test: add string-decoder fuzz test

PR-URL: #22709
Fixes: #22626
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-string-decoder-fuzz.js

@@ -0,0 +1,48 @@
+'use strict';
+require('../common');
+const { StringDecoder } = require('string_decoder');
+const util = require('util');
+const assert = require('assert');
+
+// Tests that, for random sequences of bytes, our StringDecoder gives the
+// same result as a direction conversion using Buffer.toString().
+// In particular, it checks that StringDecoder aligns with V8’s own output.
+
+function rand(max) {
+  return Math.floor(Math.random() * max);
+}
+
+function randBuf(maxLen) {
+  const buf = Buffer.allocUnsafe(rand(maxLen));
+  for (let i = 0; i < buf.length; i++)
+    buf[i] = rand(256);
+  return buf;
+}
+
+const encodings = [
+  'utf16le', 'utf8', 'ascii', 'hex', 'base64', 'latin1'
+];
+
+function runSingleFuzzTest() {
+  const enc = encodings[rand(encodings.length)];
+  const sd = new StringDecoder(enc);
+  const bufs = [];
+  const strings = [];
+
+  const N = rand(10);
+  for (let i = 0; i < N; ++i) {
+    const buf = randBuf(50);
+    bufs.push(buf);
+    strings.push(sd.write(buf));
+  }
+  strings.push(sd.end());
+
+  assert.strictEqual(strings.join(''), Buffer.concat(bufs).toString(enc),
+                     `Mismatch:\n${util.inspect(strings)}\n` +
+                     util.inspect(bufs.map((buf) => buf.toString('hex'))) +
+                     `\nfor encoding ${enc}`);
+}
+
+const start = Date.now();
+while (Date.now() - start < 100)
+  runSingleFuzzTest();