fix: infinite redirection loop when accessing user's preferred locale-prefixed path with middleware present

[masnormen]

What? / Why?

This PR will fix #55648.

When the defaultLocale of a Nextjs project is different than the user's preferredLocale, and user accesses that locale path, Next js somehow will infinitely redirect to the same page.

1. Say the defaultLocale of Nextjs project is 'en'. And the project has other locale 'id'
2. User's preferredLocale (Accept-Language header) is 'id'
3. User access /id
4. User gets redirected to /id again infinitely

This happens only when using a middleware.

How?

The function getLocaleRedirect seems to be called twice when we fetch GET /id. One triggered somewhere when handling the middleware call (?), and one triggered somewhere in invokeRender in router-server.

The first call correctly detect that the user is already on the right URL ('/id', as in the preferredLocale). But the second call somehow fails to determine it.

I've noticed that only this second call of getLocaleRedirect is problematic, and the second call can be distinguished by the header x-invoke-path. Infinite redirect is solved by detecting if the invoke path is the same as current locale.

[ijjk]

Allow CI Workflow Run

• approve CI run for commit: 7ab17c4

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

[ijjk]

Stats from current PR

Default Build

General Overall increase ⚠️

	vercel/next.js canary	masnormen/next.js fix/locale-detection-middleware-infinite-redirect	Change
buildDuration	13.8s	13.8s	N/A
buildDurationCached	8.5s	6.3s	N/A
nodeModulesSize	197 MB	197 MB	⚠️ +3.6 kB
nextStartRea..uration (ms)	406ms	403ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	masnormen/next.js fix/locale-detection-middleware-infinite-redirect	Change
2453-HASH.js gzip	30.5 kB	30.5 kB	N/A
3304.HASH.js gzip	181 B	181 B	✓
3f784ff6-HASH.js gzip	53.7 kB	53.7 kB	N/A
8299-HASH.js gzip	5.04 kB	5.04 kB	N/A
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	243 B	242 B	N/A
main-HASH.js gzip	32.1 kB	32.2 kB	N/A
webpack-HASH.js gzip	1.7 kB	1.7 kB	N/A
Overall change	45.4 kB	45.4 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	masnormen/next.js fix/locale-detection-middleware-infinite-redirect	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	masnormen/next.js fix/locale-detection-middleware-infinite-redirect	Change
_app-HASH.js gzip	196 B	197 B	N/A
_error-HASH.js gzip	184 B	184 B	✓
amp-HASH.js gzip	505 B	505 B	✓
css-HASH.js gzip	324 B	325 B	N/A
dynamic-HASH.js gzip	2.5 kB	2.5 kB	N/A
edge-ssr-HASH.js gzip	258 B	258 B	✓
head-HASH.js gzip	352 B	352 B	✓
hooks-HASH.js gzip	370 B	371 B	N/A
image-HASH.js gzip	4.2 kB	4.2 kB	✓
index-HASH.js gzip	259 B	259 B	✓
link-HASH.js gzip	2.67 kB	2.67 kB	N/A
routerDirect..HASH.js gzip	314 B	312 B	N/A
script-HASH.js gzip	386 B	386 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	6.56 kB	6.56 kB	✓

Client Build Manifests

	vercel/next.js canary	masnormen/next.js fix/locale-detection-middleware-infinite-redirect	Change
_buildManifest.js gzip	483 B	485 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	masnormen/next.js fix/locale-detection-middleware-infinite-redirect	Change
index.html gzip	528 B	526 B	N/A
link.html gzip	541 B	539 B	N/A
withRouter.html gzip	523 B	522 B	N/A
Overall change	0 B	0 B	✓

Edge SSR bundle Size

	vercel/next.js canary	masnormen/next.js fix/locale-detection-middleware-infinite-redirect	Change
edge-ssr.js gzip	95 kB	95 kB	N/A
page.js gzip	3.05 kB	3.06 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	masnormen/next.js fix/locale-detection-middleware-infinite-redirect	Change
middleware-b..fest.js gzip	624 B	624 B	✓
middleware-r..fest.js gzip	151 B	151 B	✓
middleware.js gzip	25.5 kB	25.5 kB	N/A
edge-runtime..pack.js gzip	839 B	839 B	✓
Overall change	1.61 kB	1.61 kB	✓

Next Runtimes

	vercel/next.js canary	masnormen/next.js fix/locale-detection-middleware-infinite-redirect	Change
app-page-exp...dev.js gzip	171 kB	171 kB	✓
app-page-exp..prod.js gzip	96.7 kB	96.7 kB	✓
app-page-tur..prod.js gzip	98.5 kB	98.5 kB	✓
app-page-tur..prod.js gzip	92.9 kB	92.9 kB	✓
app-page.run...dev.js gzip	150 kB	150 kB	✓
app-page.run..prod.js gzip	91.4 kB	91.4 kB	✓
app-route-ex...dev.js gzip	21.3 kB	21.3 kB	✓
app-route-ex..prod.js gzip	15 kB	15 kB	✓
app-route-tu..prod.js gzip	15 kB	15 kB	✓
app-route-tu..prod.js gzip	14.8 kB	14.8 kB	✓
app-route.ru...dev.js gzip	20.9 kB	20.9 kB	✓
app-route.ru..prod.js gzip	14.8 kB	14.8 kB	✓
pages-api-tu..prod.js gzip	9.51 kB	9.51 kB	✓
pages-api.ru...dev.js gzip	9.79 kB	9.79 kB	✓
pages-api.ru..prod.js gzip	9.51 kB	9.51 kB	✓
pages-turbo...prod.js gzip	22.3 kB	22.3 kB	✓
pages.runtim...dev.js gzip	23 kB	23 kB	✓
pages.runtim..prod.js gzip	22.3 kB	22.3 kB	✓
server.runti..prod.js gzip	50.6 kB	50.6 kB	✓
Overall change	950 kB	950 kB	✓

build cache

	vercel/next.js canary	masnormen/next.js fix/locale-detection-middleware-infinite-redirect	Change
0.pack gzip	1.56 MB	1.56 MB	N/A
index.pack gzip	105 kB	105 kB	N/A
Overall change	0 B	0 B	✓

Diff details

Diff for middleware.js

Diff too large to display

Commit: 4c461b2

[ijjk]

Tests Passed

[masnormen]

is it possible that the test is flaky? it seems that the test gives different results everytime I run it locally. even when i removed my modification.

[samcx]

@masnormen That one is indeed a flaky test! Taking a look at your .

[ijjk]

Hi, we shouldn't be relying on x-invoke-path here as that is a local router-server implementation detail that we want to eventually get rid of, I opened #62606 which has an alternative fix for this carrying over your test as well.

[masnormen]

Hi, we shouldn't be relying on x-invoke-path here as that is a local router-server implementation detail that we want to eventually get rid of, I opened #62606 which has an alternative fix for this carrying over your test as well.

Hi. Yeah that's great too. Using headers directly does feel too hacky.