+    await expectPromise(thrown).toRejectWith(message);
+
+    // Returns early when throwing errors through async iterable
+    expect(returned).to.equal(true);
+    expect(await doubles.next()).to.deep.equal({
+      value: undefined,
+      done: true,
+    });
+  });
+
   it('passes through caught errors through async generators', async () => {
     async function* source() {
       try {

src/execution/mapAsyncIterable.ts

@@ -30,17 +30,19 @@ export function mapAsyncIterable<T, U, R = undefined>(
     try {
       return { value: await callback(value), done: false };
     } catch (error) {
-      /* c8 ignore start */
-      // FIXME: add test case
-      if (typeof iterator.return === 'function') {
-        try {
-          await iterator.return();
-        } catch (_e) {
-          /* ignore error */
-        }
-      }
+      await returnIgnoringErrors();
       throw error;
-      /* c8 ignore stop */
+    }
+  }
+
+  async function returnIgnoringErrors(): Promise<void> {
+    if (typeof iterator.return === 'function') {
+      try {
+        await iterator.return(); /* c8 ignore start */
+      } catch (_error) {
+        // FIXME: add test case
+        /* ignore error */
+      } /* c8 ignore stop */
     }
   }
 
@@ -58,6 +60,11 @@ export function mapAsyncIterable<T, U, R = undefined>(
       if (typeof iterator.throw === 'function') {
         return mapResult(iterator.throw(error));
       }
+
+      if (typeof iterator.return === 'function') {
+        await returnIgnoringErrors();
+      }
+
       throw error;
     },
     [Symbol.asyncIterator]() {