// The trusted bus host (lead decision 70). It forks
// packages/bus/src/process.mjs, boots it over IPC, keeps the reader
// capability it gets back, and hands it to the notifier child over IPC.
// Capabilities never appear in argv, stdout or the environment. While the
// host runs, `<dataRoot>/bus-host/host.json` (0600) names it for `mosaic bus
// stop|status` and for the human commands' default business.
//
// startHost() is also the in-process API S6 uses: bindLaunch(record) binds
// a launched run's process identity and returns its capability.

import { fork } from "node:child_process";
import { once } from "node:events";
import { existsSync, mkdirSync, readFileSync, renameSync, rmSync, writeFileSync } from "node:fs";
import { join } from "node:path";
import { fileURLToPath } from "node:url";
import { CliError } from "./errors.mjs";

const BROKER = fileURLToPath(new URL("../../bus/src/process.mjs", import.meta.url));
const NOTIFIER = fileURLToPath(new URL("./notifier-process.mjs", import.meta.url));
export const BOOT_TIMEOUT_MS = 30000;
const START_TIMEOUT_MS = 15000;
const CLOSE_TIMEOUT_MS = 20000;

export const hostDir = (dataRoot) => join(dataRoot, "bus-host");
export const hostFile = (dataRoot) => join(hostDir(dataRoot), "host.json");

// `/proc/<pid>/stat` field 22, the start time in clock ticks. Null when the
// process is gone or has exited and waits to be reaped (state Z).
export function startTimeOf(pid) {
  try {
    const raw = readFileSync(`/proc/${pid}/stat`, "utf8");
    const fields = raw.slice(raw.lastIndexOf(")") + 2).split(" ");
    return fields[0] === "Z" ? null : fields[19];
  } catch {
    return null;
  }
}

export function cmdlineOf(pid) {
  try {
    return readFileSync(`/proc/${pid}/cmdline`, "utf8").split("\0").filter(Boolean);
  } catch {
    return null;
  }
}

// The state file, or null. `live` is true only when the pid still runs with
// the recorded start time, so a recycled pid never counts as the host.
export function readHostState(dataRoot) {
  const file = hostFile(dataRoot);
  if (!existsSync(file)) return null;
  let state;
  try {
    state = JSON.parse(readFileSync(file, "utf8"));
  } catch {
    throw new CliError(`bus host state is unreadable: ${file}`, 3);
  }
  if (!state || !Number.isSafeInteger(state.pid) || typeof state.startTime !== "string" || typeof state.business !== "string") {
    throw new CliError(`bus host state is malformed: ${file}`, 3);
  }
  return { ...state, live: startTimeOf(state.pid) === state.startTime };
}

function writeHostState(dataRoot, state) {
  mkdirSync(hostDir(dataRoot), { recursive: true, mode: 0o700 });
  const file = hostFile(dataRoot);
  const tmp = `${file}.${process.pid}.tmp`;
  writeFileSync(tmp, `${JSON.stringify(state, null, 2)}\n`, { mode: 0o600, flag: "wx" });
  renameSync(tmp, file);
}

// The first IPC message from child, or a refusal when it exits or the wait runs out.
function firstReply(child, timeoutMs, what) {
  return new Promise((resolve, reject) => {
    const done = (fn, v) => {
      clearTimeout(timer);
      child.off("message", onMessage);
      child.off("exit", onExit);
      fn(v);
    };
    const onMessage = (m) => done(resolve, m);
    const onExit = (code) => done(reject, new CliError(`${what} exited (${code}) before it replied`, 1));
    const timer = setTimeout(() => done(reject, new CliError(`${what} did not reply within ${Math.round(timeoutMs / 1000)} s`, 1)), timeoutMs);
    child.on("message", onMessage);
    child.on("exit", onExit);
  });
}

async function ended(child, timeoutMs) {
  if (child.exitCode !== null || child.signalCode !== null) return child.exitCode;
  const timer = setTimeout(() => child.kill("SIGKILL"), timeoutMs);
  const [code] = await once(child, "exit");
  clearTimeout(timer);
  return code;
}

// Calls onDeath(name, code, signal) once for each child that exits. A child
// that exited before this runs (the broker while the notifier starts, say)
// has already emitted its exit event, so its exit state is checked here.
export function watchChildren(children, onDeath) {
  for (const [name, child] of Object.entries(children)) {
    if (!child) continue;
    if (child.exitCode !== null || child.signalCode !== null) onDeath(name, child.exitCode, child.signalCode);
    else child.once("exit", (code, signal) => onDeath(name, code, signal));
  }
}

// boot: the {op:'boot'} config from bootConfig(). notifier: null, or
// {binding, base?, pollMs?}; base and pollMs exist for the tests, and the
// command line never sets them. Resolves once both children are up.
export async function startHost({ boot, business, notifier = null, bootTimeoutMs = BOOT_TIMEOUT_MS, log = (l) => process.stderr.write(`mosaic-bus: ${l}\n`) }) {
  const dataRoot = boot.dataRoot;
  const prior = readHostState(dataRoot);
  if (prior?.live) throw new CliError(`a bus host already runs for ${prior.business} (pid ${prior.pid})`, 3);

  const broker = fork(BROKER, [], { stdio: ["ignore", "inherit", "inherit", "ipc"] });
  const reply = firstReply(broker, bootTimeoutMs, "broker");
  broker.send({ op: "boot", config: boot });
  let ready;
  try {
    ready = await reply;
  } catch (e) {
    broker.kill("SIGTERM");
    await ended(broker, 5000);
    throw e;
  }
  if (ready?.ok !== true) {
    await ended(broker, 5000);
    throw new CliError(`broker refused to start: ${typeof ready?.error === "string" ? ready.error : "startup-refused"}`, 3);
  }
  const reader = ready.readers.find((r) => r.business === business);

