streams: implement Readable.from async iterator utility

doc/api/stream.md

@@ -46,8 +46,8 @@ There are four fundamental stream types within Node.js:
 * [`Transform`][] - `Duplex` streams that can modify or transform the data as it
   is written and read (for example, [`zlib.createDeflate()`][]).
 
-Additionally, this module includes the utility functions [pipeline][] and
-[finished][].
+Additionally, this module includes the utility functions [pipeline][],
+[finished][] and [Readable.from][].
 
 ### Object Mode
 
@@ -1480,6 +1480,31 @@ async function run() {
 run().catch(console.error);
 ```
 
+### Readable.from(iterable, [options])
+
+* `iterable` {Iterable} Object implementing the `Symbol.asyncIterator` or
+  `Symbol.iterator` iterable protocol.
+* `options` {Object} Options provided to `new stream.Readable([options])`.
+  By default, `Readable.from()` will set `options.objectMode` to `true`, unless
+  this is explicitly opted out by setting `options.objectMode` to `false`.
+
+A utility method for creating Readable Streams out of iterators.
+
+```js
+const { Readable } = require('stream');
+
+async function * generate() {
+  yield 'hello';
+  yield 'streams';
+}
+
+const readable = Readable.from(generate());
+
+readable.on('data', (chunk) => {
+  console.log(chunk);
+});
+```
+
 ## API for Stream Implementers
 
 <!--type=misc-->
@@ -2395,6 +2420,89 @@ primarily for examples and testing, but there are some use cases where
 
 <!--type=misc-->
 
+### Streams Compatibility with Async Generators and Async Iterators
+
+With the support of async generators and iterators in JavaScript, async
+generators are effectively a first-class language-level stream construct at
+this point.
+
+Some common interop cases of using Node.js streams with async generators
+and async iterators are provided below.
+
+#### Consuming Readable Streams with Async Iterators
+
+```js
+(async function() {
+  for await (const chunk of readable) {
+    console.log(chunk);
+  }
+})();
+```
+
+#### Creating Readable Streams with Async Generators
+
+We can construct a Node.js Readable Stream from an asynchronous generator
+using the `Readable.from` utility method:
+
+```js
+const { Readable } = require('stream');
+
+async function * generate() {
+  yield 'a';
+  yield 'b';
+  yield 'c';
+}
+
+const readable = Readable.from(generate());
+
+readable.on('data', (chunk) => {
+  console.log(chunk);
+});
+```
+
+#### Piping to Writable Streams from Async Iterators
+
+In the scenario of writing to a writeable stream from an async iterator,
+it is important to ensure the correct handling of backpressure and errors.
+
+```js
+const { once } = require('events');
+
+const writeable = fs.createWriteStream('./file');
+
+(async function() {
+  for await (const chunk of iterator) {
+    // Handle backpressure on write
+    if (!writeable.write(value))
+      await once(writeable, 'drain');
+  }
+  writeable.end();
+  // Ensure completion without errors
+  await once(writeable, 'finish');
+})();
+```
+
+In the above, errors on the write stream would be caught and thrown by the two
+`once` listeners, since `once` will also handle `'error'` events.
+
+Alternatively the readable stream could be wrapped with `Readable.from` and
+then piped via `.pipe`:
+
+```js
+const { once } = require('events');
+
+const writeable = fs.createWriteStream('./file');
+
+(async function() {
+  const readable = Readable.from(iterator);
+  readable.pipe(writeable);
+  // Ensure completion without errors
+  await once(writeable, 'finish');
+})();
+```
+
+<!--type=misc-->
+
 ### Compatibility with Older Node.js Versions
 
 <!--type=misc-->
@@ -2531,6 +2639,7 @@ contain multi-byte characters.
 [Compatibility]: #stream_compatibility_with_older_node_js_versions
 [HTTP requests, on the client]: http.html#http_class_http_clientrequest
 [HTTP responses, on the server]: http.html#http_class_http_serverresponse
+[Readable.from]: #readable.from
 [TCP sockets]: net.html#net_class_net_socket
 [child process stdin]: child_process.html#child_process_subprocess_stdin
 [child process stdout and stderr]: child_process.html#child_process_subprocess_stdout

lib/_stream_readable.js

@@ -1139,3 +1139,42 @@ function endReadableNT(state, stream) {
     }
   }
 }
+
+Readable.from = function(iterable, opts) {
+  let iterator;
+  if (iterable && iterable[Symbol.asyncIterator])
+    iterator = iterable[Symbol.asyncIterator]();
