Merge branch '51226-check-yarn-lock' into 'master'

Add danger check for duplicate yarn dependencies

Closes #51226

See merge request gitlab-org/gitlab-ce!21591

Dangerfile

@@ -8,5 +8,6 @@ danger.import_dangerfile(path: 'danger/database')
 danger.import_dangerfile(path: 'danger/documentation')
 danger.import_dangerfile(path: 'danger/frozen_string')
 danger.import_dangerfile(path: 'danger/commit_messages')
+danger.import_dangerfile(path: 'danger/duplicate_yarn_dependencies')
 danger.import_dangerfile(path: 'danger/prettier')
 danger.import_dangerfile(path: 'danger/eslint')

danger/duplicate_yarn_dependencies/Dangerfile

+# frozen_string_literal: true
+
+return unless helper.all_changed_files.include? 'yarn.lock'
+
+duplicate = `node_modules/.bin/yarn-deduplicate --list --strategy fewer yarn.lock`
+             .split(/$/)
+             .map(&:strip)
+             .reject(&:empty?)
+
+return if duplicate.empty?
+
+warn 'This merge request has introduced duplicated yarn dependencies.'
+
+markdown(<<~MARKDOWN)
+  ## Duplicate yarn dependencies
+
+  The following dependencies should be de-duplicated:
+
+  * #{duplicate.map { |path| "`#{path}`" }.join("\n* ")}
+
+  Please run the following command and commit the changes to `yarn.lock`:
+
+  ```
+  node_modules/.bin/yarn-deduplicate --strategy fewer yarn.lock \\
+    && yarn install
+  ```
+MARKDOWN

package.json

@@ -158,7 +158,8 @@
     "nodemon": "^1.18.4",
     "prettier": "1.15.2",
     "vue-jest": "^3.0.1",
-    "webpack-dev-server": "^3.1.10"
+    "webpack-dev-server": "^3.1.10",
+    "yarn-deduplicate": "^1.0.5"
   },
   "engines": {
     "yarn": "^1.10.0"