Fix getStaticPaths modules being cached in dev mode (#10852)

packages/next/server/static-paths-worker.ts

-import { join } from 'path'
 import { buildStaticPaths } from '../build/utils'
-import { getPagePath } from '../next-server/server/require'
 import { loadComponents } from '../next-server/server/load-components'
-import { PAGES_MANIFEST, SERVER_DIRECTORY } from '../next-server/lib/constants'
+
+// store initial require modules so we don't clear them below
+const initialCache = new Set(Object.keys(require.cache))
 
 // we call getStaticPaths in a separate process to ensure
 // side-effects aren't relied on in dev that will break
@@ -16,10 +16,11 @@ export async function loadStaticPaths(
   // we need to clear any modules manually here since the
   // require-cache-hot-loader doesn't affect require cache here
   // since we're in a separate process
-  delete require.cache[join(distDir, SERVER_DIRECTORY, PAGES_MANIFEST)]
-
-  const pagePath = await getPagePath(pathname, distDir, serverless, true)
-  delete require.cache[pagePath]
+  Object.keys(require.cache).forEach(mod => {
+    if (!initialCache.has(mod)) {
+      delete require.cache[mod]
+    }
+  })
 
   const components = await loadComponents(
     distDir,

test/integration/prerender/test/index.test.js

@@ -983,6 +983,46 @@ describe('SSG Prerender', () => {
     runTests(true)
   })
 
+  describe('dev mode getStaticPaths', () => {
+    beforeAll(async () => {
+      await fs.writeFile(
+        nextConfig,
+        // we set cpus to 1 so that we make sure the requests
+        // aren't being cached at the jest-worker level
+        `module.exports = { experimental: { cpus: 1 } }`,
+        'utf8'
+      )
+      await fs.remove(join(appDir, '.next'))
+      appPort = await findPort()
+      app = await launchApp(appDir, appPort)
+    })
+    afterAll(async () => {
+      await fs.remove(nextConfig)
+      await killApp(app)
+    })
+
+    it('should not cache getStaticPaths errors', async () => {
+      const errMsg = /The `fallback` key must be returned from getStaticPaths/
+      await check(() => renderViaHTTP(appPort, '/blog/post-1'), /post-1/)
+
+      const blogPage = join(appDir, 'pages/blog/[post]/index.js')
+      const origContent = await fs.readFile(blogPage, 'utf8')
+      await fs.writeFile(
+        blogPage,
+        origContent.replace('fallback: true,', '/* fallback: true, */')
+      )
+
+      try {
+        await check(() => renderViaHTTP(appPort, '/blog/post-1'), errMsg)
+
+        await fs.writeFile(blogPage, origContent)
+        await check(() => renderViaHTTP(appPort, '/blog/post-1'), /post-1/)
+      } finally {
+        await fs.writeFile(blogPage, origContent)
+      }
+    })
+  })
+
   describe('serverless mode', () => {
     beforeAll(async () => {
       await fs.writeFile(