Support returning async iterables from resolver functions

Support returning async iterables from resolver functions

src/execution/__tests__/lists-test.ts

@@ -3,10 +3,18 @@ import { describe, it } from 'mocha';
 
 import { expectJSON } from '../../__testUtils__/expectJSON';
 
+import type { PromiseOrValue } from '../../jsutils/PromiseOrValue';
+
 import { parse } from '../../language/parser';
 
+import type { GraphQLFieldResolver } from '../../type/definition';
+import { GraphQLList, GraphQLObjectType } from '../../type/definition';
+import { GraphQLString } from '../../type/scalars';
+import { GraphQLSchema } from '../../type/schema';
+
 import { buildSchema } from '../../utilities/buildASTSchema';
 
+import type { ExecutionResult } from '../execute';
 import { execute, executeSync } from '../execute';
 
 describe('Execute: Accepts any iterable as list value', () => {
@@ -66,6 +74,175 @@ describe('Execute: Accepts any iterable as list value', () => {
   });
 });
 
+describe('Execute: Accepts async iterables as list value', () => {
+  function complete(rootValue: unknown, as: string = '[String]') {
+    return execute({
+      schema: buildSchema(`type Query { listField: ${as} }`),
+      document: parse('{ listField }'),
+      rootValue,
+    });
+  }
+
+  function completeObjectList(
+    resolve: GraphQLFieldResolver<{ index: number }, unknown>,
+  ): PromiseOrValue<ExecutionResult> {
+    const schema = new GraphQLSchema({
+      query: new GraphQLObjectType({
+        name: 'Query',
+        fields: {
+          listField: {
+            resolve: async function* listField() {
+              yield await Promise.resolve({ index: 0 });
+              yield await Promise.resolve({ index: 1 });
+              yield await Promise.resolve({ index: 2 });
+            },
+            type: new GraphQLList(
+              new GraphQLObjectType({
+                name: 'ObjectWrapper',
+                fields: {
+                  index: {
+                    type: GraphQLString,
+                    resolve,
+                  },
+                },
+              }),
+            ),
+          },
+        },
+      }),
+    });
+    return execute({
+      schema,
+      document: parse('{ listField { index } }'),
+    });
+  }
+
+  it('Accepts an AsyncGenerator function as a List value', async () => {
+    async function* listField() {
+      yield await Promise.resolve('two');
+      yield await Promise.resolve(4);
+      yield await Promise.resolve(false);
+    }
+
+    expectJSON(await complete({ listField })).toDeepEqual({
+      data: { listField: ['two', '4', 'false'] },
+    });
+  });
+
+  it('Handles an AsyncGenerator function that throws', async () => {
+    async function* listField() {
+      yield await Promise.resolve('two');
+      yield await Promise.resolve(4);
+      throw new Error('bad');
+    }
+
+    expectJSON(await complete({ listField })).toDeepEqual({
+      data: { listField: ['two', '4', null] },
+      errors: [
+        {
+          message: 'bad',
+          locations: [{ line: 1, column: 3 }],
+          path: ['listField', 2],
+        },
+      ],
+    });
+  });
+
+  it('Handles an AsyncGenerator function where an intermediate value triggers an error', async () => {
+    async function* listField() {
+      yield await Promise.resolve('two');
+      yield await Promise.resolve({});
+      yield await Promise.resolve(4);
+    }
+
+    expectJSON(await complete({ listField })).toDeepEqual({
+      data: { listField: ['two', null, '4'] },
+      errors: [
+        {
+          message: 'String cannot represent value: {}',
+          locations: [{ line: 1, column: 3 }],
+          path: ['listField', 1],
+        },
+      ],
+    });
+  });
+
+  it('Handles errors from `completeValue` in AsyncIterables', async () => {
+    async function* listField() {
+      yield await Promise.resolve('two');
+      yield await Promise.resolve({});
+    }
+
+    expectJSON(await complete({ listField })).toDeepEqual({
+      data: { listField: ['two', null] },
+      errors: [
+        {
+          message: 'String cannot represent value: {}',
+          locations: [{ line: 1, column: 3 }],
+          path: ['listField', 1],
+        },
+      ],
+    });
+  });
+
+  it('Handles promises from `completeValue` in AsyncIterables', async () => {
+    expectJSON(
+      await completeObjectList(({ index }) => Promise.resolve(index)),
+    ).toDeepEqual({
+      data: { listField: [{ index: '0' }, { index: '1' }, { index: '2' }] },
+    });
+  });
+
+  it('Handles rejected promises from `completeValue` in AsyncIterables', async () => {
+    expectJSON(
+      await completeObjectList(({ index }) => {
+        if (index === 2) {
+          return Promise.reject(new Error('bad'));
+        }
+        return Promise.resolve(index);
+      }),
+    ).toDeepEqual({
+      data: { listField: [{ index: '0' }, { index: '1' }, { index: null }] },
+      errors: [
+        {
+          message: 'bad',
+          locations: [{ line: 1, column: 15 }],
+          path: ['listField', 2, 'index'],
+        },
+      ],
+    });
+  });
+  it('Handles nulls yielded by async generator', async () => {
+    async function* listField() {
+      yield await Promise.resolve(1);
+      yield await Promise.resolve(null);
+      yield await Promise.resolve(2);
+    }
+    const errors = [
+      {
+        message: 'Cannot return null for non-nullable field Query.listField.',
+        locations: [{ line: 1, column: 3 }],
+        path: ['listField', 1],
+      },
+    ];
+
+    expect(await complete({ listField }, '[Int]')).to.deep.equal({
+      data: { listField: [1, null, 2] },
+    });
+    expect(await complete({ listField }, '[Int]!')).to.deep.equal({
+      data: { listField: [1, null, 2] },
+    });
+    expectJSON(await complete({ listField }, '[Int!]')).toDeepEqual({
+      data: { listField: null },
+      errors,
+    });
+    expectJSON(await complete({ listField }, '[Int!]!')).toDeepEqual({
+      data: null,
+      errors,
+    });
+  });
+});
+
 describe('Execute: Handles list nullability', () => {
   async function complete(args: { listField: unknown; as: string }) {
     const { listField, as } = args;