+  else if (iterable && iterable[Symbol.iterator])
+    iterator = iterable[Symbol.iterator]();
+  else
+    throw new ERR_INVALID_ARG_TYPE('iterable', ['Iterable'], iterable);
+
+  const readable = new Readable({
+    objectMode: true,
+    ...opts
+  });
+  // Reading boolean to protect against _read
+  // being called before last iteration completion.
+  let reading = false;
+  readable._read = function() {
+    if (!reading) {
+      reading = true;
+      next();
+    }
+  };
+  async function next() {
+    try {
+      const { value, done } = await iterator.next();
+      if (done) {
+        readable.push(null);
+      } else if (readable.push(await value)) {
+        next();
+      } else {
+        reading = false;
+      }
+    } catch (err) {
+      readable.destroy(err);
+    }
+  }
+  return readable;
+};

test/parallel/test-events-once.js

@@ -90,4 +90,4 @@ Promise.all([
   catchesErrors(),
   stopListeningAfterCatchingError(),
   onceError()
-]);
+]).then(common.mustCall());

test/parallel/test-readable-from.js

@@ -0,0 +1,163 @@
+'use strict';
+
+const { mustCall } = require('../common');
+const { once } = require('events');
+const { Readable } = require('stream');
+const { strictEqual } = require('assert');
+
+async function toReadableBasicSupport() {
+  async function * generate() {
+    yield 'a';
+    yield 'b';
+    yield 'c';
+  }
+
+  const stream = Readable.from(generate());
+
+  const expected = ['a', 'b', 'c'];
+
+  for await (const chunk of stream) {
+    strictEqual(chunk, expected.shift());
+  }
+}
+
+async function toReadableSyncIterator() {
+  function * generate() {
+    yield 'a';
+    yield 'b';
+    yield 'c';
+  }
+
+  const stream = Readable.from(generate());
+
+  const expected = ['a', 'b', 'c'];
+
+  for await (const chunk of stream) {
+    strictEqual(chunk, expected.shift());
+  }
+}
+
+async function toReadablePromises() {
+  const promises = [
+    Promise.resolve('a'),
+    Promise.resolve('b'),
+    Promise.resolve('c')
+  ];
+
+  const stream = Readable.from(promises);
+
+  const expected = ['a', 'b', 'c'];
+
+  for await (const chunk of stream) {
+    strictEqual(chunk, expected.shift());
+  }
+}
+
+async function toReadableString() {
+  const stream = Readable.from('abc');
+
+  const expected = ['a', 'b', 'c'];
+
+  for await (const chunk of stream) {
+    strictEqual(chunk, expected.shift());
+  }
+}
+
+async function toReadableOnData() {
+  async function * generate() {
+    yield 'a';
+    yield 'b';
+    yield 'c';
+  }
+
+  const stream = Readable.from(generate());
+
+  let iterations = 0;
+  const expected = ['a', 'b', 'c'];
+
+  stream.on('data', (chunk) => {
+    iterations++;
+    strictEqual(chunk, expected.shift());
+  });
+
+  await once(stream, 'end');
+
+  strictEqual(iterations, 3);
+}
+
+async function toReadableOnDataNonObject() {
+  async function * generate() {
+    yield 'a';
+    yield 'b';
+    yield 'c';
+  }
+
+  const stream = Readable.from(generate(), { objectMode: false });
+
+  let iterations = 0;
+  const expected = ['a', 'b', 'c'];
+
+  stream.on('data', (chunk) => {
+    iterations++;
+    strictEqual(chunk instanceof Buffer, true);
+    strictEqual(chunk.toString(), expected.shift());
+  });
+
+  await once(stream, 'end');
+
+  strictEqual(iterations, 3);
+}
+
+async function destroysTheStreamWhenThrowing() {
+  async function * generate() {
+    throw new Error('kaboom');
+  }
+
+  const stream = Readable.from(generate());
+
+  stream.read();
+
+  try {
+    await once(stream, 'error');
+  } catch (err) {
+    strictEqual(err.message, 'kaboom');
+    strictEqual(stream.destroyed, true);
+  }
+}
+
+async function asTransformStream() {
+  async function * generate(stream) {
+    for await (const chunk of stream) {
+      yield chunk.toUpperCase();
+    }
+  }
+
+  const source = new Readable({
+    objectMode: true,
+    read() {
+      this.push('a');
+      this.push('b');
+      this.push('c');
+      this.push(null);
+    }
+  });
+
+  const stream = Readable.from(generate(source));
+
+  const expected = ['A', 'B', 'C'];
+
+  for await (const chunk of stream) {
+    strictEqual(chunk, expected.shift());
+  }
+}
+
+Promise.all([
+  toReadableBasicSupport(),
+  toReadableSyncIterator(),
+  toReadablePromises(),
+  toReadableString(),
+  toReadableOnData(),
+  toReadableOnDataNonObject(),
+  destroysTheStreamWhenThrowing(),
+  asTransformStream()
+]).then(mustCall());