Mutex and Condition

Atomics.Mutex and Atomics.Condition, shared structs with no fields, and the UnlockToken that using unlocks.

Atomics.Mutex and Atomics.Condition are shared structs with no fields, so they can be stored in another shared struct's fields and sent to workers. A lock gives an UnlockToken, which using unlocks at the end of its block. A Condition waits with that token: it unlocks the Mutex while it waits and locks it again before it returns.

shared struct Queue {
  items = new SharedArray(16);
  size = 0;
  lock = new Atomics.Mutex();
  changed = new Atomics.Condition();
}

// on a worker: wait until there is something to take
{
  using token = Atomics.Mutex.lock(queue.lock);
  Atomics.Condition.waitFor(queue.changed, token, Infinity, () => queue.size > 0);
  // take an item, then tell the others
  Atomics.Condition.notify(queue.changed);
}

Atomics.load, store, exchange and compareExchange also take a shared struct and a field name (§2.3), for lock-free code on its fields. The code is in src/runtime/sync.ts and src/runtime/atomics.ts.