src/execution/execute.ts

@@ -1,6 +1,7 @@
 import { devAssert } from '../jsutils/devAssert';
 import { inspect } from '../jsutils/inspect';
 import { invariant } from '../jsutils/invariant';
+import { isAsyncIterable } from '../jsutils/isAsyncIterable';
 import { isIterableObject } from '../jsutils/isIterableObject';
 import { isObjectLike } from '../jsutils/isObjectLike';
 import { isPromise } from '../jsutils/isPromise';
@@ -703,6 +704,66 @@ function completeValue(
   );
 }
 
+/**
+ * Complete a async iterator value by completing the result and calling
+ * recursively until all the results are completed.
+ */
+async function completeAsyncIteratorValue(
+  exeContext: ExecutionContext,
+  itemType: GraphQLOutputType,
+  fieldNodes: ReadonlyArray<FieldNode>,
+  info: GraphQLResolveInfo,
+  path: Path,
+  iterator: AsyncIterator<unknown>,
+): Promise<ReadonlyArray<unknown>> {
+  let containsPromise = false;
+  const completedResults = [];
+  let index = 0;
+  // eslint-disable-next-line no-constant-condition
+  while (true) {
+    const fieldPath = addPath(path, index, undefined);
+    try {
+      // eslint-disable-next-line no-await-in-loop
+      const { value, done } = await iterator.next();
+      if (done) {
+        break;
+      }
+
+      try {
+        // TODO can the error checking logic be consolidated with completeListValue?
+        const completedItem = completeValue(
+          exeContext,
+          itemType,
+          fieldNodes,
+          info,
+          fieldPath,
+          value,
+        );
+        if (isPromise(completedItem)) {
+          containsPromise = true;
+        }
+        completedResults.push(completedItem);
+      } catch (rawError) {
+        completedResults.push(null);
+        const error = locatedError(
+          rawError,
+          fieldNodes,
+          pathToArray(fieldPath),
+        );
+        handleFieldError(error, itemType, exeContext);
+        continue;
+      }
+    } catch (rawError) {
+      completedResults.push(null);
+      const error = locatedError(rawError, fieldNodes, pathToArray(fieldPath));
+      handleFieldError(error, itemType, exeContext);
+      break;
+    }
+    index += 1;
+  }
+  return containsPromise ? Promise.all(completedResults) : completedResults;
+}
+
 /**
  * Complete a list value by completing each item in the list with the
  * inner type
@@ -715,6 +776,21 @@ function completeListValue(
   path: Path,
   result: unknown,
 ): PromiseOrValue<ReadonlyArray<unknown>> {
+  const itemType = returnType.ofType;
+
+  if (isAsyncIterable(result)) {
+    const iterator = result[Symbol.asyncIterator]();
+
+    return completeAsyncIteratorValue(
+      exeContext,
+      itemType,
+      fieldNodes,
+      info,
+      path,
+      iterator,
+    );
+  }
+
   if (!isIterableObject(result)) {
     throw new GraphQLError(
       `Expected Iterable, but did not find one for field "${info.parentType.name}.${info.fieldName}".`,
@@ -723,7 +799,6 @@ function completeListValue(
 
   // This is specified as a simple map, however we're optimizing the path
   // where the list contains no Promises by avoiding creating another Promise.
-  const itemType = returnType.ofType;
   let containsPromise = false;
   const completedResults = Array.from(result, (item, index) => {
     // No need to modify the info object containing the path,