Keep node script instance only one.

// IMPORTANT: THIS SCRIPT ONLY WORKS IN SYSTEM WHICH SUPPORT UNIX SOCKET!

import fs from 'node:fs'
import http from 'node:http'
import path from 'node:path'

async function lock_server(lock_service_path) {
    const server = http.createServer((req, res) => { res.end("ok") })
    // make sure this server doesn't keep the process running
    server.unref()

    server.on('error', function (e) {
        if (e.code === "EADDRINUSE") {
            // Try to comminute with the server, if it did not answer, we continure by remove the socket and re-create it.
            // Are U OK?
            const clientRequest = http.request({
                socketPath: lock_service_path,
                path: '/',
                method: 'GET',
                timeout: 100, // Should be very short, we are in the same host.
            }, (res) => {
                res.on('data', (data) => {
                    console.log(data.toString())
                    // Exit process when duplicate server is running.
                    console.log("Sentinel server already running - can't run more than one instance")
                    process.exit(0)
                })

                res.on('error', async (e) => {
                    // So this server is failed, remove the socket file and retry listen.
                    await fs.promises.unlink(lock_service_path)
                    server.listen(lock_service_path, () => {
                        console.log(`Sentinel server restart listening on ${lock_service_path}`)
                    })
                })
            })

            clientRequest.on('error', (e) => {
                console.log(e)
                // So this server is failed, remove the socket file and retry listen.
                fs.unlinkSync(lock_service_path)
                server.listen(lock_service_path, () => {
                    console.log(`Sentinel server restart listening on ${lock_service_path}`)
                })
            })

            clientRequest.end()
        } else {
            console.log(e)
        }
    })

    server.listen(lock_service_path, () => {
        console.log(`Sentinel server listening on ${lock_service_path}`)
    })

    return server
}


// Example usage:
await lock_server(path.join('.', '.lock_service'))

// Just do a nap.
const sleep = ms => new Promise(resolve => setTimeout(resolve, ms))
await sleep(10*1000)