Add support for 412 Precondition Failed status for static files (#7944)

* Add support for Precondition Failed status for static files

* Add test

* Trigger tests

* Update next-server.ts

packages/next-server/server/next-server.ts

@@ -648,6 +648,9 @@ export default class Server {
     } catch (err) {
       if (err.code === 'ENOENT' || err.statusCode === 404) {
         this.render404(req, res, parsedUrl)
+      } else if (err.statusCode === 412) {
+        res.statusCode = 412
+        return this.renderError(err, req, res, path)
       } else {
         throw err
       }

test/integration/production/test/index.test.js

@@ -126,6 +126,21 @@ describe('Production Usage', () => {
       expect(res.status).toBe(405)
     })
 
+    it('should return 412 method on static file if If-Unmodified-Since is invalid', async () => {
+      const buildId = readFileSync(join(__dirname, '../.next/BUILD_ID'), 'utf8')
+
+      const res = await fetch(
+        `http://localhost:${appPort}/_next/static/${buildId}/pages/index.js`,
+        {
+          method: 'GET',
+          headers: {
+            'if-unmodified-since': 'Fri, 12 Jul 2019 20:00:13 GMT'
+          }
+        }
+      )
+      expect(res.status).toBe(412)
+    })
+
     it('should set Content-Length header', async () => {
       const url = `http://localhost:${appPort}`
       const res = await fetch(url)