Prepare custom routes loading for adding additional routes (#13857)

Extracted from #13333, the same exact code lives in that PR as well, but we can merge this separately if it makes reviewing #13333 easier

This PR does 3 things 
- deduplicate code from build and next-dev-server that loads custom routes from next.config.js  (`loadCustomRoutes`)
- in `loadCustomRoutes`, load these rewrites, headers and redirects configs concurrently instead of sequentially.
- in next-server, make `this.customRoutes` always defined, this allows us to remove the big `if` around its initialization code in `generateRoutes`, which in turn makes it possible to reuse this code for other routing than user defined routes, which is how #13333 adds its redirects.

packages/next/build/index.ts

@@ -10,13 +10,11 @@ import nanoid from 'next/dist/compiled/nanoid/index.js'
 import { pathToRegexp } from 'next/dist/compiled/path-to-regexp'
 import path from 'path'
 import formatWebpackMessages from '../client/dev/error-overlay/format-webpack-messages'
-import checkCustomRoutes, {
+import loadCustomRoutes, {
   getRedirectStatus,
-  Header,
   Redirect,
-  Rewrite,
   RouteType,
-} from '../lib/check-custom-routes'
+} from '../lib/load-custom-routes'
 import {
   PAGES_404_GET_INITIAL_PROPS_ERROR,
   PUBLIC_DIR_MIDDLEWARE_CONFLICT,
@@ -112,25 +110,8 @@ export default async function build(dir: string, conf = null): Promise<void> {
   const { target } = config
   const buildId = await generateBuildId(config.generateBuildId, nanoid)
   const distDir = path.join(dir, config.distDir)
-  const headers: Header[] = []
-  const rewrites: Rewrite[] = []
-  const redirects: Redirect[] = []
-
-  if (typeof config.experimental.redirects === 'function') {
-    const _redirects = await config.experimental.redirects()
-    checkCustomRoutes(_redirects, 'redirect')
-    redirects.push(..._redirects)
-  }
-  if (typeof config.experimental.rewrites === 'function') {
-    const _rewrites = await config.experimental.rewrites()
-    checkCustomRoutes(_rewrites, 'rewrite')
-    rewrites.push(..._rewrites)
-  }
-  if (typeof config.experimental.headers === 'function') {
-    const _headers = await config.experimental.headers()
-    checkCustomRoutes(_headers, 'header')
-    headers.push(..._headers)
-  }
+
+  const { headers, rewrites, redirects } = await loadCustomRoutes(config)
 
   if (ciEnvironment.isCI && !ciEnvironment.hasNextSupport) {
     const cacheDir = path.join(distDir, 'cache')

packages/next/build/utils.ts

@@ -5,7 +5,12 @@ import textTable from 'next/dist/compiled/text-table'
 import path from 'path'
 import { isValidElementType } from 'react-is'
 import stripAnsi from 'next/dist/compiled/strip-ansi'
-import { Redirect, Rewrite, Header } from '../lib/check-custom-routes'
+import {
+  Redirect,
+  Rewrite,
+  Header,
+  CustomRoutes,
+} from '../lib/load-custom-routes'
 import {
   SSG_GET_INITIAL_PROPS_CONFLICT,
   SERVER_PROPS_GET_INIT_PROPS_CONFLICT,
@@ -279,11 +284,7 @@ export function printCustomRoutes({
   redirects,
   rewrites,
   headers,
-}: {
-  redirects: Redirect[]
-  rewrites: Rewrite[]
-  headers: Header[]
-}) {
+}: CustomRoutes) {
   const printRoutes = (
     routes: Redirect[] | Rewrite[] | Header[],
     type: 'Redirects' | 'Rewrites' | 'Headers'

packages/next/lib/check-custom-routes.ts → packages/next/lib/load-custom-routes.ts

@@ -122,7 +122,7 @@ function tryParsePath(route: string, handleUrl?: boolean): ParseAttemptResult {
 
 export type RouteType = 'rewrite' | 'redirect' | 'header'
 
-export default function checkCustomRoutes(
+function checkCustomRoutes(
   routes: Redirect[] | Header[] | Rewrite[],
   type: RouteType
 ): void {
@@ -302,3 +302,52 @@ export default function checkCustomRoutes(
     throw new Error(`Invalid ${type}${numInvalidRoutes === 1 ? '' : 's'} found`)
   }
 }
+
+export interface CustomRoutes {
+  headers: Header[]
+  rewrites: Rewrite[]
+  redirects: Redirect[]
+}
+
+async function loadRedirects(config: any) {
+  if (typeof config.experimental.redirects !== 'function') {
+    return []
+  }
+  const _redirects = await config.experimental.redirects()
+  checkCustomRoutes(_redirects, 'redirect')
+  return _redirects
+}
+
+async function loadRewrites(config: any) {
+  if (typeof config.experimental.rewrites !== 'function') {
+    return []
+  }
+  const _rewrites = await config.experimental.rewrites()
+  checkCustomRoutes(_rewrites, 'rewrite')
+  return _rewrites
+}
+
+async function loadHeaders(config: any) {
+  if (typeof config.experimental.headers !== 'function') {
+    return []
+  }
+  const _headers = await config.experimental.headers()
+  checkCustomRoutes(_headers, 'header')
+  return _headers
+}
+
+export default async function loadCustomRoutes(
+  config: any
+): Promise<CustomRoutes> {
+  const [headers, rewrites, redirects] = await Promise.all([
+    loadHeaders(config),
+    loadRewrites(config),
+    loadRedirects(config),
+  ])
+
+  return {
+    headers,
+    rewrites,
+    redirects,
+  }
+}