  let notify = null;
  if (notifier) {
    notify = fork(NOTIFIER, [], { stdio: ["ignore", "inherit", "inherit", "ipc"] });
    const started = firstReply(notify, START_TIMEOUT_MS, "notifier");
    notify.send({ op: "start", path: ready.path, cap: reader.cap, business, dataRoot, binding: notifier.binding, base: notifier.base, pollMs: notifier.pollMs });
    let ok;
    try {
      ok = await started;
    } catch (e) {
      notify.kill("SIGTERM");
      await ended(notify, 5000);
      if (broker.connected) broker.send({ op: "close" }, () => {});
      await ended(broker, CLOSE_TIMEOUT_MS);
      throw e;
    }
    if (ok?.ok !== true) {
      await ended(notify, 5000);
      if (broker.connected) broker.send({ op: "close" }, () => {});
      await ended(broker, CLOSE_TIMEOUT_MS);
      throw new CliError(`notifier refused to start: ${typeof ok?.error === "string" ? ok.error : "notifier-refused"}`, 3);
    }
  }

  const state = { hostVersion: 1, pid: process.pid, startTime: startTimeOf(process.pid), business, startedAt: new Date().toISOString(), notifier: notifier ? notifier.binding : null };
  rmSync(hostFile(dataRoot), { force: true });
  writeHostState(dataRoot, state);

  let closing = false;
  let finish;
  const done = new Promise((r) => (finish = r));

  // Replies from process.mjs carry no request id, so requests go one at a time.
  let queue = Promise.resolve();
  function bindLaunch(record) {
    const run = queue.then(async () => {
      if (closing) throw new CliError("bus host is closing", 1);
      const r = firstReply(broker, 10000, "broker");
      broker.send({ op: "bindLaunch", record });
      const m = await r;
      if (m?.ok !== true) throw new CliError(`launch bind refused: ${m?.error ?? "bind-refused"}`, 3);
      return m.launch;
    });
    queue = run.catch(() => {});
    return run;
  }

  async function close(code = 0) {
    if (closing) return done;
    closing = true;
    let result = code;
    if (notify) {
      if (notify.connected) notify.send({ op: "stop" }, () => {});
      if ((await ended(notify, CLOSE_TIMEOUT_MS)) !== 0 && result === 0) result = 1;
    }
    await queue;
    if (broker.connected) broker.send({ op: "close" }, () => {});
    if ((await ended(broker, CLOSE_TIMEOUT_MS)) !== 0 && result === 0) result = 1;
    const now = readHostState(dataRoot);
    if (now && now.pid === state.pid && now.startTime === state.startTime) rmSync(hostFile(dataRoot), { force: true });
    finish(result);
    return done;
  }

  // A child that dies on its own takes the host down with exit 1: the unit
  // restarts the pair rather than run a broker without its notifier.
  // Runs after `queue` exists, since close() awaits it.
  watchChildren({ broker, notifier: notify }, (name, code, signal) => {
    if (closing) return;
    log(`${name} exited (${signal ?? code}); stopping the host`);
    close(1);
  });

  return Object.freeze({ path: ready.path, business, bindLaunch, close, done, pids: Object.freeze({ broker: broker.pid, notifier: notify?.pid ?? null }) });
}

// `mosaic bus stop`: SIGTERM to the recorded host, after checking that the
// pid still runs with the recorded start time and is a `bus start` process.
export async function stopHost(dataRoot, { timeoutMs = 60000 } = {}) {
  const state = readHostState(dataRoot);
  if (!state || !state.live) return { stopped: false, reason: state ? "stale state file; no host runs" : "no host runs" };
  const argv = cmdlineOf(state.pid) ?? [];
  const i = argv.findIndex((a) => a.endsWith("/packages/cli/src/cli.mjs"));
  if (i < 0 || argv[i + 1] !== "bus" || argv[i + 2] !== "start") {
    throw new CliError(`pid ${state.pid} is not a bus host; refusing to signal it`, 3);
  }
  process.kill(state.pid, "SIGTERM");
  const deadline = Date.now() + timeoutMs;
  while (Date.now() < deadline) {
    if (startTimeOf(state.pid) !== state.startTime) return { stopped: true, business: state.business, pid: state.pid };
    await new Promise((r) => setTimeout(r, 200));
  }
  throw new CliError(`bus host pid ${state.pid} did not stop within ${Math.round(timeoutMs / 1000)} s`, 1);
}

// `mosaic bus status`: what the files and /proc say. Never removes a lock.
export function hostStatus(dataRoot) {
  const state = readHostState(dataRoot);
  const lock = join(dataRoot, "bus", "writer.lock");
  // The bus store writes {pid, at}. A dead pid means a broker was killed
  // hard; the bus README leaves removing the lock to the operator.
  let owner = null;
  if (existsSync(lock)) {
    try {
      owner = JSON.parse(readFileSync(lock, "utf8"));
    } catch {
      owner = {};
    }
  }
  const lockPid = Number.isSafeInteger(owner?.pid) ? owner.pid : null;
  return {
    host: state ? { business: state.business, pid: state.pid, startedAt: state.startedAt, notifier: state.notifier ?? null, live: state.live } : null,
    socket: existsSync(join(dataRoot, "bus", "broker.sock")),
    writerLock: owner ? { pid: lockPid, at: typeof owner.at === "string" ? owner.at : null, live: lockPid !== null && startTimeOf(lockPid) !== null } : null,
  };
}