Shared Memory and Cross-Origin Isolation

1// 1. The gate is a runtime boolean, not a build-time assumption.
2export const canShare =
3  typeof SharedArrayBuffer !== 'undefined' && self.crossOriginIsolated === true
4 
5export function makePipe(slots: number, slotBytes: number) {
6  if (!canShare) return transferablePipe(slots, slotBytes) // exercised path, not a stub
7 
8  // [0] = writeIndex, [1] = readIndex, then the slot data
9  const control = new SharedArrayBuffer(2 * Int32Array.BYTES_PER_ELEMENT)
10  const data = new SharedArrayBuffer(slots * slotBytes)
11  const idx = new Int32Array(control)
12  const bytes = new Uint8Array(data)
13  return { control, data, idx, bytes, slots, slotBytes }
14}
15 
16// --- in the worker (producer) ---
17function publish(p: ReturnType<typeof makePipe> & { idx: Int32Array }, frame: Uint8Array) {
18  const w = Atomics.load(p.idx, 0)
19  const r = Atomics.load(p.idx, 1)
20  if ((w + 1) % p.slots === r) return false      // full: drop, do not overwrite
21 
22  p.bytes.set(frame, w * p.slotBytes)            // 1. fill the slot (plain writes)
23  Atomics.store(p.idx, 0, (w + 1) % p.slots)     // 2. THEN publish the index
24  Atomics.notify(p.idx, 0)                       // 3. wake anyone waiting
25  return true
26}
27 
28// --- on the main thread (consumer) ---
29// Atomics.wait() THROWS here, by design: a blocked main thread is a frozen
30// page and a frozen accessibility tree. Poll from rAF instead.
31function drain(p: ReturnType<typeof makePipe> & { idx: Int32Array }, draw: (b: Uint8Array) => void) {
32  const tick = () => {
33    const w = Atomics.load(p.idx, 0)
34    let r = Atomics.load(p.idx, 1)
35    while (r !== w) {
36      draw(p.bytes.subarray(r * p.slotBytes, (r + 1) * p.slotBytes))
37      r = (r + 1) % p.slots
38      Atomics.store(p.idx, 1, r)
39    }
40    requestAnimationFrame(tick)
41  }
42  requestAnimationFrame(tick)
43}

The ordering in publish is the whole correctness argument: fill, then atomically store the index. Reverse those two lines and the consumer can read a half-written frame — with no error, on some devices, sometimes.