doc: WorkerPool async_hooks example warns about max listeners

[loganfsmyth]

📗 API Reference Docs Problem

• Version: 15.0.1

• Platform: All

• Subsystem: async_hooks

Location

The example WorkerPool using async_hooks

Affected URL(s):

• https://nodejs.org/api/async_hooks.html#async_hooks_using_asyncresource_for_a_worker_thread_pool

Description

The code I'm explicitly concerned about is

  runTask(task, callback) {
    if (this.freeWorkers.length === 0) {
      // No free threads, wait until a worker thread becomes free.
      this.once(kWorkerFreedEvent, () => this.runTask(task, callback));
      return;
    }

    // ...

Since this code snippet is presented explicitly as how to do something and is very much code that developers could easily end up copy-pasting into their own codebase, I figured it was worth raising as a potential issue.

This code has 2 issues:

• This quickly triggers a MaxListenersExceededWarning since it adds a listener for every queued task and that number is unbounded
• This is O(N^2) because when the kWorkerFreedEvent fires, it fires every callback (even though N-1 will just be re-queued), and once listeners have to do a linear search to remove themselves when they fire

Thoughts?

✍️

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  submit a pull request.