Retry with full jitter

When an API goes down, every client fails at once and, with fixed backoff, retries at the same instant — a thundering herd that keeps the service down. AWS's 'full jitter' fix is to pick a random wait between zero and the backoff ceiling.

Implement retryWithJitter(task, options) where options may contain attempts (default 3), baseDelay (default 100) and random (default Math.random). Call task(attemptIndex) with the zero-based attempt number. Resolve with its value as soon as an attempt succeeds. After a failed attempt at index i, if further attempts remain, call random() exactly once and wait Math.floor(random() * baseDelay * 2 ** i) milliseconds before the next attempt. After the final attempt fails, reject with that last error and do not call random again.

What it has to do

• Call task with the zero-based attempt index.
• Resolve as soon as an attempt succeeds, without waiting again.
• Call random exactly once per wait, and never after the final attempt.
• Wait Math.floor(random() * baseDelay * 2 ** attemptIndex) milliseconds between attempts.
• Reject with the error from the final attempt.

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