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ArrayBuffer.prototype.transfer() proposal for JavaScript
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README.md

ArrayBuffer.prototype.transfer() proposal

This is a proposal to add a new method, transfer(), to JavaScript's ArrayBuffer class. It has not yet been presented to the JavaScript standards committee.

The problem

When dealing with binary data asynchronously, there is a conflict between efficiency and safety.

Consider a function such as the following:

function validateAndWrite(arrayBuffer) {
  // Do some asynchronous validation.
  await validate(arrayBuffer);

  // Assuming we've got here, it's valid; write it to disk.
  await fs.writeFile("data.bin", arrayBuffer);
}

This function, as written, is fairly efficient. But it is not safe, or predictable: the caller can modify any data they pass in at any time, and the function will just keep using the same data. Consider:

const data = new Uint8Array([0x01, 0x02, 0x03]);
validateAndWrite(data.buffer);
setTimeout(() => {
  data[0] = data[1] = data[2] = 0x00;
}, 50);

If validate() takes 49 milliseconds, then it's quite possible that validateAndWrite() ends up validating the original contents of the buffer (0x01 0x02 0x03) but writing the new contents of the buffer (0x00 0x00 0x00). That is, any security or robustness guarantees validate() was meant to provide are illusory.

(This gets worse if you consider how fs.writeFile() might be implemented; I have unconfirmed reports that Node.js's version directly reads the ArrayBuffer's memory from another thread, so that it's possible to get a race condition and end up writing 0x01 0x00 0x00 or similar.)

The defensive way to write this code is to make a copy of the buffer before processing it:

function validateAndWriteSafeButSlow(arrayBuffer) {
  // Copy first!
  const copy = arrayBuffer.slice();

  await validate(copy);
  await fs.writeFile("data.bin", copy);
}

However, this adds extra time and memory consumption to all binary-data-consuming functions; given that binary data often comes in large chunks, this is not generally desireable.

Detaching and transferring

Some web platform APIs, notably the various postMessage() methods and the BYOB reader mode for ReadableStream, have a solution for this dillema. When you pass an ArrayBuffer (or wrapper around one, such as a typed array) to one of these APIs, they take ownership of the data block encapsulated in the ArrayBuffer.

Concretely, they detach the ArrayBuffer object itself. Essentially, it no longer "owns" the memory, and attempting to use it will throw (sorta). These APIs then use the backing bytes for their own purposes.

This proposal is for exposing that same capability to JavaScript

ArrayBuffer.prototype.transfer()

To do this, we propose a new method, ab.transfer(), which detaches ab and returns a new ArrayBuffer pointing to the same data block. This would allow our above validateAndWrite() function to be safe and fast:

function validateAndWriteSafeAndFast(arrayBuffer) {
  // Transfer first!
  const transferred = arrayBuffer.transfer();

  await validate(transferred);
  await fs.writeFile("data.bin", transferred);
}

Now attempts to modify an ArrayBuffer after passing it to validateAndWriteSafeAndFast() will throw, making it clear that the function has taken ownership of the data and does not expect further modification of it.

As a bonus, you can use this for providing a strong signal to the engine that it can free an ArrayBuffer's memory, without needing to find all references to the ArrayBuffer object and null them out:

arrayBuffer.transfer(); // don't save the result anywhere

Bonus proposal: ArrayBuffer.prototype.realloc(newByteLength)

As a secondary API, while we're in the area, we propose a related API, called realloc(). This transfers the contents of the ArrayBuffer into a new one with a new length. It is expected to have similar semantics to the C realloc() function, allowing in-place expansion or contraction when possible.

The main use case for this is trimming an ArrayBuffer, while avoiding copies when possible.

For example, consider reading a file. You are given a low-level system API file.readInto(buffer, offset, count) that attempts to read count bytes from file into buffer starting at offset, and resolves with a promise for the number of bytes read:

const buffer = new ArrayBuffer(1024 * 1024);
const bytesRead = await file.readInto(buffer, 0, buffer.byteLength);

If the file is small, bytesRead might be much less than 1 MiB, and so you're wasting memory. You could fix this, but it would require a copy:

const tempBuffer = new ArrayBuffer(1024 * 1024);
const bytesRead = await file.readInto(tempBuffer, 0, tempBuffer.byteLength);
const buffer = tempBuffer.slice(0, bytesRead);

But with ArrayBuffer.prototype.realloc(), the implementation may be able to avoid the copy:

const tempBuffer = new ArrayBuffer(1024 * 1024);
const bytesRead = await file.readInto(tempBuffer, 0, tempBuffer.byteLength);
const buffer = tempBuffer.realloc(bytesRead);

If implementation conditions align correctly, no copies are performed here: buffer points to the same region of memory as tempBuffer, but now any bytes between bytesRead and 1024 * 1024 are freed, since they can be accessed neither via buffer (whose length is bytesRead) nor via tempBuffer (which is now detached).

I mainly envision this being used as an alternative for tempBuffer.slice(0, bytesRead) that developers can use whenever they want to resize, and don't need two copies of the data lying around. In that case they can just use realloc() and perhaps get a performance or memory improvement if the engine is in a good position to optimize.

This API is less important than the transfer() API, and I look forward to the committee's feedback as to whether we should pursue it or not.

FAQs

What about SharedArrayBuffer?

It doesn't really make sense to add transfer() to SharedArrayBuffer.prototype. Certainly the use case, of avoiding concurrent access to the same data, doesn't make sense in this context. Additionally, we have no concept of detaching for SharedArrayBuffer instances.

History and acknowledgments

This proposal derives from Luke Wagner's ArrayBuffer.transfer strawperson, split into two distinct methods: transfer() and realloc(). That strawperson in turn derives from a suggestion of Dmitry Lomov. Thanks to them both!

At that time one of the major envisioned use cases for the proposal was to provide resizable memory for asm.js. That use case has largely been subsumed by WebAssembly, and so the proposal was abandoned. I picked it up because I believe there is still a strong use case for transferring and trimming ArrayBuffers even in pure JavaScript code.

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