test: add `makeDuplexPair()` helper

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

PR-URL: https://github.com/nodejs/node/pull/16269
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

1 files changed, 45 insertions, 0 deletions

diff --git a/test/common/duplexpair.js b/test/common/duplexpair.js
new file mode 100644
index 0000000000..ea5bd86a04
--- /dev/null
+++ b/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;