test: writable stream ending state

Add a test for _writableState.ending, when ending becomes true,
but the stream is not finished/ended yet.

PR-URL: #8707
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Related: #8686

lib/_stream_writable.js

@@ -43,6 +43,7 @@ function WritableState(options, stream) {
   // cast to ints.
   this.highWaterMark = ~~this.highWaterMark;
 
+  // drain event flag.
   this.needDrain = false;
   // at the start of calling end()
   this.ending = false;

test/parallel/test-stream-writableState-ending.js

@@ -0,0 +1,34 @@
+'use strict';
+
+require('../common');
+
+const assert = require('assert');
+const stream = require('stream');
+
+const writable = new stream.Writable();
+
+function testStates(ending, finished, ended) {
+  assert.strictEqual(writable._writableState.ending, ending);
+  assert.strictEqual(writable._writableState.finished, finished);
+  assert.strictEqual(writable._writableState.ended, ended);
+}
+
+writable._write = (chunk, encoding, cb) => {
+  // ending, finished, ended start in false.
+  testStates(false, false, false);
+  cb();
+};
+
+writable.on('finish', () => {
+  // ending, finished, ended = true.
+  testStates(true, true, true);
+});
+
+writable.end('testing function end()', () => {
+  // ending, finished, ended = true.
+  testStates(true, true, true);
+});
+
+// ending, ended = true.
+// finished = false.
+testStates(true, false, true);