Return underlying AsyncIterators when execute result is returned

src/execution/__tests__/stream-test.js

@@ -1,6 +1,7 @@
 import { expect } from 'chai';
 import { describe, it } from 'mocha';
 
+import invariant from '../../jsutils/invariant';
 import isAsyncIterable from '../../jsutils/isAsyncIterable';
 import { parse } from '../../language/parser';
 
@@ -72,6 +73,36 @@ const query = new GraphQLObjectType({
         yield {};
       },
     },
+    asyncIterableListDelayed: {
+      type: new GraphQLList(friendType),
+      async *resolve() {
+        for (const friend of friends) {
+          // pause an additional ms before yielding to allow time
+          // for tests to return or throw before next value is processed.
+          // eslint-disable-next-line no-await-in-loop
+          await new Promise((r) => setTimeout(r, 1));
+          yield friend;
+        }
+      },
+    },
+    asyncIterableListNoReturn: {
+      type: new GraphQLList(friendType),
+      resolve() {
+        let i = 0;
+        return {
+          [Symbol.asyncIterator]: () => ({
+            async next() {
+              const friend = friends[i++];
+              if (friend) {
+                await new Promise((r) => setTimeout(r, 1));
+                return { value: friend, done: false };
+              }
+              return { value: undefined, done: true };
+            },
+          }),
+        };
+      },
+    },
     asyncIterableListDelayedClose: {
       type: new GraphQLList(friendType),
       async *resolve() {
@@ -626,4 +657,114 @@ describe('Execute: stream directive', () => {
       },
     ]);
   });
+  it('Returns underlying async iterables when dispatcher is returned', async () => {
+    const document = parse(`
+      query { 
+        asyncIterableListDelayed @stream(initialCount: 1) {
+          name
+          id
+        }
+      }
+    `);
+    const schema = new GraphQLSchema({ query });
+
+    const executeResult = await execute(schema, document, {});
+    invariant(isAsyncIterable(executeResult));
+
+    const result1 = await executeResult.next();
+    expect(result1).to.deep.equal({
+      done: false,
+      value: {
+        data: {
+          asyncIterableListDelayed: [
+            {
+              id: '1',
+              name: 'Luke',
+            },
+          ],
+        },
+        hasNext: true,
+      },
+    });
+
+    executeResult.return();
+
+    // this result had started processing before return was called
+    const result2 = await executeResult.next();
+    expect(result2).to.deep.equal({
+      done: false,
+      value: {
+        data: {
+          id: '2',
+          name: 'Han',
+        },
+        hasNext: true,
+        path: ['asyncIterableListDelayed', 1],
+      },
+    });
+
+    // third result is not returned because async iterator has returned
+    const result3 = await executeResult.next();
+    expect(result3).to.deep.equal({
+      done: false,
+      value: {
+        hasNext: false,
+      },
+    });
+  });
+  it('Can return async iterable when underlying iterable does not have a return method', async () => {
+    const document = parse(`
+      query { 
+        asyncIterableListNoReturn @stream(initialCount: 1) {
+          name
+          id
+        }
+      }
+    `);
+    const schema = new GraphQLSchema({ query });
+
+    const executeResult = await execute(schema, document, {});
+    invariant(isAsyncIterable(executeResult));
+
+    const result1 = await executeResult.next();
+    expect(result1).to.deep.equal({
+      done: false,
+      value: {
+        data: {
+          asyncIterableListNoReturn: [
+            {
+              id: '1',
+              name: 'Luke',
+            },
+          ],
+        },
+        hasNext: true,
+      },
+    });
+
+    executeResult.return();
+
+    // this result had started processing before return was called
+    const result2 = await executeResult.next();
+    expect(result2).to.deep.equal({
+      done: false,
+      value: {
+        data: {
+          id: '2',
+          name: 'Han',
+        },
+        hasNext: true,
+        path: ['asyncIterableListNoReturn', 1],
+      },
+    });
+
+    // third result is not returned because async iterator has returned
+    const result3 = await executeResult.next();
+    expect(result3).to.deep.equal({
+      done: false,
+      value: {
+        hasNext: false,
+      },
+    });
+  });
 });

src/execution/execute.js

@@ -1645,11 +1645,15 @@ type DispatcherResult = {|
  */
 export class Dispatcher {
   _subsequentPayloads: Array<Promise<IteratorResult<DispatcherResult, void>>>;
+  _iterators: Array<AsyncIterator<mixed>>;
+  _isDone: boolean;
   _initialResult: ?ExecutionResult;
   _hasReturnedInitialResult: boolean;
 
   constructor() {
     this._subsequentPayloads = [];
+    this._iterators = [];
+    this._isDone = false;
     this._hasReturnedInitialResult = false;
   }
 
@@ -1718,13 +1722,16 @@ export class Dispatcher {
     itemType: GraphQLOutputType,
   ): void {
     const subsequentPayloads = this._subsequentPayloads;
+    const iterators = this._iterators;
+    iterators.push(iterator);
     function next(index) {
       const fieldPath = addPath(path, index);
       const patchErrors = [];
       subsequentPayloads.push(
         iterator.next().then(
           ({ value: data, done }) => {
             if (done) {
+              iterators.splice(iterators.indexOf(iterator), 1);
               return { value: undefined, done: true };
             }
 
@@ -1795,6 +1802,14 @@ export class Dispatcher {
   }
 
   _race(): Promise<IteratorResult<ExecutionPatchResult, void>> {
+    if (this._isDone) {
+      return Promise.resolve({
+        value: {
+          hasNext: false,
+        },
+        done: false,
+      });
+    }
     return new Promise((resolve) => {
       this._subsequentPayloads.forEach((promise) => {
         promise.then(() => {
@@ -1851,13 +1866,24 @@ export class Dispatcher {
     return this._race();
   }
 
+  _return(): Promise<IteratorResult<AsyncExecutionResult, void>> {
+    return Promise.all(
+      // $FlowFixMe[prop-missing]
+      this._iterators.map((iterator) => iterator.return?.()),
+    ).then(() => {
+      this._isDone = true;
+      return { value: undefined, done: true };
+    });
+  }
+
   get(initialResult: ExecutionResult): AsyncIterable<AsyncExecutionResult> {
     this._initialResult = initialResult;
     return ({
       [SYMBOL_ASYNC_ITERATOR]() {
         return this;
       },
       next: () => this._next(),
+      return: () => this._return(),
     }: any);
   }
 }