Support incrementalCacheHandlerPath for standalone output (#48694)

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### What?
This is a follow-up to #46290 by
@ijjk

Currently, nextjs build resolves provided incrementalCacheHandlerPath
over destDir (.next by default). And for standalone builds the custom
cache can't be resolved and leads to runtime error.

### Why?
This fix basically introduces support of incrementalCacheHandlerPath for
the standalone build.

### How?
incrementalCacheHandlerPath is now always resolved relatively to
distDir, which should work for both default and standalone builds.

Also, for convenience (and better testability)
incrementalCacheHandlerPath can now be provided as a relative path to
the project directory.

packages/next/src/build/index.ts

@@ -826,6 +826,8 @@ export default async function build(
 
       const manifestPath = path.join(distDir, SERVER_DIRECTORY, PAGES_MANIFEST)
 
+      const { incrementalCacheHandlerPath } = config.experimental
+
       const requiredServerFiles = nextBuildSpan
         .traceChild('generate-required-server-files')
         .traceFn(() => ({
@@ -841,12 +843,8 @@ export default async function build(
             experimental: {
               ...config.experimental,
               trustHostHeader: ciEnvironment.hasNextSupport,
-              incrementalCacheHandlerPath: config.experimental
-                .incrementalCacheHandlerPath
-                ? path.relative(
-                    distDir,
-                    config.experimental.incrementalCacheHandlerPath
-                  )
+              incrementalCacheHandlerPath: incrementalCacheHandlerPath
+                ? path.relative(distDir, incrementalCacheHandlerPath)
                 : undefined,
             },
           },
@@ -1918,9 +1916,13 @@ export default async function build(
 
             // ensure we trace any dependencies needed for custom
             // incremental cache handler
-            if (config.experimental.incrementalCacheHandlerPath) {
+            if (incrementalCacheHandlerPath) {
               toTrace.push(
-                require.resolve(config.experimental.incrementalCacheHandlerPath)
+                require.resolve(
+                  path.isAbsolute(incrementalCacheHandlerPath)
+                    ? incrementalCacheHandlerPath
+                    : path.join(dir, incrementalCacheHandlerPath)
+                )
               )
             }
 

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

@@ -30,7 +30,7 @@ import type { MiddlewareRouteMatch } from '../shared/lib/router/utils/middleware
 import type { RouteMatch } from './future/route-matches/route-match'
 
 import fs from 'fs'
-import { join, relative, resolve, sep } from 'path'
+import { join, relative, resolve, sep, isAbsolute } from 'path'
 import { IncomingMessage, ServerResponse } from 'http'
 import { addRequestMeta, getRequestMeta } from './request-meta'
 import {
@@ -385,9 +385,9 @@ export default class NextNodeServer extends BaseServer {
     const { incrementalCacheHandlerPath } = this.nextConfig.experimental
 
     if (incrementalCacheHandlerPath) {
-      CacheHandler = require(this.minimalMode
-        ? join(this.distDir, incrementalCacheHandlerPath)
-        : incrementalCacheHandlerPath)
+      CacheHandler = require(isAbsolute(incrementalCacheHandlerPath)
+        ? incrementalCacheHandlerPath
+        : join(this.distDir, incrementalCacheHandlerPath))
       CacheHandler = CacheHandler.default || CacheHandler
     }
 

test/production/standalone-mode/required-server-files/cache-handler.js

@@ -0,0 +1,20 @@
+const cache = new Map()
+
+module.exports = class CacheHandler {
+  constructor() {
+    console.log('initialized custom cache-handler')
+  }
+
+  async get(key) {
+    console.log('cache-handler get', key)
+    return cache.get(key)
+  }
+
+  async set(key, data) {
+    console.log('cache-handler set', key)
+    cache.set(key, {
+      value: data,
+      lastModified: Date.now(),
+    })
+  }
+}

test/production/standalone-mode/required-server-files/required-server-files.test.ts

@@ -37,12 +37,16 @@ describe('should set-up next', () => {
         pages: new FileRef(join(__dirname, 'pages')),
         lib: new FileRef(join(__dirname, 'lib')),
         'middleware.js': new FileRef(join(__dirname, 'middleware.js')),
+        'cache-handler.js': new FileRef(join(__dirname, 'cache-handler.js')),
         'data.txt': new FileRef(join(__dirname, 'data.txt')),
         '.env': new FileRef(join(__dirname, '.env')),
         '.env.local': new FileRef(join(__dirname, '.env.local')),
         '.env.production': new FileRef(join(__dirname, '.env.production')),
       },
       nextConfig: {
+        experimental: {
+          incrementalCacheHandlerPath: './cache-handler.js',
+        },
         eslint: {
           ignoreDuringBuilds: true,
         },
@@ -297,6 +301,16 @@ describe('should set-up next', () => {
     ).toContain('"compress":false')
   })
 
+  it('`incrementalCacheHandlerPath` should have correct path', async () => {
+    expect(
+      await fs.pathExists(join(next.testDir, 'standalone/cache-handler.js'))
+    ).toBe(true)
+
+    expect(
+      await fs.readFileSync(join(next.testDir, 'standalone/server.js'), 'utf8')
+    ).toContain('"incrementalCacheHandlerPath":"../cache-handler.js"')
+  })
+
   it('should output middleware correctly', async () => {
     expect(
       await fs.pathExists(