create one file cache per instance

[Rich-Harris]

If you have multiple instances of sirv with different options...

const static_handler = sirv('static', {
  etag: true,
  setHeaders: res => {
    res.setHeader('cache-control', `public, max-age=0, must-revalidate`);
  }
});

const assets_handler = sirv('build/assets', {
  maxAge: 31536000,
  immutable: true
});

http.createServer((req, res) => {
  assets_handler(req, res, () => {
    static_handler((req, res) => {
      ssr(req, res);
    });
  });
}).listen(3000);

...they currently share a FILES cache, meaning that files can be intercepted by the wrong handler. For example, a request for an asset like /global.css will be handled by assets_handler which is only supposed to deal with immutable (hashed) files. Because of that, options like setHeaders are disregarded.

(I know this usage of setHeaders looks redundant, but I've found that explicitly adding must-revalidate rather than relying on maxAge: 0 is necessary, otherwise the browser doesn't send if-none-match.)

This PR fixes it by making a FILES cache per handler. Haven't added a test yet because I wasn't sure how that should look, since all the existing tests assume a single handler.

[lukeed]

Thanks, good catch 👍

And I think it makes sense to auto-append must-revalidate if options.maxAge === 0

[paulocoghi]

What I did to maintain the single file cache for all instances was to use the full file path as the cache index.

This way, even if multiple files have the same URL in different instances, they certainly won't have the same absolute address in the file system.