streams: refactor ReadableStream asyncIterator creation and a few fixes

Closes: #23041

- Rewrite `ReadableAsyncIterator` class into
`ReadableStreamAsyncIteratorPrototype` which contains no constructor and
inherits from `%AsyncIteratorPrototype%`.

- Rewrite `AsyncIteratorRecord` into dumb function.

PR-URL: #23042
Fixes: #23041
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

lib/_stream_readable.js

@@ -42,7 +42,7 @@ const { emitExperimentalWarning } = require('internal/util');
 
 // Lazy loaded to improve the startup performance.
 let StringDecoder;
-let ReadableAsyncIterator;
+let createReadableStreamAsyncIterator;
 
 util.inherits(Readable, Stream);
 
@@ -990,9 +990,11 @@ Readable.prototype.wrap = function(stream) {
 
 Readable.prototype[Symbol.asyncIterator] = function() {
   emitExperimentalWarning('Readable[Symbol.asyncIterator]');
-  if (ReadableAsyncIterator === undefined)
-    ReadableAsyncIterator = require('internal/streams/async_iterator');
-  return new ReadableAsyncIterator(this);
+  if (createReadableStreamAsyncIterator === undefined) {
+    createReadableStreamAsyncIterator =
+      require('internal/streams/async_iterator');
+  }
+  return createReadableStreamAsyncIterator(this);
 };
 
 Object.defineProperty(Readable.prototype, 'readableHighWaterMark', {

lib/internal/streams/async_iterator.js

@@ -8,12 +8,9 @@ const kLastPromise = Symbol('lastPromise');
 const kHandlePromise = Symbol('handlePromise');
 const kStream = Symbol('stream');
 
-const AsyncIteratorRecord = class AsyncIteratorRecord {
-  constructor(value, done) {
-    this.done = done;
-    this.value = value;
-  }
-};
+function createIterResult(value, done) {
+  return { value, done };
+}
 
 function readAndResolve(iter) {
   const resolve = iter[kLastResolve];
@@ -26,7 +23,7 @@ function readAndResolve(iter) {
       iter[kLastPromise] = null;
       iter[kLastResolve] = null;
       iter[kLastReject] = null;
-      resolve(new AsyncIteratorRecord(data, false));
+      resolve(createIterResult(data, false));
     }
   }
 }
@@ -43,7 +40,7 @@ function onEnd(iter) {
     iter[kLastPromise] = null;
     iter[kLastResolve] = null;
     iter[kLastReject] = null;
-    resolve(new AsyncIteratorRecord(null, true));
+    resolve(createIterResult(null, true));
   }
   iter[kEnded] = true;
 }
@@ -69,39 +66,13 @@ function wrapForNext(lastPromise, iter) {
   };
 }
 
-const ReadableAsyncIterator = class ReadableAsyncIterator {
-  constructor(stream) {
-    this[kStream] = stream;
-    this[kLastResolve] = null;
-    this[kLastReject] = null;
-    this[kError] = null;
-    this[kEnded] = false;
-    this[kLastPromise] = null;
-
-    stream.on('readable', onReadable.bind(null, this));
-    stream.on('end', onEnd.bind(null, this));
-    stream.on('error', onError.bind(null, this));
-
-    // the function passed to new Promise
-    // is cached so we avoid allocating a new
-    // closure at every run
-    this[kHandlePromise] = (resolve, reject) => {
-      const data = this[kStream].read();
-      if (data) {
-        this[kLastPromise] = null;
-        this[kLastResolve] = null;
-        this[kLastReject] = null;
-        resolve(new AsyncIteratorRecord(data, false));
-      } else {
-        this[kLastResolve] = resolve;
-        this[kLastReject] = reject;
-      }
-    };
-  }
+const AsyncIteratorPrototype = Object.getPrototypeOf(
+  Object.getPrototypeOf(async function* () {}).prototype);
 
+const ReadableStreamAsyncIteratorPrototype = Object.setPrototypeOf({
   get stream() {
     return this[kStream];
-  }
+  },
 
   next() {
     // if we have detected an error in the meanwhile
@@ -112,7 +83,7 @@ const ReadableAsyncIterator = class ReadableAsyncIterator {
     }
 
     if (this[kEnded]) {
-      return Promise.resolve(new AsyncIteratorRecord(null, true));
+      return Promise.resolve(createIterResult(null, true));
     }
 
     // if we have multiple next() calls
@@ -129,7 +100,7 @@ const ReadableAsyncIterator = class ReadableAsyncIterator {
       // without triggering the next() queue
       const data = this[kStream].read();
       if (data !== null) {
-        return Promise.resolve(new AsyncIteratorRecord(data, false));
+        return Promise.resolve(createIterResult(data, false));
       }
 
       promise = new Promise(this[kHandlePromise]);
@@ -138,7 +109,7 @@ const ReadableAsyncIterator = class ReadableAsyncIterator {
     this[kLastPromise] = promise;
 
     return promise;
-  }
+  },
 
   return() {
     // destroy(err, cb) is a private API
@@ -150,10 +121,45 @@ const ReadableAsyncIterator = class ReadableAsyncIterator {
           reject(err);
           return;
         }
-        resolve(new AsyncIteratorRecord(null, true));
+        resolve(createIterResult(null, true));
       });
     });
-  }
+  },
+}, AsyncIteratorPrototype);
+
+const createReadableStreamAsyncIterator = (stream) => {
+  const iterator = Object.create(ReadableStreamAsyncIteratorPrototype, {
+    [kStream]: { value: stream, writable: true },
+    [kLastResolve]: { value: null, writable: true },
+    [kLastReject]: { value: null, writable: true },
+    [kError]: { value: null, writable: true },
+    [kEnded]: { value: false, writable: true },
+    [kLastPromise]: { value: null, writable: true },
+    // the function passed to new Promise
+    // is cached so we avoid allocating a new
+    // closure at every run
+    [kHandlePromise]: {
+      value: (resolve, reject) => {
+        const data = iterator[kStream].read();
+        if (data) {
+          iterator[kLastPromise] = null;
+          iterator[kLastResolve] = null;
+          iterator[kLastReject] = null;
+          resolve(createIterResult(data, false));
+        } else {
+          iterator[kLastResolve] = resolve;
+          iterator[kLastReject] = reject;
+        }
+      },
+      writable: true,
+    },
+  });
+
+  stream.on('readable', onReadable.bind(null, iterator));
+  stream.on('end', onEnd.bind(null, iterator));
+  stream.on('error', onError.bind(null, iterator));
+
+  return iterator;
 };
 
-module.exports = ReadableAsyncIterator;
+module.exports = createReadableStreamAsyncIterator;

test/parallel/test-stream-readable-async-iterators.js

@@ -5,6 +5,28 @@ const { Readable } = require('stream');
 const assert = require('assert');
 
 async function tests() {
+  {
+    const AsyncIteratorPrototype = Object.getPrototypeOf(
+      Object.getPrototypeOf(async function* () {}).prototype);
+    const rs = new Readable({});
+    assert.strictEqual(
+      Object.getPrototypeOf(Object.getPrototypeOf(rs[Symbol.asyncIterator]())),
+      AsyncIteratorPrototype);
+  }
+
+  await (async function() {
+    const readable = new Readable({ objectMode: true, read() {} });
+    readable.push(0);
+    readable.push(1);
+    readable.push(null);
+
+    const iter = readable[Symbol.asyncIterator]();
+    assert.strictEqual((await iter.next()).value, 0);
+    for await (const d of iter) {
+      assert.strictEqual(d, 1);
+    }
+  })();
+
   await (async function() {
     console.log('read without for..await');
     const max = 5;