Strip back pipeline to Rubocop linting only (#562)

* Only lint Rubocop for now (and only modified files)

deleted files are excluded

* Scope action for PRs

* Disable tests for now

* Run command in one line

* Fix RuboCop command

* Further improve linting setup

* Simplify linter setup

* Print changed ruby files

* Fix further errors and improve workflow file

* Change random Rails files to see effects

* Fix spaces in changed files

* Include target branch lookup

* Remove unnecessary ${{}} syntax

* Fetch target branch

* Improve commit retrieval

* Redo whole workflow

* Revert dummy changes in Rails files

* Use checkout@v4 (instead of @V3)

* Explicitly set fail level & force excluding files

See command line flags here:
https://docs.rubocop.org/rubocop/usage/basic_usage.html#command-line-flags

* Add more events to trigger workflow

See the docs here:
https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request

* Add printout to display RuboCop version used

.github/workflows/linter.yml

@@ -1,87 +1,45 @@
-on: [push]
+name: Linting
+
+# Trigger each time HEAD branch is updated in a pull request
+# see https://github.com/orgs/community/discussions/26366
+on:
+  pull_request:
+    types: [opened, reopened, synchronize, ready_for_review]
 
 jobs:
+
+  # Resources:
+  # number [1] being most important
+  # [1] https://github.com/actions/checkout/issues/520#issuecomment-1167205721
+  # [2] https://robertfaldo.medium.com/commands-to-run-rubocop-and-specs-you-changed-in-your-branch-e6d2f2e4110b
+  # [3] https://community.atlassian.com/t5/Bitbucket-questions/Git-diff-show-different-files-than-PR-Pull-Request/qaq-p/2331786
   rubocop:
+    name: RuboCop
     runs-on: ubuntu-latest
-    name: A job to check rubocop linter errors
     steps:
-      - uses: actions/checkout@v2
+      - name: 'Checkout PR branch (with test merge-commit)'
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 2 # to also fetch parent of PR
+
+      # Adapted from this great comment [1]. Git diff adapted from [2].
+      # "|| test $? = 1;" is used to ignore the exit code of grep when no files
+      # are found matching the pattern. For the "three dots" ... syntax, see [3].
+      - name: Get changed files
+        run: |
+          files=$(git diff --name-only --diff-filter=ACMR -r HEAD^1...HEAD | grep '\.rb$' || test $? = 1;)
+          printf "🎴 Changed ruby files: \n$files"
+          echo "CHANGED_FILES=$(echo ${files} | xargs)" >> $GITHUB_ENV
+      
       - name: Set up Ruby 3
+        if: env.CHANGED_FILES != ''
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: 3.1.4
-      - name: Install gems # usual step to install the gems.
+          bundler-cache: true
+
+      - name: Run RuboCop
+        if: env.CHANGED_FILES != ''
         run: |
-          bin/bundle config path vendor/bundle
-          bin/bundle config set without 'default doc job cable storage ujs test db'
-          bin/bundle install --jobs 4 --retry 3
-      - name: Linter count
-        id: hello
-        uses: henrixapp/[email protected]
-        with:
-          name: Rubocop
-          command: bin/bundle exec rubocop app config lib spec
-          total_regexp: \d+ offenses detected
-          errors_regexp: \d+ offenses detected
-          warnings_regexp: \d+ offenses detected
-          compare_branch: mampf-next
-          mode: changed
-          include: .rb
-  eslint:
-    runs-on: ubuntu-latest
-    name: A job to check eslint linter errors
-    steps:
-      - uses: actions/checkout@v2
-      - name: Linter count
-        id: hello
-        uses: henrixapp/[email protected]
-        with:
-          name: EsLint
-          command: npx eslint
-          total_regexp: \d+ problems
-          errors_regexp: \d+ errors
-          warnings_regexp: \d+ warnings
-          compare_branch: mampf-next
-          mode: changed
-          include: .js
-  coffee:
-    runs-on: ubuntu-latest
-    name: A job to check coffee linter errors
-    steps:
-      - uses: actions/checkout@v2
-      - name: Linter count
-        id: hello
-        uses: henrixapp/[email protected]
-        with:
-          name: Coffee
-          command: npx coffeelint
-          total_regexp: \d+ errors
-          errors_regexp: \d+ errors
-          warnings_regexp: \d+ warnings
-          compare_branch: mampf-next
-          mode: changed
-          include: .coffee
-  # erblint:
-  #   runs-on: ubuntu-latest
-  #   name: A job to check erblint linter errors
-  #   steps:
-  #   - uses: actions/checkout@v2
-  #   - name: Set up Ruby 2.7
-  #     uses: ruby/setup-ruby@v1
-  #     with:
-  #       ruby-version: 2.7
-  #   - name: Install gems # usual step to install the gems.
-  #     run: |
-  #       bin/bundle config path vendor/bundle
-  #       bin/bundle config set without 'default doc job cable storage ujs test db'
-  #       bin/bundle install --jobs 4 --retry 3
-  #   - name: Linter count
-  #     id: hello
-  #     uses: henrixapp/[email protected]
-  #     with:
-  #       name: Erblint
-  #       command:  bin/bundle exec erblint  . 
-  #       total_regexp: \d+ error(s)
-  #       errors_regexp: \d+ error(s)
-  #       warnings_regexp: \d+ error(s)
-  #       compare_branch: mampf-next
+          echo "🚨 Running RuboCop version: $(bundle info rubocop | head -1)"
+          bundle exec rubocop --format github --fail-level 'convention' --force-exclusion -- $CHANGED_FILES