stream: improve multiple callback error message

When a transform stream's callback is called more than once,
an error is emitted with a somewhat confusing message. This
commit hopes to improve the quality of the error message.

Fixes: #12513
PR-URL: #12520
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_stream_transform.js

@@ -88,8 +88,10 @@ function afterTransform(stream, er, data) {
 
   var cb = ts.writecb;
 
-  if (!cb)
-    return stream.emit('error', new Error('no writecb in Transform class'));
+  if (!cb) {
+    return stream.emit('error',
+                       new Error('write callback called multiple times'));
+  }
 
   ts.writechunk = null;
   ts.writecb = null;

test/parallel/test-stream-transform-callback-twice.js

@@ -0,0 +1,14 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const { Transform } = require('stream');
+const stream = new Transform({
+  transform(chunk, enc, cb) { cb(); cb(); }
+});
+
+stream.on('error', common.mustCall((err) => {
+  assert.strictEqual(err.toString(),
+                     'Error: write callback called multiple times');
+}));
+
+stream.write('foo');