plugging-promise-race

Testing & Quality

Reference for the `Promise.race` cross-iteration handler-leak bug. Loads on demand when writing or reviewing concurrency code that uses `Promise.race`, `Promise.any`, or hand-rolled concurrency limiters.

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plugging-promise-race

Never re-race the same pool of promises across loop iterations. Each call to Promise.race([A, B, …]) attaches fresh .then handlers to every arm. A promise that survives N iterations accumulates N handler sets. See nodejs/node#17469 and @watchable/unpromise.

Patterns

Safe — both arms created per call:

const value = await Promise.race([
  fetchSomething(),
  new Promise((_, r) => setTimeout(() => r(new Error('timeout')), 5000)),
])

Leaky — pool survives across iterations, accumulating handlers:

while (queue.length) {
  const winner = await Promise.race(pool) // ← N handlers per arm by iteration N
  pool = pool.filter(p => p !== winner)
}

Same hazard for Promise.any and any long-lived arm such as an interrupt signal.

The fix

Use a single-waiter "slot available" signal. Each task's .then resolves a one-shot promiseWithResolvers that the loop awaits, then replaces. No persistent pool, nothing to stack.

let signal = Promise.withResolvers<Task>()
function startTask(task: Task) {
  task.run().then(() => {
    const prev = signal
    signal = Promise.withResolvers<Task>()
    prev.resolve(task)
  })
}
while (queue.length) {
  // launch up to N tasks
  while (running < N && queue.length) startTask(queue.shift()!)
  const finished = await signal.promise
  running -= 1
}

The arm being awaited is always fresh; nothing accumulates handlers.

Quick check

Before merging concurrency code, ask: does any arm of a Promise.race/Promise.any outlive the call? If yes, refactor to the single-waiter signal.