fix(30724): clear "x-middleware-next" header when chaining middlewares

[hi-ogawa]

Fixes #30724

The linked issue wasn't triaged yet, so I wasn't sure if the current behavior was by design.
But, it seemed the issue was relatively simple, so I submit a possible fix here.
I hope this fix aligns to the intended behavior, but if that's not the case, I would appreciate any guidance to improve my code.
Thanks for the review!

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[javivelasco]

Thanks for this contribution! I'd change the semantics a bit though, seems more straightforward.

[ijjk]

Stats from current PR

Default Build (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
buildDuration	18.6s	18.2s	-381ms
buildDurationCached	3.7s	3.5s	-212ms
nodeModulesSize	335 MB	335 MB	⚠️ +130 B

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.016	3.008	-0.01
/ avg req/sec	828.83	831.04	+2.21
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.468	1.474	⚠️ +0.01
/error-in-render avg req/sec	1702.46	1695.91	⚠️ -6.55

Client Bundles (main, webpack, commons)

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	28.3 kB	28.3 kB	✓
webpack-HASH.js gzip	1.45 kB	1.45 kB	✓
Overall change	72.2 kB	72.2 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
_app-HASH.js gzip	1.37 kB	1.37 kB	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	327 B	327 B	✓
dynamic-HASH.js gzip	2.38 kB	2.38 kB	✓
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	635 B	635 B	✓
image-HASH.js gzip	4.44 kB	4.44 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	1.87 kB	1.87 kB	✓
routerDirect..HASH.js gzip	321 B	321 B	✓
script-HASH.js gzip	383 B	383 B	✓
withRouter-HASH.js gzip	318 B	318 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	13.3 kB	13.3 kB	✓

Client Build Manifests

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
_buildManifest.js gzip	460 B	460 B	✓
Overall change	460 B	460 B	✓

Rendered Page Sizes

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
index.html gzip	524 B	524 B	✓
link.html gzip	535 B	535 B	✓
withRouter.html gzip	517 B	517 B	✓
Overall change	1.58 kB	1.58 kB	✓

---

Default Build with SWC (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
buildDuration	19.1s	19s	-86ms
buildDurationCached	3.6s	3.6s	⚠️ +42ms
nodeModulesSize	335 MB	335 MB	⚠️ +130 B

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.855	2.911	⚠️ +0.06
/ avg req/sec	875.7	858.67	⚠️ -17.03
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.368	1.439	⚠️ +0.07
/error-in-render avg req/sec	1826.84	1737.77	⚠️ -89.07

Client Bundles (main, webpack, commons)

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.3 kB	42.3 kB	✓
main-HASH.js gzip	28.5 kB	28.5 kB	✓
webpack-HASH.js gzip	1.43 kB	1.43 kB	✓
Overall change	72.5 kB	72.5 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
_app-HASH.js gzip	1.35 kB	1.35 kB	✓
_error-HASH.js gzip	180 B	180 B	✓
amp-HASH.js gzip	305 B	305 B	✓
css-HASH.js gzip	321 B	321 B	✓
dynamic-HASH.js gzip	2.38 kB	2.38 kB	✓
head-HASH.js gzip	342 B	342 B	✓
hooks-HASH.js gzip	622 B	622 B	✓
image-HASH.js gzip	4.46 kB	4.46 kB	✓
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	1.91 kB	1.91 kB	✓
routerDirect..HASH.js gzip	314 B	314 B	✓
script-HASH.js gzip	375 B	375 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	13.2 kB	13.2 kB	✓

Client Build Manifests

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	hi-ogawa/next.js 30724-clear-next-header-when-chaining-middlewares	Change
index.html gzip	521 B	521 B	✓
link.html gzip	533 B	533 B	✓
withRouter.html gzip	515 B	515 B	✓
Overall change	1.57 kB	1.57 kB	✓

Commit: 9f992a7

[Izhaki]

Why was this change made?

It means that a middleware cannot longer set headers without breaking the middleware chain as we have below:

        if (!result.response.headers.has('x-middleware-next')) {
          break
        }

This will break some of the examples, like this one.

It also means that a middleware cannot simply add a cookie and keep the rest as is. For example, you may want to add a csrf cookie if missing, but not break the middleware chain or return a custom Response.