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Lock Detection** - Automatically removes locks older than 60 seconds\n- **Process Exit Cleanup** - Locks are always released, even on errors\n- **Zero Dependencies** - No external runtime dependencies\n- **TypeScript First** - Complete type definitions with strict mode\n- **Production Tested** - Battle-tested in BernierLLC tools monorepo\n\n## Installation\n\n```bash\nnpm install @bernierllc/file-lock\n```\n\n## Usage\n\n### Basic Lock Acquisition\n\n```typescript\nimport { acquireLock } from '@bernierllc/file-lock';\nimport * as fs from 'fs';\n\nconst unlock = await acquireLock('./important-file.json', {\n  maxRetries: 10,\n  timeoutMs: 30000\n});\n\ntry {\n  // Critical section - exclusive access to file\n  const data = await fs.promises.readFile('./important-file.json', 'utf8');\n  const obj = JSON.parse(data);\n  obj.updated = Date.now();\n  await fs.promises.writeFile('./important-file.json', JSON.stringify(obj, null, 2));\n} finally {\n  await unlock();\n}\n```\n\n### Convenience Wrapper\n\n```typescript\nimport { withFileLock } from '@bernierllc/file-lock';\nimport * as fs from 'fs';\n\nconst result = await withFileLock('./config.json', async () => {\n  const config = JSON.parse(await fs.promises.readFile('./config.json', 'utf8'));\n  config.lastUpdated = Date.now();\n  await fs.promises.writeFile('./config.json', JSON.stringify(config, null, 2));\n  return config;\n}, {\n  maxRetries: 5,\n  initialDelayMs: 100\n});\n\nconsole.log('Updated config:', result);\n```\n\n### Protecting Package Status Updates\n\n```typescript\nimport { withFileLock } from '@bernierllc/file-lock';\nimport * as fs from 'fs';\n\nasync function updatePackageStatus(packageName: string, updates: any) {\n  const statusFile = './PACKAGES_STATUS.json';\n\n  await withFileLock(statusFile, async () => {\n    const status = JSON.parse(await fs.promises.readFile(statusFile, 'utf8'));\n\n    status.packages[packageName] = {\n      ...status.packages[packageName],\n      ...updates,\n      lastUpdated: new Date().toISOString()\n    };\n\n    await fs.promises.writeFile(statusFile, JSON.stringify(status, null, 2));\n  }, {\n    maxRetries: 10,\n    timeoutMs: 30000\n  });\n}\n\n// Safe for concurrent execution\nawait Promise.all([\n  updatePackageStatus('validators-api', { status: 'completed' }),\n  updatePackageStatus('validators-email', { status: 'completed' }),\n  updatePackageStatus('validators-html', { status: 'completed' })\n]);\n```\n\n### Custom Retry Configuration\n\n```typescript\nimport { withFileLock } from '@bernierllc/file-lock';\n\nawait withFileLock('./shared-resource.json', async () => {\n  // Your critical section\n}, {\n  maxRetries: 20,           // More retry attempts\n  initialDelayMs: 50,       // Start with shorter delay\n  maxDelayMs: 10000,        // Allow longer max delay\n  timeoutMs: 60000,         // 1 minute total timeout\n  jitter: true              // Apply jitter (default)\n});\n```\n\n## API Reference\n\n### `acquireLock(filePath, options?)`\n\nAcquires an exclusive lock on a file.\n\n**Parameters:**\n\n- `filePath: string` - Path to file to lock\n- `options?: LockOptions` - Optional configuration\n\n**Returns:** `Promise<UnlockFunction>` - Function to release the lock\n\n**Throws:**\n\n- Error if lock cannot be acquired within timeout\n- Error if file path is invalid\n\n### `withFileLock(filePath, fn, options?)`\n\nExecutes a function with file lock protection (convenience wrapper).\n\n**Parameters:**\n\n- `filePath: string` - Path to file to lock\n- `fn: () => Promise<T>` - Async function to execute with lock held\n- `options?: LockOptions` - Optional configuration\n\n**Returns:** `Promise<T>` - Result of fn\n\n**Throws:**\n\n- Error if lock cannot be acquired\n- Propagates any error thrown by fn\n\n### `LockOptions` Interface\n\n```typescript\ninterface LockOptions {\n  maxRetries?: number;        // Maximum retry attempts (default: 10)\n  initialDelayMs?: number;    // Initial retry delay in ms (default: 100)\n  maxDelayMs?: number;        // Maximum retry delay in ms (default: 5000)\n  timeoutMs?: number;         // Total timeout in ms (default: 30000)\n  jitter?: boolean;           // Apply jitter to delays (default: true)\n}\n```\n\n### `LockData` Interface\n\n```typescript\ninterface LockData {\n  pid: number;                // Process ID of lock holder\n  timestamp: number;          // Lock acquisition timestamp\n  file: string;               // Path to locked file\n}\n```\n\n### `UnlockFunction` Type\n\n```typescript\ntype UnlockFunction = () => Promise<void>;\n```\n\n## Configuration\n\n### Default Values\n\n```typescript\n{\n  maxRetries: 10,          // 10 retry attempts\n  initialDelayMs: 100,     // Start with 100ms delay\n  maxDelayMs: 5000,        // Cap at 5 second delay\n  timeoutMs: 30000,        // 30 second total timeout\n  jitter: true             // Apply ±20% jitter\n}\n```\n\n### Exponential Backoff Schedule\n\nWith default settings and jitter disabled:\n\n- Attempt 1: 100ms\n- Attempt 2: 200ms\n- Attempt 3: 400ms\n- Attempt 4: 800ms\n- Attempt 5: 1600ms\n- Attempt 6+: 5000ms (capped)\n\nWith jitter enabled (default), each delay varies by ±20%.