readline: add support for async iteration

Co-authored-by: Ivan Filenko <ivan.filenko@protonmail.com>
Fixes: #18603
Refs: #18904
PR-URL: #23916
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>

doc/api/readline.md

@@ -309,6 +309,43 @@ rl.write(null, { ctrl: true, name: 'u' });
 The `rl.write()` method will write the data to the `readline` `Interface`'s
 `input` *as if it were provided by the user*.
 
+### rl\[Symbol.asyncIterator\]()
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* Returns: {AsyncIterator}
+
+Create an `AsyncIterator` object that iterates through each line in the input
+stream as a string. This method allows asynchronous iteration of
+`readline.Interface` objects through `for`-`await`-`of` loops.
+
+Errors in the input stream are not forwarded.
+
+If the loop is terminated with `break`, `throw`, or `return`,
+[`rl.close()`][] will be called. In other words, iterating over a
+`readline.Interface` will always consume the input stream fully.
+
+A caveat with using this experimental API is that the performance is
+currently not on par with the traditional `'line'` event API, and thus it is
+not recommended for performance-sensitive applications. We expect this
+situation to improve in the future.
+
+```js
+async function processLineByLine() {
+  const rl = readline.createInterface({
+    // ...
+  });
+
+  for await (const line of rl) {
+    // Each line in the readline input will be successively available here as
+    // `line`.
+  }
+}
+```
+
 ## readline.clearLine(stream, dir)
 <!-- YAML
 added: v0.7.7
@@ -517,12 +554,38 @@ rl.on('line', (line) => {
 
