API: Catch empty commit messages

changelogs/unreleased/api-empty-commit-message.yml

+---
+title: 'API: Catch empty commit messages'
+merge_request: 21322
+author: Robert Schilling
+type: fixed

lib/api/files.rb

@@ -59,7 +59,7 @@ module API
       params :simple_file_params do
         requires :file_path, type: String, desc: 'The url encoded path to the file. Ex. lib%2Fclass%2Erb'
         requires :branch, type: String, desc: 'Name of the branch to commit into. To create a new branch, also provide `start_branch`.'
-        requires :commit_message, type: String, desc: 'Commit message'
+        requires :commit_message, type: String, regexp: /^\S+$/, desc: 'Commit message'
         optional :start_branch, type: String, desc: 'Name of the branch to start the new commit from'
         optional :author_email, type: String, desc: 'The email of the author'
         optional :author_name, type: String, desc: 'The name of the author'

spec/requests/api/files_spec.rb

@@ -337,6 +337,18 @@ describe API::Files do
       expect(response).to have_gitlab_http_status(400)
     end
 
+    it 'returns a 400 bad request if the commit message is empty' do
+      invalid_params = {
+        branch: 'master',
+        content: 'puts 8',
+        commit_message: ''
+      }
+
+      post api(route(file_path), user), invalid_params
+
+      expect(response).to have_gitlab_http_status(400)
+    end
+
     it "returns a 400 if editor fails to create file" do
       allow_any_instance_of(Repository).to receive(:create_file)
         .and_raise(Gitlab::Git::CommitError, 'Cannot create file')