\n\n### Stale Lock Detection\n\nLocks older than **60 seconds** are automatically considered stale and removed. This prevents indefinite blocking when processes crash without cleanup.\n\n## Performance\n\n**Production metrics from BernierLLC tools monorepo:**\n\n- **Uncontended lock**: <1ms\n- **Light contention (2-3 processes)**: <100ms average\n- **Heavy contention (5+ processes)**: <500ms average\n- **Maximum timeout**: 30s (configurable)\n- **Success rate**: 100% (zero corruption incidents)\n\n## Lock File Format\n\nLock files use `.lock` suffix and contain JSON metadata:\n\n```json\n{\n  \"pid\": 12345,\n  \"timestamp\": 1696234567890,\n  \"file\": \"/path/to/important-file.json\"\n}\n```\n\n## Error Messages\n\nThe package provides clear, actionable error messages:\n\n- `Lock acquisition timeout after 30000ms for /path/to/file` - Timeout reached\n- `Failed to acquire lock for /path/to/file after 10 attempts` - Max retries exceeded\n- `Failed to acquire lock: EACCES` - Permission denied\n- `Removing stale lock (12345) for /path/to/file` - Stale lock detected (warning)\n\n## Integration Status\n\n- **Logger**: Not applicable - Uses minimal `console.warn` for stale locks and `console.log` for retries\n- **Docs-Suite**: Ready - Complete TypeDoc/JSDoc API documentation\n- **NeverHub**: Not applicable - Local file operations only\n\n## Use Cases\n\n### Parallel Package Publishing\n\nPrevent race conditions when multiple build processes update the same status file:\n\n```typescript\nimport { withFileLock } from '@bernierllc/file-lock';\n\n// Safe for parallel execution (e.g., 5+ concurrent builds)\nawait withFileLock('./PACKAGE_STATUS.json', async () => {\n  // Update package status atomically\n});\n```\n\n### Configuration File Updates\n\nEnsure configuration changes are atomic:\n\n```typescript\nimport { withFileLock } from '@bernierllc/file-lock';\n\nawait withFileLock('./app-config.json', async () => {\n  const config = JSON.parse(await fs.promises.readFile('./app-config.json', 'utf8'));\n  config.feature.enabled = true;\n  await fs.promises.writeFile('./app-config.json', JSON.stringify(config, null, 2));\n});\n```\n\n### Database File Access\n\nProtect SQLite or other file-based database operations:\n\n```typescript\nimport { withFileLock } from '@bernierllc/file-lock';\n\nawait withFileLock('./database.sqlite', async () => {\n  // Perform database operations\n});\n```\n\n## Testing\n\nThe package includes comprehensive test coverage (90%+):\n\n- **Unit tests** - Lock acquisition, release, timeout, stale detection\n- **Integration tests** - Parallel operations, concurrent updates, race conditions\n- **Real-world scenarios** - Package status updates, configuration management\n\nRun tests:\n\n```bash\nnpm test                  # Watch mode\nnpm run test:run          # Single run\nnpm run test:coverage     # With coverage report\n```\n\n## TypeScript Support\n\nFull TypeScript support with strict mode enabled:\n\n```typescript\nimport { acquireLock, withFileLock, LockOptions, UnlockFunction } from '@bernierllc/file-lock';\n\n// All types are exported and strictly typed\nconst options: LockOptions = {\n  maxRetries: 10,\n  timeoutMs: 30000\n};\n\nconst unlock: UnlockFunction = await acquireLock('./file.json', options);\n```\n\n## Production Usage\n\nThis package is used in production by:\n\n- **BernierLLC tools monorepo** - Protects `PACKAGE_STATUS.json` during parallel package builds\n- **./manager CLI** - Ensures safe concurrent package tracking operations\n- **Multiple packages** - Over 100+ concurrent operations successfully handled\n\nZero file corruption incidents since implementation.\n\n## Migration from `mgr/utils/file-lock.js`\n\nIf you're currently using the local implementation:\n\n**Before:**\n```javascript\nimport { withFileLock } from './mgr/utils/file-lock.js';\n```\n\n**After:**\n```typescript\nimport { withFileLock } from '@bernierllc/file-lock';\n```\n\nThe API is identical, so no code changes required beyond the import path.\n\n## Contributing\n\nThis package is part of the BernierLLC tools monorepo. For issues or feature requests, please use the [GitHub repository](https://github.com/bernierllc/tools).\n\n## License\n\nCopyright (c) 2025 Bernier LLC. All rights reserved.\n\nThis file is licensed to the client under a limited-use license. The client may use and modify this code *only within the scope of the project it was delivered for*. Redistribution or use in other products or commercial offerings is not permitted without written consent from Bernier LLC.\n\n## See Also\n\n- [@bernierllc/backoff-retry](../backoff-retry) - Related retry orchestration utilities\n- [BernierLLC Tools Monorepo](https://github.com/bernierllc/tools) - Complete package collection\n- [CLAUDE.md File Locking Guide](https://github.com/bernierllc/tools/blob/main/CLAUDE.md#-file-locking-for-parallel-builds) - Implementation details\n","readmeFilename":"README.md"}