fix: improve ArrayBuffer brand check in ensureBuffer (#429)

`instanceof` is notoriously unreliable in JS, because it can
be fooled by prototype manipulation and breaks down entirely
for cross-realm brand checks.

Using `Object.prototype.toString` is generally a better choice
for this.

This also adds `SharedArrayBuffer` as an alternative, as both
should be treated the same under most circumstances.

Author: addaleax

src/ensure_buffer.ts

@@ -18,7 +18,11 @@ export function ensureBuffer(potentialBuffer: Buffer | ArrayBufferView | ArrayBu
     return Buffer.from(potentialBuffer.buffer);
   }
 
-  if (potentialBuffer instanceof ArrayBuffer) {
+  if (
+    ['[object ArrayBuffer]', '[object SharedArrayBuffer]'].includes(
+      Object.prototype.toString.call(potentialBuffer)
+    )
+  ) {
     return Buffer.from(potentialBuffer);
   }
 

test/node/ensure_buffer_test.js

@@ -1,3 +1,4 @@
+/* global SharedArrayBuffer */
 'use strict';
 
 const { Buffer } = require('buffer');
@@ -19,7 +20,7 @@ describe('ensureBuffer tests', function () {
     expect(bufferOut).to.equal(bufferIn);
   });
 
-  it('should wrap a UInt8Array with a buffer', function () {
+  it('should wrap a Uint8Array with a buffer', function () {
     const arrayIn = Uint8Array.from([1, 2, 3]);
     let bufferOut;
 
@@ -31,6 +32,34 @@ describe('ensureBuffer tests', function () {
     expect(bufferOut.buffer).to.equal(arrayIn.buffer);
   });
 
+  it('should wrap a ArrayBuffer with a buffer', function () {
+    const arrayBufferIn = Uint8Array.from([1, 2, 3]).buffer;
+    let bufferOut;
+
+    expect(function () {
+      bufferOut = ensureBuffer(arrayBufferIn);
+    }).to.not.throw(Error);
+
+    expect(bufferOut).to.be.an.instanceOf(Buffer);
+    expect(bufferOut.buffer).to.equal(arrayBufferIn);
+  });
+
+  it('should wrap a SharedArrayBuffer with a buffer', function () {
+    if (typeof SharedArrayBuffer === 'undefined') {
+      this.skip();
+      return;
+    }
+    const arrayBufferIn = new SharedArrayBuffer(3);
+    let bufferOut;
+
+    expect(function () {
+      bufferOut = ensureBuffer(arrayBufferIn);
+    }).to.not.throw(Error);
+
+    expect(bufferOut).to.be.an.instanceOf(Buffer);
+    expect(bufferOut.buffer).to.equal(arrayBufferIn);
+  });
+
   [0, 12, -1, '', 'foo', null, undefined, ['list'], {}, /x/].forEach(function (item) {
     it(`should throw if input is ${typeof item}: ${item}`, function () {
       expect(function () {