Layers: Prefer local server utils over extended layers

[JamieCurnow]

Layers: Prefer local server utils over extended layers

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• nuxt-framework-cecwxh (Open reproduction)

[JamieCurnow]

I've done a bit of research here...

So. When there are layers, nuxt._layers becomes an array of the root project, followed by the layers. This can be demonstrated with:

 hooks: {
    ready(nuxt) {
      console.log('ready: ', nuxt.options._layers);
    },

So the first layer is the root, the second layer is an actual layer.

Then when nuxt creates nitro, it uses these layers in the nitro scanDirs options:
https://github.com/nuxt/nuxt/blob/main/packages/nuxt/src/core/nitro.ts#L96

The scanDirs in this object are passed into a call to createNitro() here:
https://github.com/nuxt/nuxt/blob/main/packages/nuxt/src/core/nitro.ts#L397

At that point they look like this:

[
  '/home/projects/nuxt-framework-cecwxh/server', // root "layer"
  '/home/projects/nuxt-framework-cecwxh/layer/server' // other layers
]

Which can be seen by:

hooks: {
  'nitro:config'(config) {
    console.log(config.scanDirs);
  }
}

These scanDirs get looped over and a /utils/* appended to them here:
https://github.com/unjs/nitro/blob/v2/src/core/config/resolvers/imports.ts#L30

Then they get passed in as imports.dirs to unImport here:
https://github.com/unjs/nitro/blob/v2/src/core/config/resolvers/paths.ts#L67

unImport does priority by order of dirs with the last dir in the array taking more priority - ref:
unjs/unimport#141

I tried to reverse the order of the scan imports by doing:

hooks: {
  'nitro:config'(config) {
    console.log('scanDirs before nitro init', config.scanDirs);
    config.scanDirs?.reverse();
  },
}

However... This doesn't work because the root dir gets unshifted onto the array by nitro anyway and then deduped so it ends up at the start of the array again...

createNitro calls loadOptions() here:
https://github.com/unjs/nitro/blob/v2/src/core/nitro.ts#L24

loadOptions loops over the configResolvers can calls them which includes resolvePathOptions here:
https://github.com/unjs/nitro/blob/v2/src/core/config/loader.ts#L32

resolvePathOptions does an unshift() on scanDirs to add the root to the start of the array here:
https://github.com/unjs/nitro/blob/v2/src/core/config/resolvers/paths.ts#L67

There is no hook between this happening, and nitro calling unImport.init() so you can never end up with the root project imports being after the layer imports, and because of the priority of unImport, that means that the layer imports always override the root project imports.

I'm not sure what the fix for this should be, potentially change the unshift to a push() in nitro? And then reverse the order of _layers in nuxt? Tricky one... But I'm happy to make a PR (or two) with some guidance @danielroe @pi0 ?