Allow next argument to requestHandler

[Rich-Harris]

Suppose you have a situation like this:

import http from 'http';
import snowpack from 'snowpack';
import sirv from 'sirv';

const snowpackServer = await snowpack.unstable__startServer(...);
const publicHandler = sirv('public');

const server = http.createServer((req, res) => {
  // in here, we want to serve
  //  1. files in public, or
  //  2. files handled by snowpack, or
  //  3. content rendered by a server-side rendering framework
});

Importantly, the SSR framework is responsible for rendering custom 404 pages.

Serving the contents of public then falling through to other handlers is straightforward enough, but Snowpack doesn't allow requests to fall through to another layer:

const server = http.createServer((req, res) => {
  publicHandler(req, res, () => {
    snowpackServer.requestHandler(req, res, () => {
      // we never get here
      renderPage(req, res);
    });
  });
});

Instead, you have to ensure that Snowpack is the final handler...

const server = http.createServer((req, res) => {
  publicHandler(req, res, () => {
    renderPage(req, res, () => {
      snowpackServer.requestHandler(req, res);
    });
  });
});

...but this denies the SSR framework the opportunity to render a custom 404.

What if Snowpack only served a 404 response if the third argument to requestHandler wasn't provided?

[FredKSchott]

Makes sense!

New prioritized issue created to track: #1295 (Until GH gives us a "transfer discussion to issue" button, I'm stuck copy-pasting)