We're running into some tricky issues trying to design readable byte streams,
related to preventing observable data races but also minimizing the number of
copies. I wrote up a more full explanation of the problem at [1] for those
interested. Apparently similar problems have been encountered by a variety of
web specs, including web audio, EME, MSE, web crypto, and DataCues.
The basic problem is that we need a way to detach a section of an array buffer,
so that another thread can write to it without being observed, and then
un-detach it when the other thread is done. This would allow us to write code
like
```js
const ab = new ArrayBuffer(1024);
readableByteStream.readInto(ab, 0, 256).then(bytesRead => {
// here ab bytes 0-256 can be accessed
// bytes 0-bytesRead (often 0-256) will have the data
});
// here ab bytes 0-256 are detached---cannot be accessed, presumably throwing
upon access
```
What do we think? Are there technical reasons that make detaching sections of
an array buffer a bad idea or hard to implement? Or is this a plausible future?
NOTE: The proposed-for-ES2016 [ArrayBuffer.transfer][2] seems likely motivated
by similar issues. It can get us part of the way there, but the result is
awkward:
```js
const ab = new ArrayBuffer(1024);
readableByteStream.readInto(ab, 0, 256).then(({ result, bytesRead }) => {
// here ab is (still) completely detached
// but result is a 1024-byte ArrayBuffer with bytes 0-bytesRead having the
data
});
// here ab is completely detached, and no longer usable
```
This gets kind of ridiculous when you imagine reading more than one segment
into the array buffer: for each segment you have to pass in the
currently-non-detached array buffer, and you get back a new array buffer
variable you need to pass along---even though the entire time you are working
with the same backing memory, just transferred between the different JS
variables.
[1]: https://gist.github.com/domenic/de5e63ae8b1b8b8c3f6c
[2]:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer/transfer
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