stream: make Duplex inherits from DuplexBase

Add ability to subclass `stream.Duplex` without inheriting the
"no-half-open enforcer" regardless of the value of the `allowHalfOpen`
option.

PR-URL: https://github.com/nodejs/node/pull/18974
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Chen Gang <gangc.cxy@foxmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

lib/_stream_duplex.js

@@ -29,33 +29,15 @@
 module.exports = Duplex;
 
 const util = require('util');
-const Readable = require('_stream_readable');
+const DuplexBase = require('internal/streams/duplex_base');
-const Writable = require('_stream_writable');
 
-util.inherits(Duplex, Readable);
+util.inherits(Duplex, DuplexBase);
-
-{
-  // avoid scope creep, the keys array can then be collected
-  const keys = Object.keys(Writable.prototype);
-  for (var v = 0; v < keys.length; v++) {
-    const method = keys[v];
-    if (!Duplex.prototype[method])
-      Duplex.prototype[method] = Writable.prototype[method];
-  }
-}
 
 function Duplex(options) {
   if (!(this instanceof Duplex))
     return new Duplex(options);
 
-  Readable.call(this, options);
+  DuplexBase.call(this, options);
-  Writable.call(this, options);
-
-  if (options && options.readable === false)
-    this.readable = false;
-
-  if (options && options.writable === false)
-    this.writable = false;
 
   this.allowHalfOpen = true;
   if (options && options.allowHalfOpen === false) {

lib/internal/streams/duplex_base.js

@@ -0,0 +1,30 @@
+'use strict';
+
+const util = require('util');
+const Readable = require('_stream_readable');
+const Writable = require('_stream_writable');
+
+function DuplexBase(options) {
+  Readable.call(this, options);
+  Writable.call(this, options);
+
+  if (options && options.readable === false)
+    this.readable = false;
+
+  if (options && options.writable === false)
+    this.writable = false;
+}
+
+util.inherits(DuplexBase, Readable);
+
+{
+  // Avoid scope creep, the keys array can then be collected.
+  const keys = Object.keys(Writable.prototype);
+  for (var v = 0; v < keys.length; v++) {
+    const method = keys[v];
+    if (!DuplexBase.prototype[method])
+      DuplexBase.prototype[method] = Writable.prototype[method];
+  }
+}
+
+module.exports = DuplexBase;

node.gyp

@@ -146,6 +146,7 @@
       'lib/internal/streams/lazy_transform.js',
       'lib/internal/streams/async_iterator.js',
       'lib/internal/streams/BufferList.js',
+      'lib/internal/streams/duplex_base.js',
       'lib/internal/streams/duplexpair.js',
       'lib/internal/streams/legacy.js',
       'lib/internal/streams/destroy.js',