fix($core): keep createTemp from desyncing webpack

Calling emptyDirSync every time createTemp is called - i.e, every time App.process is called; i.e., every time source files update - throws off webpack-dev-server, causing it to lose track of the source files and subdirectories. This leads to a variety of sporadic errors and breaks auto-refresh of frontmatter and configuration. Fix this by avoiding emptying temporary directories when we can detect that they were previously initialized by the same instance of the VuePress process. Fixes #1283. Fixes #2233. Fixes #2254. Fixes #2437.

Author: spinda

packages/@vuepress/core/lib/node/createTemp.js

@@ -1,5 +1,10 @@
+const cryptoRandomString = require('crypto-random-string')
 const { fs, path, chalk, logger } = require('@vuepress/shared-utils')
 
+// Generate a unique file name whose presence in a temporary directory indicates
+// that the directory was initialized by this instance of VuePress.
+const shibbolethFileName = `.vuepress-${cryptoRandomString({ length: 16, type: 'hex' })}`
+
 /**
  * Create a dynamic temp utility context that allow to lanuch
  * multiple apps with isolated context at the same time.
@@ -17,10 +22,15 @@ module.exports = function createTemp (tempPath) {
  tempPath = path.resolve(tempPath)
  }
 
- if (!fs.existsSync(tempPath)) {
- fs.ensureDirSync(tempPath)
- } else {
+ // Ensure the temporary directory exists and was initialized by this instance
+ // of VuePress, by checking for the presence of the shibboleth file. Avoid
+ // emptying the temporary directory if it was previously initialized by this
+ // instance of VuePress; otherwise, webpack-dev-server can lose track of the
+ // paths it's watching.
+ const shibbolethFilePath = path.join(tempPath, shibbolethFileName)
+ if (!fs.existsSync(shibbolethFilePath)) {
  fs.emptyDirSync(tempPath)
+ fs.writeFileSync(shibbolethFilePath, '')
  }
 
  logger.debug(`Temp directory: ${chalk.gray(tempPath)}`)

packages/@vuepress/core/package.json

@@ -38,6 +38,7 @@
  "copy-webpack-plugin": "^5.0.2",
  "core-js": "^3.6.4",
  "cross-spawn": "^6.0.5",
+ "crypto-random-string": "^3.2.0",
  "css-loader": "^2.1.1",
  "file-loader": "^3.0.1",
  "js-yaml": "^3.13.1",