stream: 'data' argument on callback of Transform._flush()

Add a `data` argument on Transform._flush() callback to be API consistent with Transform._transform().

Fixes: #3707

lib/_stream_transform.js

@@ -115,8 +115,8 @@ function Transform(options) {
 
   this.once('prefinish', function() {
     if (typeof this._flush === 'function')
-      this._flush(function(er) {
-        done(stream, er);
+      this._flush(function(er, data) {
+        done(stream, er, data);
       });
     else
       done(stream);
@@ -173,10 +173,13 @@ Transform.prototype._read = function(n) {
 };
 
 
-function done(stream, er) {
+function done(stream, er, data) {
   if (er)
     return stream.emit('error', er);
 
+  if (data !== null && data !== undefined)
+    stream.push(data);
+
   // if there's nothing in the write buffer, then that means
   // that nothing more will ever be provided
   var ws = stream._writableState;

test/parallel/test-stream-transform-flush-data.js

@@ -0,0 +1,27 @@
+'use strict';
+
+require('../common');
+
+const assert = require('assert');
+const Transform = require('stream').Transform;
+
+
+const expected = 'asdf';
+
+
+function _transform(d, e, n) {
+  n();
+}
+function _flush(n) {
+  n(null, expected);
+}
+
+var t = new Transform({
+  transform: _transform,
+  flush: _flush
+});
+
+t.end(Buffer.from('blerg'));
+t.on('data', (data) => {
+  assert.strictEqual(data.toString(), expected);
+});