Support systemd socket activation in react-router-serve

[ktosiek]

I would like to run react-router-serve as a socket-activated service. I want this mostly for zero-downtime restarts in small deployments: systemd can keep a socket open (and Linux will handle a small queue of connections waiting to be received).

I believe this can be handled inside react-router-serve, without external dependencies: the core of socket activation protocol implementation is pretty much:

if (env.LISTEN_PID == process.pid) {
  servers =  servers.concat(
    Array.from({length: parseInt(env.LISTEN_FDS)}, (v, i) => i + 3))
      .map(fd => app.listen({fd},  () => console.log(`Listening on fd ${fd}`);
}

I will try to prepare a PR, but I wonder if this kind of feature would be accepted?

Edit: did the PR, let's see what happens ^_^: #13848