New Middleware API signature

[javivelasco]

This PR updates the middleware signature to instead expose an async function with an interface:

export async function middleware(request, event) {
  // ....
}

The usage of event.respondWith will throw with an error showing the new API and so will do calling request.respondWith and request.waitUntil to help with the migration. This is much simpler since we don't have to deal with the definition of synchronous event handlers.

Also, this PR brings updates to improve the way we handle subrequests and avoid infinite loops.

Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Integration tests added
• Errors have helpful link attached, see contributing.md

[styfle]

Can we preserve the old API so users can upgrade safely without runtime failures?

Seems like this is going to be a problem for beta users.

[styfle]

Update: I guess it will be fine as long as the user doesn't upgrade Next.js

[ijjk]

Stats from current PR

Default Build (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	javivelasco/next.js new-api-signature	Change
buildDuration	23.2s	23.2s	-8ms
buildDurationCached	4.6s	4.5s	-80ms
nodeModulesSize	475 MB	475 MB	⚠️ +5.4 kB

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	javivelasco/next.js new-api-signature	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.914	3.955	⚠️ +0.04
/ avg req/sec	638.71	632.05	⚠️ -6.66
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	2.085	2.112	⚠️ +0.03
/error-in-render avg req/sec	1198.8	1183.67	⚠️ -15.13

Client Bundles (main, webpack, commons)

	vercel/next.js canary	javivelasco/next.js new-api-signature	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	javivelasco/next.js new-api-signature	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	javivelasco/next.js new-api-signature	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	javivelasco/next.js new-api-signature	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	javivelasco/next.js new-api-signature	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	✓

---

Default Build with SWC (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	javivelasco/next.js new-api-signature	Change
buildDuration	19.5s	19.3s	-231ms
buildDurationCached	4.5s	4.6s	⚠️ +120ms
nodeModulesSize	475 MB	475 MB	⚠️ +5.4 kB

Page Load Tests Overall increase ✓

	vercel/next.js canary	javivelasco/next.js new-api-signature	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.976	3.979	0
/ avg req/sec	628.73	628.26	⚠️ -0.47
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	2.171	2.126	-0.04
/error-in-render avg req/sec	1151.39	1176.19	+24.8

Client Bundles (main, webpack, commons)

	vercel/next.js canary	javivelasco/next.js new-api-signature	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	javivelasco/next.js new-api-signature	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	javivelasco/next.js new-api-signature	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	javivelasco/next.js new-api-signature	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	javivelasco/next.js new-api-signature	Change
index.html gzip	534 B	534 B	✓
link.html gzip	547 B	547 B	✓
withRouter.html gzip	528 B	528 B	✓
Overall change	1.61 kB	1.61 kB	✓

Commit: f165e39