fix: pass only first language to next-server to match platform redirects support

cypress/e2e/default/i18n.cy.ts

@@ -35,4 +35,34 @@ describe('Localization', () => {
 
     cy.findByText('The current locale is en')
   })
+
+  it(`function handler doesn't produce locale when first language in 'Accept-Language' header is not matching any of locales`, () => {
+    cy.request({
+      url: `/`,
+      followRedirect: false,
+      headers: {
+        // jp is not in i18n config, fr is in config
+        // Netlify Redirects only match on first language
+        // https://docs.netlify.com/routing/redirects/redirect-options/#redirect-by-country-or-language
+        // > Language-based redirects always match against the first language reported by the browser in the Accept-Language header regardless of quality value weighting.
+        // while Next.js matches on every language: https://github.com/vercel/next.js/blob/5d9597879c46b383d595d6f7b37fd373325b7544/test/unit/accept-headers.test.ts
+        'Accept-Language': 'jp, fr;q=0.9',
+        // make sure we don't use cached results
+        cookie: '__prerender_bypass=1',
+        // above cookie header cause us to hit non-ODB function variant
+        // below header allow us to force ODB-only code path despite not running ODB (this is just for testing purposes)
+        'x-next-just-first-accept-language': '1',
+      },
+    }).then((response) => {
+      // make sure we didn't hit SSR handler - not ODB and nothing else handles this request
+      // once platform starts supporting more languages in Accept-Language header - this test
+      // will start failing because then we will get platform level redirect
+      expect(response.headers['x-nf-render-mode']).to.eq('ssr')
+
+      // we don't expect for function handler to respond with a redirect
+      // locale redirects should be handled by Netlify redirects
+      // otherwise we could wrongly cache locale redirect generated by ODB
+      expect(response.status).to.eq(200)
+    })
+  })
 })

packages/runtime/src/templates/getHandler.ts

@@ -130,6 +130,16 @@ const makeHandler = ({ conf, app, pageRoot, NextServer, staticManifest = [], mod
     const query = new URLSearchParams(event.queryStringParameters).toString()
     event.path = query ? `${event.path}?${query}` : event.path
 
+    if (event.headers['accept-language'] && (mode === 'odb' || event.headers['x-next-just-first-accept-language'])) {
+      // keep just first language to match Netlify redirect limitation:
+      // https://docs.netlify.com/routing/redirects/redirect-options/#redirect-by-country-or-language
+      // > Language-based redirects always match against the first language reported by the browser in the Accept-Language header regardless of quality value weighting.
+      // If we wouldn't keep just first language, it's possible for `next-server` to generate locale redirect that could be cached by ODB
+      // because it matches on every language listed: https://github.com/vercel/next.js/blob/5d9597879c46b383d595d6f7b37fd373325b7544/test/unit/accept-headers.test.ts
+      // 'x-next-just-first-accept-language' header is escape hatch to be able to hit this code for tests (both automated and manual)
+      event.headers['accept-language'] = event.headers['accept-language'].replace(/\s*,.*$/, '')
+    }
+
     const { headers, ...result } = await getBridge(event, context).launcher(event, context)
 
     // Convert all headers to multiValueHeaders