Allow specifying alternative preClose / modifying close flow

[HDegroote]

Prerequisites

• [X] I have written a descriptive issue title
• [X] I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

The current exit flow is:

• Fastify stops receiving connections
• fastify-websocket's preClose handler is called, and all open sessions are closed
• fastify-websocket's close handler is called, and the websocket server shuts down

I'd propose to change it to something like:

• Fastify stops receiving connections
• A user-defined preClose handler is called, falling back to fastify-websocket's default preClose handler
• The preClose handler is also responsible for closing the ws server, so no need for the close handler

Motivation

I have a usecase where I would like the websocket server to shutdown as follows:

• Stop accepting new connections
• Send a 'will shut down in X seconds' message to the active connections
• Wait either X seconds, or for all connections to be closed, then shut down the server

This is not hard to implement in my own preClose handler. I managed to get around not being able to provide my own preClose handler by registering it before registering fastify-websocketServer so it runs earlier, but that's quite hacky, and my current solution causes the close handler to error out on server.close(done), because the ws server is already closed.

For context, this is the gist of my preClose hook.

app.addHook('preClose', async function () {
    const closeProm = new Promise(resolve => wsServer.close(resolve))
    closeProm.catch(() => {})

    for (const socket of wsServer.clients) {
      socket.send(`Server closing. Socket will shut down in ${sShutdownMargin}s`)
    }

    await Promise.race([
      new Promise(resolve => setTimeout(resolve, sShutdownMargin * 1000)),
      closeProm // If all connections close before the timeout
    ])

    const goingAwayCode = 1001
    for (const client of wsServer.clients) {
      client.close(goingAwayCode, 'Server is going offline')
    }

   await closeProm

If you're open to this feature, I'm fine with PR'ing whichever variation you deem fit. Or if you want to go a bit further, fastify-websocket could also support this kind of shutdown natively, by adding something like sShutdownMargin and onShutdownMessage options

Example

fastify.register(require('@fastify/websocket'), {
  options: {
    preClose: myOwnPreClose    
  }

or

fastify.register(require('@fastify/websocket'), {
  options: {
    sShutdownMargin: 10,
    onShutdownMessage: 'Shutting down in 10 seconds'    
  }

[mcollina]

Would you like to send a Pull Request to address this issue? Remember to add unit tests. Providing a user-defined preClose option is the way to go.

[HDegroote]

Cool, thanks! Yes, I'll create a PR.

Are you fine with the proposed update to the default closing flow, where it no longer has a close hook but instead closes the ws server during the preClose ?