add defer/stream support for subscriptions

src/subscription/__tests__/flattenAsyncIterator-test.js

@@ -0,0 +1,135 @@
+import { expect } from 'chai';
+import { describe, it } from 'mocha';
+
+import flattenAsyncIterator from '../flattenAsyncIterator';
+
+describe('flattenAsyncIterator', () => {
+  it('does not modify an already flat async generator', async () => {
+    async function* source() {
+      yield 1;
+      yield 2;
+      yield 3;
+    }
+
+    const result = flattenAsyncIterator(source());
+
+    expect(await result.next()).to.deep.equal({ value: 1, done: false });
+    expect(await result.next()).to.deep.equal({ value: 2, done: false });
+    expect(await result.next()).to.deep.equal({ value: 3, done: false });
+    expect(await result.next()).to.deep.equal({
+      value: undefined,
+      done: true,
+    });
+  });
+
+  it('does not modify an already flat async iterator', async () => {
+    const items = [1, 2, 3];
+
+    const iterator: any = {
+      [Symbol.asyncIterator]() {
+        return this;
+      },
+      next() {
+        return Promise.resolve({
+          done: items.length === 0,
+          value: items.shift(),
+        });
+      },
+    };
+
+    const result = flattenAsyncIterator(iterator);
+
+    expect(await result.next()).to.deep.equal({ value: 1, done: false });
+    expect(await result.next()).to.deep.equal({ value: 2, done: false });
+    expect(await result.next()).to.deep.equal({ value: 3, done: false });
+    expect(await result.next()).to.deep.equal({
+      value: undefined,
+      done: true,
+    });
+  });
+
+  it('flatten nested async generators', async () => {
+    async function* source() {
+      yield 1;
+      yield 2;
+      yield (async function* (): AsyncGenerator<number, void, void> {
+        yield 2.1;
+        yield 2.2;
+      })();
+      yield 3;
+    }
+
+    const doubles = flattenAsyncIterator(source());
+
+    const result = [];
+    for await (const x of doubles) {
+      result.push(x);
+    }
+    expect(result).to.deep.equal([1, 2, 2.1, 2.2, 3]);
+  });
+
+  it('allows returning early from a nested async generator', async () => {
+    async function* source() {
+      yield 1;
+      yield 2;
+      yield (async function* (): AsyncGenerator<number, void, void> {
+        yield 2.1;
+        // istanbul ignore next (Shouldn't be reached)
+        yield 2.2;
+      })();
+      // istanbul ignore next (Shouldn't be reached)
+      yield 3;
+    }
+
+    const doubles = flattenAsyncIterator(source());
+
+    expect(await doubles.next()).to.deep.equal({ value: 1, done: false });
+    expect(await doubles.next()).to.deep.equal({ value: 2, done: false });
+    expect(await doubles.next()).to.deep.equal({ value: 2.1, done: false });
+
+    // Early return
+    expect(await doubles.return()).to.deep.equal({
+      value: undefined,
+      done: true,
+    });
+
+    // Subsequent next calls
+    expect(await doubles.next()).to.deep.equal({
+      value: undefined,
+      done: true,
+    });
+    expect(await doubles.next()).to.deep.equal({
+      value: undefined,
+      done: true,
+    });
+  });
+
+  it('allows throwing errors from a nested async generator', async () => {
+    async function* source() {
+      yield 1;
+      yield 2;
+      yield (async function* (): AsyncGenerator<number, void, void> {
+        yield 2.1;
+        // istanbul ignore next (Shouldn't be reached)
+        yield 2.2;
+      })();
+      // istanbul ignore next (Shouldn't be reached)
+      yield 3;
+    }
+
+    const doubles = flattenAsyncIterator(source());
+
+    expect(await doubles.next()).to.deep.equal({ value: 1, done: false });
+    expect(await doubles.next()).to.deep.equal({ value: 2, done: false });
+    expect(await doubles.next()).to.deep.equal({ value: 2.1, done: false });
+
+    // Throw error
+    let caughtError;
+    try {
+      await doubles.throw('ouch');
+    } catch (e) {
+      caughtError = e;
+    }
+    expect(caughtError).to.equal('ouch');
+  });
+});

src/subscription/__tests__/subscribe-test.js

@@ -668,6 +668,153 @@ describe('Subscription Publish Phase', () => {
     });
   });
 
