// The endpoint the other machines' checkers dial (novox/hq ADR 0145).
//
// **This module's own endpoint is the instrument.** It is declared reachable over the private network
// like any other service, so it is admitted by exactly the rule that governs every internally-exposed
// service and it fails when that rule is wrong. A probe on a port that is never closed — ssh, say —
// would have passed throughout the outage this module exists to catch.
//
// It accepts a connection and closes it. Answering anything would make this a protocol, and then the
// question would be whether the protocol worked rather than whether the path did.

import { createServer } from "node:net";

const port = Number(process.env.MESH_NETWORK_CHECKER_PORT ?? "9876");

const server = createServer((socket) => {
  // Written before closing so a person dialling it by hand sees something, and so a half-open
  // connection is not mistaken for a working path by a client that only checks the handshake.
  socket.end("mesh network-checker\n");
});

server.on("error", (err: Error) => {
  // Said and fatal: a probe that cannot listen must not look like a probe that nothing dialled.
  console.error(`network-checker: cannot serve the probe on ${port}: ${err.message}`);
  process.exit(1);
});

server.listen(port, () => console.log(`network-checker: probe listening on ${port}`));

for (const signal of ["SIGTERM", "SIGINT"] as const) {
  process.on(signal, () => server.close(() => process.exit(0)));
}