automatically fetch the tip of the base branch if it is not available

.github/workflows/ci.yml

@@ -11,8 +11,6 @@ jobs:
       BUNDLE_GEMFILE: ${{ github.workspace }}/test/using_bundler/Gemfile
     steps:
       - uses: actions/checkout@v4
-      - name: Fetch head commit of base branch
-        run: git fetch --depth 1 origin ${{ github.event.pull_request.base.sha }}
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: 2.6

README.md

@@ -52,6 +52,9 @@ It's same as `-level` flag of reviewdog.
 Optional. Run Rubocop only on changed (and added) files, for speedup [`true`, `false`].
 Default: `false`.
 
+Will fetch the tip of the base branch with depth 1 from remote origin if it is not available.
+If you use different remote name or customize the checkout otherwise, make the tip of the base branch available before this action
+
 ### `reporter`
 
 Optional. Reporter of reviewdog command [`github-pr-check`, `github-check`, `github-pr-review`].

action.yml

@@ -20,7 +20,10 @@ inputs:
     description: 'Report level for reviewdog [info,warning,error]'
     default: 'error'
   only_changed:
-    description: "Run Rubocop only on changed (and added) files, for speedup [`true`, `false`]"
+    description: |
+      Run Rubocop only on changed (and added) files, for speedup [`true`, `false`].
+      Will fetch the tip of the base branch with depth 1 from remote origin if it is not available.
+      If you use different remote name or customize the checkout otherwise, make the tip of the base branch available before this action.
     default: 'false'
   reporter:
     description: |

script.sh

@@ -91,6 +91,11 @@ fi
 if [ "${INPUT_ONLY_CHANGED}" = "true" ]; then
   echo '::group:: Getting changed files list'
 
+  # check if commit is present in repository, otherwise fetch it
+  if ! git cat-file -e "${BASE_REF}"; then
+    git fetch --depth 1 origin "${BASE_REF}"
+  fi
+
   # get intersection of changed files (excluding deleted) with target files for
   # rubocop as an array
   # shellcheck disable=SC2086