test: add test case

Author: pieh

cypress/e2e/default/i18n.cy.ts

@@ -35,4 +35,28 @@ 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`, () => {
+    // make sure we don't use cached results
+    cy.setCookie('__prerender_bypass', '1')
+    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',
+        Accept: '*/*',
+        cookie: '__prerender_bypass=1',
+      },
+    }).then((response) => {
+      // we don't expect for function handler to return 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

@@ -135,6 +135,7 @@ const makeHandler = ({ conf, app, pageRoot, NextServer, staticManifest = [], mod
       // 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
       event.headers['accept-language'] = event.headers['accept-language'].replace(/\s*,.*$/, '')
     }