Serialise work per key

A shared lock over all writes is simple and slow: saving document A blocks saving document B for no reason. What you usually want is one lock per record.

Implement createKeyedLock(), returning a function run(key, task). task takes no arguments and returns a promise, and run returns a promise settling with that task's own result. Tasks sharing a key run one at a time, in the order run was called for that key, and a task never starts before the previous task for its key has settled. Tasks with different keys are independent and run concurrently. If a task rejects, only its own promise rejects — the next task for that key still runs.

What it has to do

• Never run two tasks with the same key at the same time.
• Start same-key tasks in call order.
• Let different keys run concurrently.
• Settle each run with its own task's result.
• Keep the key's queue moving after a task rejects.

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