test: add makeDuplexPair() helper

Add a utility for adding simple, streams-API based duplex pairs.

PR-URL: #16269
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/common/README.md

@@ -5,6 +5,7 @@ This directory contains modules used to test the Node.js implementation.
 ## Table of Contents
 
 * [Common module API](#common-module-api)
+* [Duplex pair helper](#duplex-pair-helper)
 * [WPT module](#wpt-module)
 
 ## Common Module API
@@ -318,6 +319,14 @@ Decrements the `Countdown` counter.
 Specifies the remaining number of times `Countdown.prototype.dec()` must be
 called before the callback is invoked.
 
+## Duplex pair helper
+
+The `common/duplexpair` module exports a single function `makeDuplexPair`,
+which returns an object `{ clientSide, serverSide }` where each side is a
+`Duplex` stream connected to the other side.
+
+There is no difference between client or server side beyond their names.
+
 ## Fixtures Module
 
 The `common/fixtures` module provides convenience methods for working with

test/common/duplexpair.js

@@ -0,0 +1,45 @@
+/* eslint-disable required-modules */
+'use strict';
+const { Duplex } = require('stream');
+const assert = require('assert');
+
+const kCallback = Symbol('Callback');
+const kOtherSide = Symbol('Other');
+
+class DuplexSocket extends Duplex {
+  constructor() {
+    super();
+    this[kCallback] = null;
+    this[kOtherSide] = null;
+  }
+
+  _read() {
+    const callback = this[kCallback];
+    if (callback) {
+      this[kCallback] = null;
+      callback();
+    }
+  }
+
+  _write(chunk, encoding, callback) {
+    assert.notStrictEqual(this[kOtherSide], null);
+    assert.strictEqual(this[kOtherSide][kCallback], null);
+    this[kOtherSide][kCallback] = callback;
+    this[kOtherSide].push(chunk);
+  }
+
+  _final(callback) {
+    this[kOtherSide].on('end', callback);
+    this[kOtherSide].push(null);
+  }
+}
+
+function makeDuplexPair() {
+  const clientSide = new DuplexSocket();
+  const serverSide = new DuplexSocket();
+  clientSide[kOtherSide] = serverSide;
+  serverSide[kOtherSide] = clientSide;
+  return { clientSide, serverSide };
+}
+
+module.exports = makeDuplexPair;