#!/usr/bin/env node
/**
 * Remote host agent: spawn/kill fleet workers so the control plane can spread replicas.
 *
 *   FLEET_AGENT_PORT=3851 node src/agent.js
 *
 * Bind loopback or a private interface. Do not publish NATS.
 */
import http from 'node:http';
import { spawn } from 'node:child_process';
import path from 'node:path';
import { fileURLToPath } from 'node:url';

const WORKER = path.join(path.dirname(fileURLToPath(import.meta.url)), 'worker.js');
const PORT = Number(process.env.FLEET_AGENT_PORT || 3851);
const BIND = process.env.FLEET_AGENT_BIND || '127.0.0.1';
const children = new Map();

function json(res, code, obj) {
  res.writeHead(code, { 'content-type': 'application/json' });
  res.end(JSON.stringify(obj));
}

async function readBody(req) {
  const chunks = [];
  for await (const c of req) chunks.push(c);
  const raw = Buffer.concat(chunks).toString('utf8');
  return raw ? JSON.parse(raw) : {};
}

export function startAgent({ port = PORT, bind = BIND } = {}) {
  const server = http.createServer(async (req, res) => {
    const url = new URL(req.url, `http://${bind}:${port}`);
    try {
      if (req.method === 'GET' && url.pathname === '/health') {
        return json(res, 200, {
          ok: true,
          role: 'fleet-agent',
          workers: [...children.keys()],
        });
      }
      if (req.method === 'POST' && url.pathname === '/spawn') {
        const body = await readBody(req);
        const instance = body.instance;
        if (!instance) return json(res, 400, { error: 'instance required' });
        if (children.has(instance)) {
          return json(res, 200, { pid: children.get(instance).pid, instance, existing: true });
        }
        const child = spawn(process.execPath, [WORKER], {
          env: { ...process.env, ...(body.env || {}) },
          stdio: ['ignore', 'pipe', 'pipe'],
        });
        children.set(instance, child);
        child.on('exit', () => children.delete(instance));
        return json(res, 200, { pid: child.pid, instance });
      }
      if (req.method === 'POST' && url.pathname === '/kill') {
        const body = await readBody(req);
        const child = children.get(body.instance);
        if (child) {
          child.kill('SIGTERM');
          children.delete(body.instance);
        }
        return json(res, 200, { killed: Boolean(child), instance: body.instance });
      }
      json(res, 404, { error: 'not found' });
    } catch (err) {
      json(res, 400, { error: err.message });
    }
  });
  server.listen(port, bind, () => {
    process.stdout.write(`fleet-agent http://${bind}:${port}/\n`);
  });
  return {
    server,
    async close() {
      for (const c of children.values()) c.kill('SIGTERM');
      children.clear();
      await new Promise((r) => server.close(r));
    },
  };
}

if (process.argv[1] && path.basename(process.argv[1]) === 'agent.js') {
  startAgent();
}