Function saveConfigWithLock

src/utils/config.ts:1173–1349

* Returns true if a write was performed; false if the write was skipped * (no changes, or auth-loss guard tripped). Callers use this to decide * whether to invalidate the cache -- invalidating after a skipped write * destroys the good cached state the auth-loss guard depends on.

(
  file: string,
  createDefault: () => A,
  mergeFn: (current: A) => A,
)

 * destroys the good cached state the auth-loss guard depends on.
 */
function saveConfigWithLock<A extends object>(
  file: string,
  createDefault: () => A,
  mergeFn: (current: A) => A,
): boolean {
  const defaultConfig = createDefault()
  const dir = dirname(file)
  const fs = getFsImplementation()

  // Ensure directory exists (mkdirSync is already recursive in FsOperations)
  fs.mkdirSync(dir)

  let release
  try {
    const lockFilePath = `${file}.lock`
    const startTime = Date.now()
    release = lockfile.lockSync(file, {
      lockfilePath: lockFilePath,
      onCompromised: err => {
        // Default onCompromised throws from a setTimeout callback, which
        // becomes an unhandled exception. Log instead -- the lock being
        // stolen (e.g. after a 10s event-loop stall) is recoverable.
        logForDebugging(`Config lock compromised: ${err}`, { level: 'error' })
      },
    })
    const lockTime = Date.now() - startTime
    if (lockTime > 100) {
      logForDebugging(
        'Lock acquisition took longer than expected - another Claude instance may be running',
      )
      logEvent('tengu_config_lock_contention', {
        lock_time_ms: lockTime,
      })
    }

    // Check for stale write - file changed since we last read it
    // Only check for global config file since lastReadFileStats tracks that specific file
    if (lastReadFileStats && file === getGlobalClaudeFile()) {
      try {
        const currentStats = fs.statSync(file)
        if (
          currentStats.mtimeMs !== lastReadFileStats.mtime ||
          currentStats.size !== lastReadFileStats.size
        ) {
          logEvent('tengu_config_stale_write', {
            read_mtime: lastReadFileStats.mtime,
            write_mtime: currentStats.mtimeMs,
            read_size: lastReadFileStats.size,
            write_size: currentStats.size,
          })
        }
      } catch (e) {
        const code = getErrnoCode(e)
        if (code !== 'ENOENT') {
          throw e
        }
        // File doesn't exist yet, no stale check needed
      }