test: verify heap buffer allocations occur

Check that small typed arrays, including `Buffer`s (unless allocated
by `Buffer.allocUnsafe()`), are indeed heap-allocated.

PR-URL: https://github.com/nodejs/node/pull/26301
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-buffer-backing-arraybuffer.js

@@ -0,0 +1,37 @@
+// Flags: --expose-internals
+'use strict';
+require('../common');
+const assert = require('assert');
+const { internalBinding } = require('internal/test/binding');
+const { arrayBufferViewHasBuffer } = internalBinding('util');
+
+const tests = [
+  { length: 0, expectOnHeap: true },
+  { length: 48, expectOnHeap: true },
+  { length: 96, expectOnHeap: false },
+  { length: 1024, expectOnHeap: false },
+];
+
+for (const { length, expectOnHeap } of tests) {
+  const arrays = [
+    new Uint8Array(length),
+    new Uint16Array(length / 2),
+    new Uint32Array(length / 4),
+    new Float32Array(length / 4),
+    new Float64Array(length / 8),
+    Buffer.alloc(length),
+    Buffer.allocUnsafeSlow(length)
+    // Buffer.allocUnsafe() is missing because it may use pooled allocations.
+  ];
+
+  for (const array of arrays) {
+    const isOnHeap = !arrayBufferViewHasBuffer(array);
+    assert.strictEqual(isOnHeap, expectOnHeap,
+                       `mismatch: ${isOnHeap} vs ${expectOnHeap} ` +
+                       `for ${array.constructor.name}, length = ${length}`);
+
+    // Consistency check: Accessing .buffer should create it.
+    array.buffer;
+    assert(arrayBufferViewHasBuffer(array));
+  }
+}