Handle path-based assetPrefix correctly in 'gatsby serve' command

[szimek]

When assetPrefix option is set to a path rather than a domain (e.g. /assets/my-app), when Gatsby builds the app it does not generate asset files in public/assets/my-app/, but still in public/ directory. This causes a problem when using gatsby serve command, as asset files have /assets/my-app/ prefix, and the express server can't find them.

Would it make sense to handle this case correctly by default (vs having some generic "rewrite" functionality) , i.e. add something like this to the express server used by serve command:

// https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/src/commands/serve.ts
const { 
  pathPrefix: configPathPrefix,
  assetPrefix: configAssetPrefix
} = config || {}
const pathPrefix = prefixPaths && configPathPrefix ? configPathPrefix : `/`
const assetPrefix = prefixPaths && configAssetPrefix && configAssetPrefix.startsWith(`/`) && !configAssetPrefix.startsWith(`//`) ? configAssetPrefix : `/`

...

router.use(assetPrefix, express.static(`public`, { dotfiles: `allow` }))

? The check if assetPrefix is a path or domain could also be done differently, e.g. with is-url-superb or URL class.

In production I can probably solve it with nginx configuration, e.g. (not tested)

location /assets/my-app/ {
    alias /public/;
}

In Next.js I can solve it with rewrites option, e.g.:

module.exports = {
  assetPrefix: '/assets/my-app',
  async rewrites() {
    return [
      { 
        source: '/assets/my-app/_next/:path*', 
        destination: '/_next/:path*' 
      }
    ]
  },
};

[violy]

my hacky solution : make a relative symlink targeting ../ in public directory onPostBuild, in gatsby-node.js — don't know if it works on all OS !

[szimek]

@violy Thank you! I actually even have a command that moves all static files into this new folder, but using a link is so much simpler 🤦‍♂️ I was looking for an alternative, because moving files breaks a lot of stuff (e.g. incremental builds).

Still, it would be great if it was supported out of the box.