How to Fix Race Condition in Deno

A race condition in Deno typically signals a straightforward problem with a clear solution. Understanding why it occurs is the first step toward a permanent fix.

Understanding Race Conditions

A race condition in Deno occurs when two or more concurrent operations access shared state, and the outcome depends on their execution timing. This leads to intermittent bugs that are notoriously difficult to reproduce because they depend on specific timing that may rarely occur in testing but frequently occurs under production load.

The Fix

// Deno/Node: mutex pattern for async operations
class AsyncMutex {
  private queue: (() => void)[] = [];
  private locked = false;

  async run<T>(fn: () => Promise<T>): Promise<T> {
    await this.acquire();
    try {
      return await fn();
    } finally {
      this.release();
    }
  }

  private acquire(): Promise<void> {
    return new Promise((resolve) => {
      if (!this.locked) { this.locked = true; resolve(); }
      else this.queue.push(resolve);
    });
  }

  private release(): void {
    const next = this.queue.shift();
    if (next) next(); else this.locked = false;
  }
}

const mutex = new AsyncMutex();
await mutex.run(async () => {
  const data = await readSharedState();
  await writeSharedState(data + 1);
});

Wrap critical sections in a mutex to serialize concurrent access. The run method ensures the lock is always released.

Detection Strategies

• Add structured logging with timestamps around critical sections to identify interleaving
• Use stress testing and concurrent load testing to increase the probability of reproducing the race
• Review all shared mutable state that is accessed from async code paths or multiple threads

Prevention

Race conditions are among the hardest bugs to catch in testing. [Bugsly](https://bugsly.dev) helps by correlating error timing patterns and identifying failures that cluster under high concurrency in your Deno application.