CNA: Add warning about permission (#14889)

close #14744

<img width="851" alt="screen_shot" src="https://user-images.githubusercontent.com/39780486/86603491-06bec800-bfdf-11ea-9928-ee85cbad86a7.png">


I have some concerns.

- `import { isWriteable } from '../next/build/is-writeable'` **not** from `create-next-app ` package.
- The warning sentence is from npm. Not for Next.js. I'm not a native English speaker. I'd like to know the natural expression for this.

packages/create-next-app/create-app.ts

@@ -19,6 +19,7 @@ import { install } from './helpers/install'
 import { isFolderEmpty } from './helpers/is-folder-empty'
 import { getOnline } from './helpers/is-online'
 import { shouldUseYarn } from './helpers/should-use-yarn'
+import { isWriteable } from './helpers/is-writeable'
 
 export class DownloadError extends Error {}
 
@@ -93,6 +94,17 @@ export async function createApp({
   }
 
   const root = path.resolve(appPath)
+
+  if (!(await isWriteable(path.dirname(root)))) {
+    console.error(
+      'The application path is not writable, please check folder permissions and try again.'
+    )
+    console.error(
+      'It is likely you do not have write permissions for this folder.'
+    )
+    process.exit(1)
+  }
+
   const appName = path.basename(root)
 
   await makeDir(root)

packages/create-next-app/helpers/is-writeable.ts

+import fs from 'fs'
+
+export async function isWriteable(directory: string): Promise<boolean> {
+  try {
+    await fs.promises.access(directory, (fs.constants || fs).W_OK)
+    return true
+  } catch (err) {
+    return false
+  }
+}

test/integration/create-next-app/index.test.js

@@ -23,9 +23,9 @@ const runStarter = (cwd, ...args) => {
   return res
 }
 
-async function usingTempDir(fn) {
+async function usingTempDir(fn, options) {
   const folder = path.join(os.tmpdir(), Math.random().toString(36).substring(2))
-  await fs.mkdirp(folder)
+  await fs.mkdirp(folder, options)
   try {
     return await fn(folder)
   } finally {
@@ -251,4 +251,28 @@ describe('create next app', () => {
       }
     })
   })
+
+  it('should exit if the folder is not writable', async () => {
+    await usingTempDir(async (cwd) => {
+      const projectName = 'not-writable'
+      expect.assertions(2)
+      try {
+        const res = await runStarter(cwd, projectName)
+
+        if (process.platform === 'win32') {
+          expect(res.exitCode).toBe(0)
+          expect(
+            fs.existsSync(path.join(cwd, projectName, 'package.json'))
+          ).toBeTruthy()
+        }
+      } catch (e) {
+        // eslint-disable-next-line jest/no-try-expect
+        expect(e.exitCode).toBe(1)
+        // eslint-disable-next-line jest/no-try-expect
+        expect(e.stderr).toMatch(
+          /you do not have write permissions for this folder/
+        )
+      }
+    }, 0o500)
+  })
 })