Don't require source files to be writeable in dev mode

packages/next/server/dev/hot-reloader.ts

@@ -24,7 +24,7 @@ import onDemandEntryHandler, {
 } from './on-demand-entry-handler'
 import { denormalizePagePath, normalizePathSep } from '../normalize-page-path'
 import getRouteFromEntrypoint from '../get-route-from-entrypoint'
-import { isWriteable } from '../../build/is-writeable'
+import { fileExists } from '../../lib/file-exists'
 import { ClientPagesLoaderOptions } from '../../build/webpack/loaders/next-client-pages-loader'
 import { ssrEntries } from '../../build/webpack/plugins/middleware-plugin'
 import { stringify } from 'querystring'
@@ -452,7 +452,7 @@ export default class HotReloader {
             }
 
             const { bundlePath, absolutePagePath, dispose } = entries[pageKey]
-            const pageExists = !dispose && (await isWriteable(absolutePagePath))
+            const pageExists = !dispose && (await fileExists(absolutePagePath))
             if (!pageExists) {
               // page was removed or disposed
               delete entries[pageKey]

test/integration/read-only-source-hmr/pages/hello.js

@@ -0,0 +1,3 @@
+const Hello = () => <p>Hello World</p>
+
+export default Hello

test/integration/read-only-source-hmr/test/index.test.js

@@ -0,0 +1,123 @@
+/* eslint-env jest */
+
+import fs from 'fs-extra'
+import {
+  check,
+  findPort,
+  getBrowserBodyText,
+  killApp,
+  launchApp,
+} from 'next-test-utils'
+import webdriver from 'next-webdriver'
+import { join } from 'path'
+
+const READ_ONLY_PERMISSIONS = 0o444
+const READ_WRITE_PERMISSIONS = 0o644
+
+const appDir = join(__dirname, '..')
+const pagePath = join(appDir, 'pages/hello.js')
+
+let appPort
+let app
+
+async function writeReadOnlyFile(path, content) {
+  const exists = await fs
+    .access(path)
+    .then(() => true)
+    .catch(() => false)
+
+  if (exists) {
+    await fs.chmod(path, READ_WRITE_PERMISSIONS)
+  }
+
+  await fs.writeFile(path, content, 'utf8')
+  await fs.chmod(path, READ_ONLY_PERMISSIONS)
+}
+
+describe('Read-only source HMR', () => {
+  beforeAll(async () => {
+    await fs.chmod(pagePath, READ_ONLY_PERMISSIONS)
+
+    appPort = await findPort()
+    app = await launchApp(appDir, appPort, {
+      env: {
+        __NEXT_TEST_WITH_DEVTOOL: 1,
+        // Events can be finicky in CI. This switches to a more reliable
+        // polling method.
+        CHOKIDAR_USEPOLLING: 'true',
+        CHOKIDAR_INTERVAL: 500,
+      },
+    })
+  })
+
+  afterAll(async () => {
+    await fs.chmod(pagePath, READ_WRITE_PERMISSIONS)
+    await killApp(app)
+  })
+
+  it('should detect changes to a page', async () => {
+    let browser
+
+    try {
+      browser = await webdriver(appPort, '/hello')
+      await check(() => getBrowserBodyText(browser), /Hello World/)
+
+      const originalContent = await fs.readFile(pagePath, 'utf8')
+      const editedContent = originalContent.replace('Hello World', 'COOL page')
+
+      await writeReadOnlyFile(pagePath, editedContent)
+      await check(() => getBrowserBodyText(browser), /COOL page/)
+
+      await writeReadOnlyFile(pagePath, originalContent)
+      await check(() => getBrowserBodyText(browser), /Hello World/)
+    } finally {
+      if (browser) {
+        await browser.close()
+      }
+    }
+  })
+
+  it('should handle page deletion and subsequent recreation', async () => {
+    let browser
+
+    try {
+      browser = await webdriver(appPort, '/hello')
+      await check(() => getBrowserBodyText(browser), /Hello World/)
+
+      const originalContent = await fs.readFile(pagePath, 'utf8')
+
+      await fs.remove(pagePath)
+
+      await writeReadOnlyFile(pagePath, originalContent)
+      await check(() => getBrowserBodyText(browser), /Hello World/)
+    } finally {
+      if (browser) {
+        await browser.close()
+      }
+    }
+  })
+
+  it('should detect a new page', async () => {
+    let browser
+    const newPagePath = join(appDir, 'pages/new.js')
+
+    try {
+      await writeReadOnlyFile(
+        newPagePath,
+        `
+        const New = () => <p>New page</p>
+
+        export default New
+      `
+      )
+
+      browser = await webdriver(appPort, '/new')
+      await check(() => getBrowserBodyText(browser), /New page/)
+    } finally {
+      if (browser) {
+        await browser.close()
+      }
+      await fs.remove(newPagePath)
+    }
+  })
+})