stream: support readable/writableHighWaterMark for Duplex streams

Fixes: #14555

doc/api/stream.md

@@ -1752,6 +1752,10 @@ constructor and implement *both* the `readable._read()` and
   * `writableObjectMode` {boolean} Defaults to `false`. Sets `objectMode`
     for writable side of the stream. Has no effect if `objectMode`
     is `true`.
+  * `readableHighWaterMark` {number} Sets `highWaterMark` for the readable side
+    of the stream. Has no effect if `highWaterMark` is provided.
+  * `writableHighWaterMark` {number} Sets `highWaterMark` for the writable side
+    of the stream. Has no effect if `highWaterMark` is provided.
 
 For example:
 

lib/_stream_readable.js

@@ -61,18 +61,32 @@ function prependListener(emitter, event, fn) {
 function ReadableState(options, stream) {
   options = options || {};
 
+  // Duplex streams are both readable and writable, but share
+  // the same options object.
+  // However, some cases require setting options to different
+  // values for the readable and the writable sides of the duplex stream.
+  // These options can be provided separately as readableXXX and writableXXX.
+  var isDuplex = stream instanceof Stream.Duplex;
+
   // object stream flag. Used to make read(n) ignore n and to
   // make all the buffer merging and length checks go away
   this.objectMode = !!options.objectMode;
 
-  if (stream instanceof Stream.Duplex)
+  if (isDuplex)
     this.objectMode = this.objectMode || !!options.readableObjectMode;
 
   // the point at which it stops calling _read() to fill the buffer
   // Note: 0 is a valid value, means "don't call _read preemptively ever"
   var hwm = options.highWaterMark;
+  var readableHwm = options.readableHighWaterMark;
   var defaultHwm = this.objectMode ? 16 : 16 * 1024;
-  this.highWaterMark = (hwm || hwm === 0) ? hwm : defaultHwm;
+
+  if (hwm || hwm === 0)
+    this.highWaterMark = hwm;
+  else if (isDuplex && (readableHwm || readableHwm === 0))
+    this.highWaterMark = readableHwm;
+  else
+    this.highWaterMark = defaultHwm;
 
   // cast to ints.
   this.highWaterMark = Math.floor(this.highWaterMark);

lib/_stream_writable.js

@@ -41,19 +41,33 @@ function nop() {}
 function WritableState(options, stream) {
   options = options || {};
 
+  // Duplex streams are both readable and writable, but share
+  // the same options object.
+  // However, some cases require setting options to different
+  // values for the readable and the writable sides of the duplex stream.
+  // These options can be provided separately as readableXXX and writableXXX.
+  var isDuplex = stream instanceof Stream.Duplex;
+
   // object stream flag to indicate whether or not this stream
   // contains buffers or objects.
   this.objectMode = !!options.objectMode;
 
-  if (stream instanceof Stream.Duplex)
+  if (isDuplex)
     this.objectMode = this.objectMode || !!options.writableObjectMode;
 
   // the point at which write() starts returning false
   // Note: 0 is a valid value, means that we always return false if
   // the entire buffer is not flushed immediately on write()
   var hwm = options.highWaterMark;
+  var writableHwm = options.writableHighWaterMark;
   var defaultHwm = this.objectMode ? 16 : 16 * 1024;
-  this.highWaterMark = (hwm || hwm === 0) ? hwm : defaultHwm;
+
+  if (hwm || hwm === 0)
+    this.highWaterMark = hwm;
+  else if (isDuplex && (writableHwm || writableHwm === 0))
+    this.highWaterMark = writableHwm;
+  else
+    this.highWaterMark = defaultHwm;
 
   // cast to ints.
   this.highWaterMark = Math.floor(this.highWaterMark);

test/parallel/test-stream-transform-split-highwatermark.js

@@ -0,0 +1,71 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+
+const { Transform, Readable, Writable } = require('stream');
+
+const DEFAULT = 16 * 1024;
+
+function testTransform(expectedReadableHwm, expectedWritableHwm, options) {
+  const t = new Transform(options);
+  assert.strictEqual(t._readableState.highWaterMark, expectedReadableHwm);
+  assert.strictEqual(t._writableState.highWaterMark, expectedWritableHwm);
+}
+
+// test overriding defaultHwm
+testTransform(666, DEFAULT, { readableHighWaterMark: 666 });
+testTransform(DEFAULT, 777, { writableHighWaterMark: 777 });
+testTransform(666, 777, {
+  readableHighWaterMark: 666,
+  writableHighWaterMark: 777,
+});
+
+// test 0 overriding defaultHwm
+testTransform(0, DEFAULT, { readableHighWaterMark: 0 });
+testTransform(DEFAULT, 0, { writableHighWaterMark: 0 });
+
+// test highWaterMark overriding
+testTransform(555, 555, {
+  highWaterMark: 555,
+  readableHighWaterMark: 666,
+});
+testTransform(555, 555, {
+  highWaterMark: 555,
+  writableHighWaterMark: 777,
+});
+testTransform(555, 555, {
+  highWaterMark: 555,
+  readableHighWaterMark: 666,
+  writableHighWaterMark: 777,
+});
+
+// test highWaterMark = 0 overriding
+testTransform(0, 0, {
+  highWaterMark: 0,
+  readableHighWaterMark: 666,
+});
+testTransform(0, 0, {
+  highWaterMark: 0,
+  writableHighWaterMark: 777,
+});
+testTransform(0, 0, {
+  highWaterMark: 0,
+  readableHighWaterMark: 666,
+  writableHighWaterMark: 777,
+});
+
+// test undefined, null, NaN
+[undefined, null, NaN].forEach((v) => {
+  testTransform(DEFAULT, DEFAULT, { readableHighWaterMark: v });
+  testTransform(DEFAULT, DEFAULT, { writableHighWaterMark: v });
+  testTransform(666, DEFAULT, { highWaterMark: v, readableHighWaterMark: 666 });
+  testTransform(DEFAULT, 777, { highWaterMark: v, writableHighWaterMark: 777 });
+});
+
+// test non Duplex streams ignore the options
+{
+  const r = new Readable({ readableHighWaterMark: 666 });
+  assert.strictEqual(r._readableState.highWaterMark, DEFAULT);
+  const w = new Writable({ writableHighWaterMark: 777 });
+  assert.strictEqual(w._writableState.highWaterMark, DEFAULT);
+}