stream: add no-half-open enforcer only if needed

The listener does not do anything if `allowHalfOpen` is enabled. Add it
only when `allowHalfOpen` is disabled.

PR-URL: #18953
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_stream_duplex.js

@@ -58,10 +58,10 @@ function Duplex(options) {
     this.writable = false;
 
   this.allowHalfOpen = true;
-  if (options && options.allowHalfOpen === false)
+  if (options && options.allowHalfOpen === false) {
     this.allowHalfOpen = false;
-
-  this.once('end', onend);
+    this.once('end', onend);
+  }
 }
 
 Object.defineProperty(Duplex.prototype, 'writableHighWaterMark', {
@@ -96,9 +96,8 @@ Object.defineProperty(Duplex.prototype, 'writableLength', {
 
 // the no-half-open enforcer
 function onend() {
-  // if we allow half-open state, or if the writable side ended,
-  // then we're ok.
-  if (this.allowHalfOpen || this._writableState.ended)
+  // If the writable side ended, then we're ok.
+  if (this._writableState.ended)
     return;
 
   // no more data can be written.

test/parallel/test-stream-duplex.js

@@ -29,6 +29,8 @@ const stream = new Duplex({ objectMode: true });
 assert(Duplex() instanceof Duplex);
 assert(stream._readableState.objectMode);
 assert(stream._writableState.objectMode);
+assert(stream.allowHalfOpen);
+assert.strictEqual(stream.listenerCount('end'), 0);
 
 let written;
 let read;