Build a counting semaphore

A pool works when you already have the full list of tasks. When work arrives over time — a user clicking upload repeatedly — you need a permit you can acquire from anywhere and release when done.

Implement a class Semaphore. new Semaphore(max) allows max holders at once. acquire() returns a promise; it resolves immediately if fewer than max permits are held, and otherwise waits until one is released. It resolves with a release function that gives the permit back and hands it to the longest-waiting caller, in first-in-first-out order. Calling a release function more than once has no effect beyond the first call, so a double release can never let an extra caller through.

What it has to do

• Allow at most max holders at any time.
• Resolve acquire() immediately while permits are free.
• Hand a released permit to waiters in FIFO order.
• Resolve with a release function rather than a plain value.
• Make release idempotent: extra calls do nothing.

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