i18n fallback : existing translated files are not built

[jsiot76]

Astro Info

Astro                    v4.4.4
Node                     v20.10.0
System                   Linux (x64)
Package Manager          npm
Output                   static
Adapter                  none
Integrations             none

If this issue only occurs in one browser, which browser is a problem?

No response

Describe the Bug

When building a project, routing fallback is working fine for missing translations, generating redirection pages, excepted for index.astro page, redirection page is also built, even if index.astro file exists in translation folder.
All pages are correctly shown when running project in dev mode.
This appeared since 4.2.0 version.

I created a stackblitz reproductible example, base on astro fixture code: astro/packages/astro/test/fixtures/i18n-routing-fallback

├── dist/
│	└── blog
│	└── end
│	└── it
│	└── pt
|	|	└── blog
|	|	└── end
|	|	└── start
|	|	|	└── index.html : translated page
|	|	└── index.html : redirection page => it should be translated page
├── src/
│	├── pages/
│	│	└── blog
│	|	│	└── [id].astro
│	├── pt/
│	│	└── blog
│	|	|	└── [id].astro
│	|	└── index.astro : translated index page => generated as redirection page
│	|	|
│	|	└── start.astro
│	└── end.astro
│	└── index.astro
│	└── start.astro
 

What's the expected result?

index.astro page should be correctly built in [locale] folder instead of index.html redirection page.

├── dist/
│	└── blog
│	└── end
│	└── it
│	└── pt
|	|	└── blog
|	|	└── end
|	|	└── start
|	|	|	└── index.html : translated page
|	|	└── index.html : correct translated page
├── src/
│	├── pages/
│	│	└── blog
│	|	│	└── [id].astro
│	├── pt/
│	│	└── blog
│	|	|	└── [id].astro
│	|	└── index.astro
│	|	|
│	|	└── start.astro
│	└── end.astro
│	└── index.astro
│	└── start.astro

Link to Minimal Reproducible Example

https://stackblitz.com/edit/withastro-astro-mrykc9?file=src%2Fpages%2Findex.astro

Participation

• I am willing to submit a pull request for this issue.

[jsiot76]

my astro.config.mjs

import { defineConfig } from 'astro/config';

export default defineConfig({
  base: 'new-site',
  i18n: {
    defaultLocale: 'en',
    locales: ['en', 'pt', 'it'],
    fallback: {
      it: 'en',
      pt: 'en',
    },
  },
});

[log101]

My initial debugging shows that the /pt index page is actually built correctly first. But in the router explosion stage, /index.astro generates multiple /pt pages under it, and one of them returns a redirect response which in turn overwrites /pt/index.html file. I suppose the problem lies within the route explosion logic.

astro/packages/astro/src/core/build/generate.ts

Lines 285 to 286 in 7fe2515

	// Now we explode the routes. A route render itself, and it can render its fallbacks (i18n routing)
	for (const route of eachRouteInRouteData(pageData)) {

I'll continue working on the problem.

[log101]

I think I found the problem, the /pt/index.html is not overwritten once but twice after being built correctly.

First Overwrite

The eachRouteInRouteData function introduced in #9119 generates routes and their fallbacks as separate routes. So, in our case it generates three routes for the /src/pages/index.astro: [/, /it, /pt], the last two are fallback routes.

astro/packages/astro/src/core/build/generate.ts

Lines 285 to 286 in 03242c6

	// Now we explode the routes. A route render itself, and it can render its fallbacks (i18n routing)
	for (const route of eachRouteInRouteData(pageData)) {

But then getPathsForRoute also returns three routes!

astro/packages/astro/src/core/build/generate.ts

Lines 327 to 335 in 03242c6

	if (route.pathname) {
	paths.push(route.pathname);
	builtPaths.add(route.pathname);
	for (const virtualRoute of route.fallbackRoutes) {
	if (virtualRoute.pathname) {
	paths.push(virtualRoute.pathname);
	builtPaths.add(virtualRoute.pathname);
	}
	}

As a result, all three paths are populated with /index.html content, which overwrites /pt/index.html.

Second Overwrite

Then, for loop continues to other routes generated by eachRouteInRouteData: /it and /pt. On /pt, because it is a fallback route, it overwrites it with a redirect. Which causes this issue.

My solution is to return only the original path at the getPathsForRoute function because generator function eachRouteInRouteData already handles fallback routes. And then when rendering fallback routes, we could check if there any matches in manifest routes. If there is a match we can skip rendering because it means there is already a non-fallback page for the path. But I'm not sure if this suits well with the fallback logic Astro has. My implentation had some failing tests, after fixing them I'll send the PR.