 ## Example: Read File Stream Line-by-Line
 
-A common use case for `readline` is to consume input from a filesystem
-[Readable][] stream one line at a time:
+A common use case for `readline` is to consume an input file one line at a
+time. The easiest way to do so is leveraging the [`fs.ReadStream`][] API as
+well as a `for`-`await`-`of` loop:
 
 ```js
+const fs = require('fs');
 const readline = require('readline');
+
+async function processLineByLine() {
+  const fileStream = fs.createReadStream('input.txt');
+
+  const rl = readline.createInterface({
+    input: fileStream,
+    crlfDelay: Infinity
+  });
+  // Note: we use the crlfDelay option to recognize all instances of CR LF
+  // ('\r\n') in input.txt as a single line break.
+
+  for await (const line of rl) {
+    // Each line in input.txt will be successively available here as `line`.
+    console.log(`Line from file: ${line}`);
+  }
+}
+
+processLineByLine();
+```
+
+Alternatively, one could use the [`'line'`][] event:
+
+```js
 const fs = require('fs');
+const readline = require('readline');
 
 const rl = readline.createInterface({
   input: fs.createReadStream('sample.txt'),
@@ -536,8 +599,11 @@ rl.on('line', (line) => {
 
 [`'SIGCONT'`]: readline.html#readline_event_sigcont
 [`'SIGTSTP'`]: readline.html#readline_event_sigtstp
+[`'line'`]: #readline_event_line
+[`fs.ReadStream`]: fs.html#fs_class_fs_readstream
 [`process.stdin`]: process.html#process_process_stdin
 [`process.stdout`]: process.html#process_process_stdout
+[`rl.close()`]: #readline_rl_close
 [Readable]: stream.html#stream_readable_streams
 [TTY]: tty.html
 [Writable]: stream.html#stream_writable_streams

lib/readline.js

@@ -33,6 +33,7 @@ const {
   ERR_INVALID_OPT_VALUE
 } = require('internal/errors').codes;
 const { debug, inherits } = require('util');
+const { emitExperimentalWarning } = require('internal/util');
 const { Buffer } = require('buffer');
 const EventEmitter = require('events');
 const {
@@ -54,11 +55,16 @@ const {
 // Lazy load StringDecoder for startup performance.
 let StringDecoder;
 
+// Lazy load Readable for startup performance.
+let Readable;
+
 const kHistorySize = 30;
 const kMincrlfDelay = 100;
 // \r\n, \n, or \r followed by something other than \n
 const lineEnding = /\r?\n|\r(?!\n)/;
 
+const kLineObjectStream = Symbol('line object stream');
+
 const KEYPRESS_DECODER = Symbol('keypress-decoder');
 const ESCAPE_DECODER = Symbol('escape-decoder');
 
@@ -190,6 +196,8 @@ function Interface(input, output, completer, terminal) {
     self._refreshLine();
   }
 
+  this[kLineObjectStream] = undefined;
+
   if (!this.terminal) {
     function onSelfCloseWithoutTerminal() {
       input.removeListener('data', ondata);
@@ -1019,6 +1027,41 @@ Interface.prototype._ttyWrite = function(s, key) {
   }
 };
 
+Interface.prototype[Symbol.asyncIterator] = function() {
+  emitExperimentalWarning('readline Interface [Symbol.asyncIterator]');
+
+  if (this[kLineObjectStream] === undefined) {
+    if (Readable === undefined) {
+      Readable = require('stream').Readable;
+    }
+    const readable = new Readable({
+      objectMode: true,
+      read: () => {
+        this.resume();
+      },
+      destroy: (err, cb) => {
+        this.off('line', lineListener);
+        this.off('close', closeListener);
+        this.close();
+        cb(err);
+      }
+    });
+    const lineListener = (input) => {
+      if (!readable.push(input)) {
+        this.pause();
+      }
+    };
+    const closeListener = () => {
+      readable.push(null);
+    };
+    this.on('line', lineListener);
+    this.on('close', closeListener);
+    this[kLineObjectStream] = readable;
+  }
+
+  return this[kLineObjectStream][Symbol.asyncIterator]();
+};
+
 /**
  * accepts a readable Stream instance and makes it emit "keypress" events
  */

test/parallel/test-readline-async-iterators-backpressure.js

@@ -0,0 +1,48 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const { Readable } = require('stream');
+const readline = require('readline');
+
+const CONTENT = 'content';
+const TOTAL_LINES = 18;
+
+(async () => {
+  const readable = new Readable({ read() {} });
+  readable.push(`${CONTENT}\n`.repeat(TOTAL_LINES));
+
+  const rli = readline.createInterface({
+    input: readable,
+    crlfDelay: Infinity
+  });
+
+  const it = rli[Symbol.asyncIterator]();
+  const highWaterMark = it.stream.readableHighWaterMark;
+
+  // For this test to work, we have to queue up more than the number of
+  // highWaterMark items in rli. Make sure that is the case.
+  assert(TOTAL_LINES > highWaterMark);
+
+  let iterations = 0;
+  let readableEnded = false;
+  for await (const line of it) {
+    assert.strictEqual(readableEnded, false);
+
+    assert.strictEqual(line, CONTENT);
+
+    const expectedPaused = TOTAL_LINES - iterations > highWaterMark;
+    assert.strictEqual(readable.isPaused(), expectedPaused);
+
+    iterations += 1;
+
