PR #92934: build: detect instrumentation/middleware/proxy with multi-segment pageExtensions

• Repository: vercel/next.js
• Author: yogeshwaran-c
• State: open | merged: False
• Link: https://github.com/vercel/next.js/pull/92934

Description (problem / solution / changelog)

What?

Make next build detect instrumentation, middleware, and proxy files when the user configures a multi-segment pageExtensions entry (e.g. 'universal.ts') and names their hook accordingly:

// next.config.js
module.exports = {
  pageExtensions: ['ts', 'tsx', 'js', 'jsx', 'universal.ts', 'universal.tsx'],
}

// src/instrumentation.universal.ts
export function register() { /* ... */ }

Today this hook is silently dropped — the file is never compiled to .next/server/instrumentation.js and register() never runs. Same for middleware.universal.ts and proxy.universal.ts.

Why?

packages/next/src/build/index.ts builds three regexes from config.pageExtensions and uses them to filter root-level files into rootPaths. That part works — instrumentation.universal.ts correctly survives the filter (the unescaped . in the extension list is treated as \.|., which still matches the literal . in the filename).

The bug is in the bucketing loop right after, which iterates rootPaths and assigns each match to one of the three target paths via:

const { name: fileBaseName } = path.parse(rootPath)
if (fileBaseName === INSTRUMENTATION_HOOK_FILENAME) instrumentationHookFilePath = rootPath

path.parse('/src/instrumentation.universal.ts').name is 'instrumentation.universal' — Node only strips the last extension. So the equality check fails and the file is dropped on the floor. The filter said yes, the bucket said no.

The same pattern applies to MIDDLEWARE_FILENAME and PROXY_FILENAME, so any user with proxy.<custom>.ts / middleware.<custom>.ts hits the identical bug.

How?

Reuse the three detection regexes that already filter rootPaths to also assign each matched file to its bucket, comparing against path.parse(rootPath).base (the full filename including extension). The filter and the bucketing now agree on the same matching rule:

if (middlewareDetectionRegExp.test(fileBase))            middlewareFilePath = rootPath
else if (proxyDetectionRegExp.test(fileBase))            proxyFilePath = rootPath
else if (instrumentationHookDetectionRegExp.test(fileBase)) instrumentationHookFilePath = rootPath

Behavior with the default pageExtensions (['ts', 'tsx', 'js', 'jsx', 'mts', 'mjs', ...]) is unchanged — those are all single-segment, and a file named instrumentation.ts matches the regex and lands in the bucket exactly as before.

Scope

This fixes the detection gap reported as the primary issue (instrumentation never registered with custom multi-segment pageExtensions). The reporter also notes a secondary observation in their case 3 (adding proxy.universal.ts makes instrumentation register but with NEXT_RUNTIME=edge). That smells like a separate downstream runtime-routing issue and is out of scope here — once detection works correctly with this PR, that scenario is worth re-evaluating against canary.

Test plan

Added test/integration/instrumentation-page-extensions/:

• Fixture: project with pageExtensions: ['ts', 'tsx', 'js', 'jsx', 'universal.ts', 'universal.tsx'], src/instrumentation.universal.ts exporting register(), and a minimal pages/index.tsx.
• Test: runs nextBuild, asserts exit code 0 and that .next/server/instrumentation.js exists — the latter only happens if detection succeeded.
• Webpack-only (gated on IS_TURBOPACK_TEST), mirroring the existing build-trace-extra-entries integration suite.

Verified locally on Windows + webpack: test fails on canary without the fix and passes after it.

Fixes #92342

Changed files

• packages/next/src/build/index.ts (modified, +12/-9)
• test/integration/instrumentation-page-extensions/app/next.config.js (added, +4/-0)
• test/integration/instrumentation-page-extensions/app/src/instrumentation.universal.ts (added, +5/-0)
• test/integration/instrumentation-page-extensions/app/src/pages/index.tsx (added, +3/-0)
• test/integration/instrumentation-page-extensions/app/tsconfig.json (added, +19/-0)
• test/integration/instrumentation-page-extensions/test/index.test.ts (added, +36/-0)