import { spawn } from "node:child_process";

/** Kernel-owned lock: a crashed coordinator cannot leave a stale ownership file.
 * The persistent file is just an inode; EOF releases the helper's lock. */
export async function withFileLock<T>(filename: string, work: () => Promise<T>): Promise<T> {
  const child = spawn("flock", ["--exclusive", "--timeout", "120", "--conflict-exit-code", "75", filename,
    process.execPath, "-e", 'process.stdout.write("locked\\n"); process.stdin.resume();'], {stdio: ["pipe", "pipe", "pipe"]});
  let diagnostics = "";
  child.stdin.on("error", () => { /* acquisition/exit handling reports helper failure */ });
  child.stderr.on("data", chunk => { diagnostics = (diagnostics + String(chunk)).slice(-2000); });
  const closed = new Promise<void>((resolve) => { child.once("close", () => resolve()); });
  try {
    await new Promise<void>((resolve, reject) => {
      let output = "";
      child.once("error", reject);
      child.once("exit", code => reject(new Error(code === 75 ? "Timed out after 120 seconds waiting for another authoring command; inspect that command before retrying" : `Cannot acquire authoring lock: ${diagnostics}`)));
      child.stdout.on("data", chunk => { output += chunk; if (output.includes("locked\n")) resolve(); });
    });
    return await work();
  } finally { child.stdin.end(); await closed; }
}