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>

Author: addaleax

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');
+constassert=require('assert');
+
+constkCallback=Symbol('Callback');
+constkOtherSide=Symbol('Other');
+
+classDuplexSocketextendsDuplex{
+constructor(){
+super();
+this[kCallback]=null;
+this[kOtherSide]=null;
+}
+
+_read(){
+constcallback=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);
+}
+}
+
+functionmakeDuplexPair(){
+constclientSide=newDuplexSocket();
+constserverSide=newDuplexSocket();
+clientSide[kOtherSide]=serverSide;
+serverSide[kOtherSide]=clientSide;
+return{ clientSide, serverSide };
+}
+
+module.exports=makeDuplexPair;