+  it('produces additional payloads for subscriptions with @defer', async () => {
+    const pubsub = new SimplePubSub();
+    const subscription = await createSubscription(
+      pubsub,
+      emailSchema,
+      parse(`
+        subscription ($priority: Int = 0) {
+          importantEmail(priority: $priority) {
+            email {
+              from
+              subject
+            }
+            ... @defer {
+              inbox {
+                unread
+                total
+              }
+            }
+          }
+        }
+      `),
+    );
+    invariant(isAsyncIterable(subscription));
+    // Wait for the next subscription payload.
+    const payload = subscription.next();
+
+    // A new email arrives!
+    expect(
+      pubsub.emit({
+        from: '[email protected]',
+        subject: 'Alright',
+        message: 'Tests are good',
+        unread: true,
+      }),
+    ).to.equal(true);
+
+    // The previously waited on payload now has a value.
+    expect(await payload).to.deep.equal({
+      done: false,
+      value: {
+        data: {
+          importantEmail: {
+            email: {
+              from: '[email protected]',
+              subject: 'Alright',
+            },
+          },
+        },
+        hasNext: true,
+      },
+    });
+
+    // Wait for the next payload from @defer
+    expect(await subscription.next()).to.deep.equal({
+      done: false,
+      value: {
+        data: {
+          inbox: {
+            unread: 1,
+            total: 2,
+          },
+        },
+        path: ['importantEmail'],
+        hasNext: false,
+      },
+    });
+
+    // Another new email arrives, after all incrementally delivered payloads are received.
+    expect(
+      pubsub.emit({
+        from: '[email protected]',
+        subject: 'Tools',
+        message: 'I <3 making things',
+        unread: true,
+      }),
+    ).to.equal(true);
+
+    // The next waited on payload will have a value.
+    expect(await subscription.next()).to.deep.equal({
+      done: false,
+      value: {
+        data: {
+          importantEmail: {
+            email: {
+              from: '[email protected]',
+              subject: 'Tools',
+            },
+          },
+        },
+        hasNext: true,
+      },
+    });
+
+    // Another new email arrives, before the incrementally delivered payloads from the last email was received.
+    expect(
+      pubsub.emit({
+        from: '[email protected]',
+        subject: 'Important',
+        message: 'Read me please',
+        unread: true,
+      }),
+    ).to.equal(true);
+
+    // Deferred payload from previous event is received.
+    expect(await subscription.next()).to.deep.equal({
+      done: false,
+      value: {
+        data: {
+          inbox: {
+            unread: 2,
+            total: 3,
+          },
+        },
+        path: ['importantEmail'],
+        hasNext: false,
+      },
+    });
+
+    // Next payload from last event
+    expect(await subscription.next()).to.deep.equal({
+      done: false,
+      value: {
+        data: {
+          importantEmail: {
+            email: {
+              from: '[email protected]',
+              subject: 'Important',
+            },
+          },
+        },
+        hasNext: true,
+      },
+    });
+
+    // The client disconnects before the deferred payload is consumed.
+    expect(await subscription.return()).to.deep.equal({
+      done: true,
+      value: undefined,
+    });
+
+    // Awaiting a subscription after closing it results in completed results.
+    expect(await subscription.next()).to.deep.equal({
+      done: true,
+      value: undefined,
+    });
+  });
+
   it('produces a payload when there are multiple events', async () => {
     const pubsub = new SimplePubSub();
     const subscription = await createSubscription(pubsub);

