test: test buffer behavior when zeroFill undefined

When ArrayBufferAllocator has an undefined zeroFill property,
Buffer.allocUnsafe() should zero fill.

Refs: 27e84ddd4e1#commitcomment-19182129

test/parallel/test-buffer-bindingobj-no-zerofill.js

@@ -0,0 +1,53 @@
+'use strict';
+
+// Flags: --expose-internals
+
+// Confirm that if a custom ArrayBufferAllocator does not define a zeroFill
+// property, that the buffer module will zero-fill when allocUnsafe() is called.
+
+require('../common');
+
+const assert = require('assert');
+const buffer = require('buffer');
+
+// Monkey-patch setupBufferJS() to have an undefined zeroFill
+const process = require('process');
+const originalBinding = process.binding;
+
+const binding = originalBinding('buffer');
+const originalSetup = binding.setupBufferJS;
+
+binding.setupBufferJS = (proto, obj) => {
+  originalSetup(proto, obj);
+  assert.strictEqual(obj.zeroFill[0], 1);
+  delete obj.zeroFill;
+};
+
+const bindingObj = {};
+
+binding.setupBufferJS(Buffer.prototype, bindingObj);
+assert.strictEqual(bindingObj.zeroFill, undefined);
+
+process.binding = (bindee) => {
+  if (bindee === 'buffer')
+    return binding;
+  return originalBinding(bindee);
+};
+
+// Load from file system because internal buffer is already loaded and we're
+// testing code that runs on first load only.
+// Do not move this require() to top of file. It is important that
+// `process.binding('buffer').setupBufferJS` be monkey-patched before this runs.
+const monkeyPatchedBuffer = require('../../lib/buffer');
+
+// On unpatched buffer. allocUnsafe() should not zero fill memory. It's always
+// possible that a segment of memory is already zeroed out, so try again and
+// again until we succeed or we time out.
+let uninitialized = buffer.Buffer.allocUnsafe(1024);
+while (uninitialized.some((val) => val !== 0))
+  uninitialized = buffer.Buffer.allocUnsafe(1024);
+
+// On monkeypatched buffer, zeroFill property is undefined. allocUnsafe() should
+// zero-fill in that case.
+const zeroFilled = monkeyPatchedBuffer.Buffer.allocUnsafe(1024);
+assert(zeroFilled.every((val) => val === 0));