Function writeBackup

src/commands/agent.ts:126–143

* Back up the current bytes at `abs` next to it without clobbering any existing * backup or writing through a symlink. Exclusive create (`wx`) fails with * EEXIST on an existing regular file OR symlink, so we walk `.bak`, `.bak.1`, * `.bak.2`, … until a free slot is found. Returns the absolute pa

(agentFs: AgentFs, abs: string, existing: string)

 * `.bak.2`, … until a free slot is found. Returns the absolute path used.
 */
async function writeBackup(agentFs: AgentFs, abs: string, existing: string): Promise<string> {
  for (let n = 0; n < 100; n++) {
    const candidate = n === 0 ? `${abs}.bak` : `${abs}.bak.${n}`;
    try {
      await agentFs.writeFile(candidate, existing, { exclusive: true });
      return candidate;
    } catch (err) {
      if (err instanceof Error && (err as NodeJS.ErrnoException).code === 'EEXIST') {
        continue;
      }
      throw err;
    }
  }
  throw new CLIError(
    `refusing to back up ${path.basename(abs)}: too many existing .bak files — clean them up and re-run.`,
    6,
  );
}

// ---------------------------------------------------------------------------
// Managed-section helpers (codex target)