fix(race): concurrent next calls with defer/stream

src/execution/__tests__/stream-test.ts

@@ -133,6 +133,22 @@ async function complete(document: DocumentNode) {
   return result;
 }
 
+async function completeAsync(document: DocumentNode, numCalls: number) {
+  const schema = new GraphQLSchema({ query });
+
+  const result = await execute({ schema, document, rootValue: {} });
+
+  invariant(isAsyncIterable(result));
+
+  const iterator = result[Symbol.asyncIterator]();
+
+  const promises = [];
+  for (let i = 0; i < numCalls; i++) {
+    promises.push(iterator.next());
+  }
+  return Promise.all(promises);
+}
+
 describe('Execute: stream directive', () => {
   it('Can stream a list field', async () => {
     const document = parse('{ scalarList @stream(initialCount: 1) }');
@@ -437,6 +453,58 @@ describe('Execute: stream directive', () => {
       },
     ]);
   });
+  it('Can stream a field that returns an async iterable', async () => {
+    const document = parse(`
+      query { 
+        asyncIterableList @stream(initialCount: 2) {
+          name
+          id
+        }
+      }
+    `);
+    const result = await completeAsync(document, 4);
+    expect(result).to.deep.equal([
+      {
+        done: false,
+        value: {
+          data: {
+            asyncIterableList: [
+              {
+                name: 'Luke',
+                id: '1',
+              },
+              {
+                name: 'Han',
+                id: '2',
+              },
+            ],
+          },
+          hasNext: true,
+        },
+      },
+      {
+        done: false,
+        value: {
+          data: {
+            name: 'Leia',
+            id: '3',
+          },
+          path: ['asyncIterableList', 2],
+          hasNext: true,
+        },
+      },
+      {
+        done: false,
+        value: {
+          hasNext: false,
+        },
+      },
+      {
+        done: true,
+        value: undefined,
+      },
+    ]);
+  });
   it('Handles error thrown in async iterable before initialCount is reached', async () => {
     const document = parse(`
       query { 

src/execution/execute.ts

@@ -1784,47 +1784,62 @@ export class Dispatcher {
         done: false,
       });
     }
-    return new Promise<{
-      promise: Promise<IteratorResult<DispatcherResult, void>>;
-    }>((resolve) => {
+    return new Promise((resolve) => {
+      let resolved = false;
       this._subsequentPayloads.forEach((promise) => {
         // eslint-disable-next-line @typescript-eslint/no-floating-promises
-        promise.then(() => {
-          // resolve with actual promise, not resolved value of promise so we can remove it from this._subsequentPayloads
-          resolve({ promise });
-        });
-      });
-    })
-      .then(({ promise }) => {
-        this._subsequentPayloads.splice(
-          this._subsequentPayloads.indexOf(promise),
-          1,
-        );
-        return promise;
-      })
-      .then(({ value, done }) => {
-        if (done && this._subsequentPayloads.length === 0) {
-          // async iterable resolver just finished and no more pending payloads
-          return {
-            value: {
-              hasNext: false,
-            },
-            done: false,
+        promise.then((payload) => {
+          if (resolved) {
+            return;
+          }
+
+          resolved = true;
+
+          if (this._subsequentPayloads.length === 0) {
+            // a different call to next has exhausted all payloads
+            resolve({ value: undefined, done: true });
+            return;
+          }
+
+          const index = this._subsequentPayloads.indexOf(promise);
+
+          if (index === -1) {
+            // a different call to next has consumed this payload
+            resolve(this._race());
+            return;
+          }
+
+          this._subsequentPayloads.splice(index, 1);
+
+          const { value, done } = payload;
+
+          if (done && this._subsequentPayloads.length === 0) {
+            // async iterable resolver just finished and no more pending payloads
+            resolve({
+              value: {
+                hasNext: false,
+              },
+              done: false,
+            });
+            return;
+          } else if (done) {
+            // async iterable resolver just finished but there are pending payloads
+            // return the next one
+            resolve(this._race());
+            return;
+          }
+
+          const returnValue: ExecutionPatchResult = {
+            ...value,
+            hasNext: this._subsequentPayloads.length > 0,
           };
-        } else if (done) {
-          // async iterable resolver just finished but there are pending payloads
-          // return the next one
-          return this._race();
-        }
-        const returnValue: ExecutionPatchResult = {
-          ...value,
-          hasNext: this._subsequentPayloads.length > 0,
-        };
-        return {
-          value: returnValue,
-          done: false,
-        };
+          resolve({
+            value: returnValue,
+            done: false,
+          });
+        });
       });
+    });
   }
 
   _next(): Promise<IteratorResult<AsyncExecutionResult, void>> {