concurrency-security

Files (1)

• SKILL.md

SKILL.md (first lines)

# Concurrency Security

Patterns for preventing race conditions, double-execution, and state corruption in concurrent systems.

## TOCTOU Prevention

Time-of-Check to Time-of-Use: the gap between reading state and acting on it.

```typescript
// WRONG: check then act - another process can change state between lines
const balance = await db.accounts.findUnique({ where: { id } })
if (balance.amount >= amount) {
  await db.accounts.update({ where: { id }, data: { amount: balance.amount - amount } })
}

// CORRECT: atomic check-and-update in a single statement
const updated = await db.$executeRaw`
  UPDATE accounts
  SET amount = amount - ${amount}
  WHERE id = ${id} AND amount >= ${amount}
  RETURNING *
`
if (updated.count === 0) throw new Error('Insufficient funds or concurrent update')
```

```typescript
// File system TOCTOU (Node.js)
// WRONG
if (fs.existsSync(filePath)) {
  fs.writeFileSync(filePath, data)  // another process may have deleted it
}

// CORRECT: use O_EXCL flag for exclusive creation
import { open } from 'fs/promises'
try {
  const fh = await open(filePath, 'wx')  // fail if file exists
  await fh.writeFile(data)
  await fh.close()
} catch (err: any) {
  if (err.code === 'EEXIST') { /* already exists, handle */ }
  throw err
}
```

## Distributed Locking with Redis

```typescript
import Redis from 'ioredis'

const redis = new Redis(process.env.REDIS_URL!)

// Simple SETNX + TTL lock
async function acquireLock(key: string, ttlMs: number): Promise<string | null> {
  const token = crypto.randomUUID()
  // SET key token NX PX ttlMs — atomic, returns OK or null
  const result = await redis.set(`lock:${key}`, token, 'NX', 'PX', ttlMs)
  return result === 'OK' ? token : null
}

async function releaseLock(key: string, token: string): Promise<void> {
  // Lua script: only delete if we own the lock (atomic compare-and-delete)
  const script = `
    if redis.call("GET", KEYS[1]) == ARGV[1] then
      return redis.call("DEL", KEYS[1])
    else
      return 0
    end
  `
  await redis.eval(script, 1, `lock:${key}`, token)
}

// Usage
async function processPayment(paymentId: string) {
  const token = await acquireLock(paymentId, 30_000)  // 30s TTL
  if (!token) throw new Error('Payment already being processed')

  try {
    await doPaymentWork(paymentId)
  } finally {
    await releaseLock(paymentId, token)
  }
}
```

### Redlock Algorithm (multi-node)

```typescript
import Redlock from 'redlock'
import Redis from 'ioredis'

// Connect to 3+ independent Redis nodes for Redlock quorum
const clients = [
  new Redis('redis://redis1:6379'),
  new Redis('redis://redis2:6379'),
  new Redis('redis://redis3:6379'),
]

const redlock = new Redlock(clients, {
  retryCount: 3,
  retryDelay: 200,
  retryJitter: 100,
})

async function criticalSection(resourceId: string) {
  await redlock.using([`resource:${resourceId}`], 10_000, async (signal) => {
    if (signal.aborted) throw signal.error

    await performAtomicOperation(resourceId)

    if (signal.aborted) throw signal.error  // check after long operations
  })
}
```

## PostgreSQL Advisory Locks

```typescript
import { Pool } from 'pg'
const pool = new Pool()

From vibeeval/vibecosystem, MIT.

More from vibeeval/vibecosystem