Correct handling of resizable ArrayBuffer?

この issue では、リポジトリのファイルやテストが指定されていません。まず、ソース TypedArray が resizable ArrayBuffer を使用しており、コピー中にそのサイズが変更された場合の Blob 構築時に期待される動作を明らかにし、次に選択した動作と、仕様でエラーまたは別の結果を要求すべきかどうかを文書化してください。

Given const b = new Blob([A, B]), where A is a TypedArray with a resizable ArrayBuffer, what is the expected behavior?

Chrome/Firefox currently both throw an error if any of the source ArrayBuffer instances are resizable; other runtimes do not.

There is an issue here if A is resized while the Blob is being constructed and the data is being copied. This can happen, for instance, if B has a getter for it's length that causes A to be resized and the implementation uses a flow like...

let total = 0;
for (const chunk of chunks) {
  total += chunk.length;  // B could resize A as a side effect here
}
const dest = allocate(total);
for (const chunk of chunks) {
  copy chunk into dest
}

Let's suppose that A and B both initially have length 10, but B's length getter resizes A to 5... what should the result be?

1. Length 15, with 5 bytes from A, 10 bytes from B, allocation gets trimmed at the end of the copy
2. Length 20, with 5 bytes from A, 10 bytes from B, and 5 zeroed bytes at the end?
3. Length 20, with 5 bytes from A, 5 zeroed bytes, 10 bytes from B
4. Error thrown because length changed?
5. Something else?

What if A is resized larger? Is the result just truncated?

Should the spec for Blob explicitly make Chrome/Firefox's behavior of throwing immediately on resizable ArrayBuffer's standard? etc.

/cc @guybedford