Re-entrant Mutex for Async Resource Locking

Snippet

Re-entrant Mutex for Async Resource Locking

In high-concurrency environments like Next.js Server Actions, race conditions can occur when multiple requests access the same shared resource. This Mutex implementation uses a promise chain to ensure that only one execution context runs the critical section at a time, preventing data corruption without blocking the main thread.

snippet.js

javascript

class AsyncMutex {
  private queue = Promise.resolve();
  private locked = false;
 
  async runExclusive<T>(callback: () => Promise<T>): Promise<T> {
    const previous = this.queue;
    let resolveLock: () => void;
    this.queue = new Promise(res => { resolveLock = res; });
 
    await previous;
    try {
      return await callback();
    } finally {
      resolveLock!();
    }
  }
}

nextjs

Breakdown

private queue = Promise.resolve();

Initializes an already resolved promise to start the execution chain.

await previous;

Wait for all previously queued operations to complete before proceeding.

resolveLock!();

Releases the lock, allowing the next task in the queue to begin.

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Re-entrant Mutex for Async Resource Locking

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