+    // We have to end the input stream asynchronously for back pressure to work.
+    // Only end when we have reached the final line.
+    if (iterations === TOTAL_LINES) {
+      readable.push(null);
+      readableEnded = true;
+    }
+  }
+
+  assert.strictEqual(iterations, TOTAL_LINES);
+})().then(common.mustCall());

test/parallel/test-readline-async-iterators-destroy.js

@@ -0,0 +1,78 @@
+'use strict';
+
+const common = require('../common');
+const fs = require('fs');
+const { join } = require('path');
+const readline = require('readline');
+const assert = require('assert');
+
+const tmpdir = require('../common/tmpdir');
+tmpdir.refresh();
+
+const filename = join(tmpdir.path, 'test.txt');
+
+const testContents = [
+  '',
+  '\n',
+  'line 1',
+  'line 1\nline 2 南越国是前203年至前111年存在于岭南地区的一个国家\nline 3\ntrailing',
+  'line 1\nline 2\nline 3 ends with newline\n'
+];
+
+async function testSimpleDestroy() {
+  for (const fileContent of testContents) {
+    fs.writeFileSync(filename, fileContent);
+
+    const readable = fs.createReadStream(filename);
+    const rli = readline.createInterface({
+      input: readable,
+      crlfDelay: Infinity
+    });
+
+    const iteratedLines = [];
+    for await (const k of rli) {
+      iteratedLines.push(k);
+      break;
+    }
+
+    const expectedLines = fileContent.split('\n');
+    if (expectedLines[expectedLines.length - 1] === '') {
+      expectedLines.pop();
+    }
+    expectedLines.splice(1);
+
+    assert.deepStrictEqual(iteratedLines, expectedLines);
+  }
+}
+
+async function testMutualDestroy() {
+  for (const fileContent of testContents) {
+    fs.writeFileSync(filename, fileContent);
+
+    const readable = fs.createReadStream(filename);
+    const rli = readline.createInterface({
+      input: readable,
+      crlfDelay: Infinity
+    });
+
+    const expectedLines = fileContent.split('\n');
+    if (expectedLines[expectedLines.length - 1] === '') {
+      expectedLines.pop();
+    }
+    expectedLines.splice(2);
+
+    const iteratedLines = [];
+    for await (const k of rli) {
+      iteratedLines.push(k);
+      for await (const l of rli) {
+        iteratedLines.push(l);
+        break;
+      }
+      assert.deepStrictEqual(iteratedLines, expectedLines);
+    }
+
+    assert.deepStrictEqual(iteratedLines, expectedLines);
+  }
+}
+
+testSimpleDestroy().then(testMutualDestroy).then(common.mustCall());

test/parallel/test-readline-async-iterators.js

@@ -0,0 +1,77 @@
+'use strict';
+
+const common = require('../common');
+const fs = require('fs');
+const { join } = require('path');
+const readline = require('readline');
+const assert = require('assert');
+
+const tmpdir = require('../common/tmpdir');
+tmpdir.refresh();
+
+const filename = join(tmpdir.path, 'test.txt');
+
+const testContents = [
+  '',
+  '\n',
+  'line 1',
+  'line 1\nline 2 南越国是前203年至前111年存在于岭南地区的一个国家\nline 3\ntrailing',
+  'line 1\nline 2\nline 3 ends with newline\n'
+];
+
+async function testSimple() {
+  for (const fileContent of testContents) {
+    fs.writeFileSync(filename, fileContent);
+
+    const readable = fs.createReadStream(filename);
+    const rli = readline.createInterface({
+      input: readable,
+      crlfDelay: Infinity
+    });
+
+    const iteratedLines = [];
+    for await (const k of rli) {
+      iteratedLines.push(k);
+    }
+
+    const expectedLines = fileContent.split('\n');
+    if (expectedLines[expectedLines.length - 1] === '') {
+      expectedLines.pop();
+    }
+    assert.deepStrictEqual(iteratedLines, expectedLines);
+    assert.strictEqual(iteratedLines.join(''), fileContent.replace(/\n/gm, ''));
+  }
+}
+
+async function testMutual() {
+  for (const fileContent of testContents) {
+    fs.writeFileSync(filename, fileContent);
+
+    const readable = fs.createReadStream(filename);
+    const rli = readline.createInterface({
+      input: readable,
+      crlfDelay: Infinity
+    });
+
+    const expectedLines = fileContent.split('\n');
+    if (expectedLines[expectedLines.length - 1] === '') {
+      expectedLines.pop();
+    }
+    const iteratedLines = [];
+    let iterated = false;
+    for await (const k of rli) {
+      // This outer loop should only iterate once.
+      assert.strictEqual(iterated, false);
+      iterated = true;
+
+      iteratedLines.push(k);
+      for await (const l of rli) {
+        iteratedLines.push(l);
+      }
+      assert.deepStrictEqual(iteratedLines, expectedLines);
+    }
+    assert.deepStrictEqual(iteratedLines, expectedLines);
+  }
+}
+
+testSimple().then(testMutual).then(common.mustCall());

tools/doc/type-parser.js

@@ -26,7 +26,7 @@ const customTypesMap = {
 
   'this': `${jsDocPrefix}Reference/Operators/this`,
 
-  'AsyncIterator': 'https://github.com/tc39/proposal-async-iteration',
+  'AsyncIterator': 'https://tc39.github.io/ecma262/#sec-asynciterator-interface',
 
   'bigint': 'https://github.com/tc39/proposal-bigint',