Support @defer directive

src/execution/__tests__/directives-perf-test.js

@@ -0,0 +1,80 @@
+// @flow strict
+
+import { expect } from 'chai';
+import { it } from 'mocha';
+
+import { parse } from '../../language/parser';
+
+import { GraphQLSchema } from '../../type/schema';
+import { GraphQLString } from '../../type/scalars';
+import { GraphQLObjectType } from '../../type/definition';
+
+import { execute } from '../execute';
+import { forAwaitEach, isAsyncIterable } from 'iterall';
+
+const schema = new GraphQLSchema({
+  query: new GraphQLObjectType({
+    name: 'TestType',
+    fields: {
+      a: { type: GraphQLString },
+      b: { type: GraphQLString },
+    },
+  }),
+});
+
+let count = 0;
+
+const rootValue = {
+  a() {
+    return 'a';
+  },
+  b() {
+    count += 1;
+    return `b${count}`;
+  },
+};
+
+function executeTestQuery(query) {
+  const document = parse(query);
+  return execute({ schema, document, rootValue });
+}
+
+it('performance test, if the same field is deferred several times, its resolve is called only once', async () => {
+  const index = 100;
+  let fragString = '';
+  let fragElementString = '';
+  for (const i of Array.from(Array(index).keys())) {
+    fragString += `
+    ...Frag${i} @defer(label: "Frag_b_defer${i}")
+    `;
+    fragElementString += `
+    fragment Frag${i} on TestType {
+          b
+        }
+        `;
+  }
+  const result = await executeTestQuery(`
+        query {
+          a
+          ${fragString}
+        }
+        ${fragElementString}
+      `);
+  expect(isAsyncIterable(result)).to.equal(true);
+  const results = [];
+  await forAwaitEach(((result: any): AsyncIterable<mixed>), value => {
+    results.push(value);
+  });
+
+  expect(results.length).to.equal(index + 1);
+  expect(results[0]).to.deep.equal({
+    data: { a: 'a' },
+  });
+  for (const i of Array.from(Array(index).keys())) {
+    expect(results[i + 1]).to.deep.equal({
+      data: { b: 'b1' },
+      label: `Frag_b_defer${i}`,
+      path: [],
+    });
+  }
+});