FileAPI/blob/Blob-array-buffer-data-isolation.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>Blob data isolation from input ArrayBuffers</title>
<link rel="help" href="https://w3c.github.io/FileAPI/#constructorBlob">

<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<script>
"use strict";

// These tests verify that Blob correctly copies its input data, so that
// subsequent modifications to the input do not affect the Blob's contents.
// This is especially important to test with small buffers that might trigger
// buffer pool optimizations in some implementations.

function readBlobAsArrayBuffer(blob) {
  return new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.onload = () => resolve(reader.result);
    reader.onerror = () => reject(reader.error);
    reader.readAsArrayBuffer(blob);
  });
}

promise_test(async () => {
  const input = new Uint8Array([1, 2, 3, 4]);
  const blob = new Blob([input]);

  input[0] = 99;

  const result = new Uint8Array(await readBlobAsArrayBuffer(blob));
  assert_array_equals(result, [1, 2, 3, 4], "Blob data should not be affected by modifying the input Uint8Array");
}, "Modifying a Uint8Array after Blob construction does not affect the Blob");

promise_test(async () => {
  const input = new Uint8Array([5, 6, 7, 8]);
  const inputBuffer = input.buffer;
  const blob = new Blob([inputBuffer]);

  input[0] = 99;

  const result = new Uint8Array(await readBlobAsArrayBuffer(blob));
  assert_array_equals(result, [5, 6, 7, 8], "Blob data should not be affected by modifying the underlying ArrayBuffer");
}, "Modifying an ArrayBuffer after Blob construction does not affect the Blob");

promise_test(async () => {
  // Test with a view into a larger buffer (common pattern that exercises offset handling)
  const largerBuffer = new ArrayBuffer(16);
  const view = new Uint8Array(largerBuffer, 4, 4);
  view.set([10, 11, 12, 13]);

  const blob = new Blob([view]);

  view[0] = 99;

  const result = new Uint8Array(await readBlobAsArrayBuffer(blob));
  assert_array_equals(result, [10, 11, 12, 13], "Blob data should not be affected by modifying a typed array view");
}, "Modifying a typed array view after Blob construction does not affect the Blob");

promise_test(async () => {
  // Very small buffer - most likely to hit buffer pool optimizations
  const input = new Uint8Array([42]);
  const blob = new Blob([input]);

  input[0] = 0;

  const result = new Uint8Array(await readBlobAsArrayBuffer(blob));
  assert_array_equals(result, [42], "Blob data should not be affected even with single-byte input");
}, "Modifying a single-byte Uint8Array after Blob construction does not affect the Blob");

promise_test(async () => {
  const blob = new Blob([new Uint8Array([1, 2, 3, 4])]);

  const result1 = await readBlobAsArrayBuffer(blob);
  new Uint8Array(result1)[0] = 99;

  const result2 = new Uint8Array(await readBlobAsArrayBuffer(blob));
  assert_array_equals(
    result2,
    [1, 2, 3, 4],
    "Blob data should not be affected by modifying a previously-read ArrayBuffer"
  );
}, "Modifying a returned ArrayBuffer does not affect subsequent reads");
</script>