fix(build): throw error if there is _next folder inside the public folder

[NriotHrreion]

Closes #2778

Root cause

In packages/vinext/src/server/prod-server.ts:1441, the function resolveAppRouterAssetPath() does not consider the case where the provided pathname that starts with _next/static is from public folder.

vinext/packages/vinext/src/server/prod-server.ts

Lines 1467 to 1470 in 255be4e

	// No `assetPrefix` (default layout), or absolute-URL `assetPrefix` with no
	// path component — both land files on disk at `dist/client/_next/static/...`
	// and emit URLs starting `/_next/static/...`.
	if (pathname.startsWith(nextStaticDir)) return pathname;

But in the upstream next.js source, it is not allowed to put a _next folder inside the public folder, or it will throw an error during npm run build.

What changed

I created an assert function assertNoPublicDirAssetConflict(). This function checks the paths. Once it found there is any _next inside public folder, it will throw an error and interrupt the build.

This assert function is executed during configResolved() (in packages/vinext/src/index.ts):

async configResolved(config) {
+  const publicDir = config.publicDir === "" ? null : config.publicDir;
+  const assetsDir =
+    config.environments.client.build.assetsDir ??
+    config.build.assetsDir ??
+    resolveAssetsDir(nextConfig.assetPrefix ?? "");
+
+  assertNoPublicDirAssetConflict({
+    root: config.root,
+    publicDir,
+    assetsDir,
+  });

  // ...

The workaround is quite simple and is aligned with the behavior of upstream next.js.

Testing

• pnpm test tests/public-dir-conflict.test.ts

Notes

The error message is as following:

const PUBLIC_NEXT_CONFLICT_ERROR =
  "You can not have a '_next' folder inside of your public folder. " +
  "This conflicts with the internal '/_next' route. " +
  "https://nextjs.org/docs/messages/public-next-folder-conflict";

I'm not sure if the link should be replaced with vinext's one.

Thanks for reviewing!

[NriotHrreion]

Oops, it seems that my formatter just replaced all import relative paths with import alias...

[pkg-pr-new]

Open in StackBlitz

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npm i https://pkg.pr.new/@vinext/types@2814

npm i https://pkg.pr.new/vinext@2814

commit: bbbcbc5

[github-actions]

Performance benchmarks

Compared bbbcbc5 against base 3c5cea4 using alternating same-runner rounds. Next.js was unchanged and skipped.

1 improved · 0 regressed · 5 within ±1.5%

Scenario	Framework	Baseline	Current	Change
Client bundle size (gzip)	vinext	134.6 KB	134.6 KB	⚫ -0.0%
Client entry size (gzip)	vinext	122.1 KB	122.2 KB	⚫ +0.0%
Dev server cold start	vinext	3.11 s	3.01 s	🟢 -3.3%
Production build time	vinext	3.37 s	3.33 s	⚫ -1.2%
RSC entry closure size (gzip)	vinext	113.8 KB	113.9 KB	⚫ +0.0%
Server bundle size (gzip)	vinext	191.4 KB	191.4 KB	⚫ +0.0%

View detailed results and traces

_{🟢 improvement · 🔴 regression · ⚫ change below 1.5% · paired base/head}