Feature: Interface Transferable / Piscina.move()

[jasnell]

Currently, values returned by a worker are not transferable. That is, they will always be cloned because we currently have no way of telling Piscina that the value should be transferred instead.

module.exports = () => {
  const buf = Buffer.alloc(10);
  return buf; // Copies the buffer, rather than transferring it
}

My proposal is to add a new Piscina.move() function that takes an input object capable of being transferred and wraps it in a new Transferable interface (in typescript terms) that effectively marks it as being able to be transferred.

const { move } = require('piscina');

module.exports = () => {
  const buf = Buffer.alloc(10);
  return move(buf);
}

The return value of move() is a Transferable, a wrapper object that (a) marks the object as being transferable and (b) provides a reference to the actual internal transferable object (e.g. the underlying ArrayBuffer in the example above.

This would only work with top level objects... for instance, the following would not cause the buffer to be transferred:

module.exports = () => {
  return { m: move(Buffer.alloc(10)) }
}

The move() function would work out of the box with objects known to be transferable by Node.js (e.g. MessagePorts and TypedArrays). Users would be able to create their own transferable objects by implementing the Transferable interface on their classes.