src/subscription/flattenAsyncIterator.js

@@ -0,0 +1,49 @@
+import { SYMBOL_ASYNC_ITERATOR } from '../polyfills/symbols';
+
+import isAsyncIterable from '../jsutils/isAsyncIterable';
+
+/**
+ * Given an AsyncIterable that could potentially yield other async iterators,
+ * flatten all yielded results into a single AsyncIterable
+ */
+export default function flattenAsyncIterator<T>(
+  iterable: AsyncGenerator<AsyncGenerator<T, void, void> | T, void, void>,
+): AsyncGenerator<T, void, void> {
+  // $FlowFixMe[prop-missing]
+  const iteratorMethod = iterable[SYMBOL_ASYNC_ITERATOR];
+  const iterator: any = iteratorMethod.call(iterable);
+  let iteratorStack: Array<AsyncGenerator<T, void, void>> = [iterator];
+
+  function next(): Promise<IteratorResult<T, void>> {
+    const currentIterator = iteratorStack[0];
+    if (!currentIterator) {
+      return Promise.resolve({ value: undefined, done: true });
+    }
+    return currentIterator.next().then((result) => {
+      if (result.done) {
+        iteratorStack.shift();
+        return next();
+      } else if (isAsyncIterable(result.value)) {
+        const childIteratorMethod = result.value[SYMBOL_ASYNC_ITERATOR];
+        const childIterator: any = childIteratorMethod.call(result.value);
+        iteratorStack.unshift(childIterator);
+        return next();
+      }
+      return result;
+    });
+  }
+  return ({
+    next,
+    return() {
+      iteratorStack = [];
+      return iterator.return();
+    },
+    throw(error?: mixed): Promise<IteratorResult<T, void>> {
+      iteratorStack = [];
+      return iterator.throw(error);
+    },
+    [SYMBOL_ASYNC_ITERATOR]() {
+      return this;
+    },
+  }: $FlowFixMe);
+}

src/subscription/subscribe.js

@@ -24,6 +24,7 @@ import type { GraphQLFieldResolver } from '../type/definition';
 import { getOperationRootType } from '../utilities/getOperationRootType';
 
 import mapAsyncIterator from './mapAsyncIterator';
+import flattenAsyncIterator from './flattenAsyncIterator';
 
 export type SubscriptionArgs = {|
   schema: GraphQLSchema,
@@ -140,8 +141,8 @@ function subscribeImpl(
   // the GraphQL specification. The `execute` function provides the
   // "ExecuteSubscriptionEvent" algorithm, as it is nearly identical to the
   // "ExecuteQuery" algorithm, for which `execute` is also used.
-  const mapSourceToResponse = (payload) => {
-    const executionResult = execute({
+  const mapSourceToResponse = (payload) =>
+    execute({
       schema,
       document,
       rootValue: payload,
@@ -150,24 +151,18 @@ function subscribeImpl(
       operationName,
       fieldResolver,
     });
-    /* istanbul ignore if - TODO: implement support for defer/stream in subscriptions */
-    if (isAsyncIterable(executionResult)) {
-      throw new Error(
-        'TODO: implement support for defer/stream in subscriptions',
-      );
-    }
-    return executionResult;
-  };
 
   // Resolve the Source Stream, then map every source value to a
   // ExecutionResult value as described above.
   return sourcePromise.then((resultOrStream) =>
     // Note: Flow can't refine isAsyncIterable, so explicit casts are used.
     isAsyncIterable(resultOrStream)
-      ? mapAsyncIterator(
-          resultOrStream,
-          mapSourceToResponse,
-          reportGraphQLError,
+      ? flattenAsyncIterator(
+          mapAsyncIterator(
+            resultOrStream,
+            mapSourceToResponse,
+            reportGraphQLError,
+          ),
         )
       : ((resultOrStream: any): ExecutionResult),
   );