Fix middleware header propagation

[styfle]

This ensures that headers are additive between multiple middleware handlers.

Since middleware is applied from the root down to the deepest leaf, we append headers in that same order.

[ijjk]

Stats from current PR

Default Build (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
buildDuration	21.4s	21.2s	-171ms
buildDurationCached	4.3s	4.1s	-170ms
nodeModulesSize	198 MB	198 MB	⚠️ +1.2 kB

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.71	3.653	-0.06
/ avg req/sec	673.82	684.41	+10.59
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.993	2.046	⚠️ +0.05
/error-in-render avg req/sec	1254.6	1221.89	⚠️ -32.71

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	27.7 kB	27.7 kB	✓
webpack-HASH.js gzip	1.45 kB	1.45 kB	✓
Overall change	71.5 kB	71.5 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
_app-HASH.js gzip	1.23 kB	1.23 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	✓
334f979574ae..6f4.css gzip	106 B	106 B	✓
Overall change	13.1 kB	13.1 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
index.html gzip	533 B	533 B	✓
link.html gzip	546 B	546 B	✓
withRouter.html gzip	527 B	527 B	✓
Overall change	1.61 kB	1.61 kB	✓

---

Default Build with SWC (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
buildDuration	22.1s	20.1s	-1.9s
buildDurationCached	4.9s	4.1s	-724ms
nodeModulesSize	198 MB	198 MB	⚠️ +1.2 kB

Page Load Tests Overall increase ✓

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
/ failed reqs	0	0	✓
/ total time (seconds)	4.479	3.712	-0.77
/ avg req/sec	558.12	673.51	+115.39
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	2.305	1.987	-0.32
/error-in-render avg req/sec	1084.76	1258.03	+173.27

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	27.8 kB	27.8 kB	✓
webpack-HASH.js gzip	1.43 kB	1.43 kB	✓
Overall change	71.6 kB	71.6 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
_app-HASH.js gzip	1.22 kB	1.22 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	621 B	621 B	✓
image-HASH.js gzip	4.46 kB	4.46 kB	✓
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	1.9 kB	1.9 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	✓
334f979574ae..6f4.css gzip	106 B	106 B	✓
Overall change	13.1 kB	13.1 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js fix-middleware-header-propegation	Change
index.html gzip	534 B	534 B	✓
link.html gzip	546 B	546 B	✓
withRouter.html gzip	528 B	528 B	✓
Overall change	1.61 kB	1.61 kB	✓

Commit: 9d26aca

[ijjk]

Failing test suites

Commit: 9d26aca

test/integration/preload-viewport/test/index.test.js

• Prefetching Links in viewport > should not prefetch already loaded scripts

Expand output

● Prefetching Links in viewport › should not prefetch already loaded scripts

expect(received).toBe(expected) // Object.is equality

Expected: false
Received: true

  397 |     expect(hrefs).toEqual([...new Set(hrefs)])
  398 |
> 399 |     // Verify encoding
      |     ^
  400 |     expect(hrefs.some((e) => e.includes(`%5Bhello%5D-`))).toBe(true)
  401 |   })
  402 |

  at Object.<anonymous> (integration/preload-viewport/test/index.test.js:399:5)