Add branch existence check to the APIv4 branches via HEAD request

changelogs/unreleased/api_branches_head.yml

+---
+title: Add branch existence check to the APIv4 branches via HEAD request
+merge_request: 13979
+author: Vitaliy @blackst0ne Klachkov
+type: added

lib/api/branches.rb

@@ -24,18 +24,23 @@ module API
         present paginate(branches), with: Entities::RepoBranch, project: user_project
       end
 
+      resource ':id/repository/branches/:branch', requirements: BRANCH_ENDPOINT_REQUIREMENTS do
         desc 'Get a single branch' do
           success Entities::RepoBranch
         end
         params do
           requires :branch, type: String, desc: 'The name of the branch'
         end
-      get ':id/repository/branches/:branch', requirements: BRANCH_ENDPOINT_REQUIREMENTS do
+        head do
+          user_project.repository.branch_exists?(params[:branch]) ? status(204) : status(404)
+        end
+        get do
           branch = user_project.repository.find_branch(params[:branch])
-        not_found!("Branch") unless branch
+          not_found!('Branch') unless branch
 
           present branch, with: Entities::RepoBranch, project: user_project
         end
+      end
 
       # Note: This API will be deprecated in favor of the protected branches API.
       # Note: The internal data model moved from `developers_can_{merge,push}` to `allowed_to_{merge,push}`

spec/requests/api/branches_spec.rb

@@ -75,6 +75,22 @@ describe API::Branches do
     let(:route) { "/projects/#{project_id}/repository/branches/#{branch_name}" }
 
     shared_examples_for 'repository branch' do
+      context 'HEAD request' do
+        it 'returns 204 No Content' do
+          head api(route, user)
+
+          expect(response).to have_gitlab_http_status(204)
+          expect(response.body).to be_empty
+        end
+
+        it 'returns 404 Not Found' do
+          head api("/projects/#{project_id}/repository/branches/unknown", user)
+
+          expect(response).to have_gitlab_http_status(404)
+          expect(response.body).to be_empty
+        end
+      end
+
       it 'returns the repository branch' do
         get api(route, current_user)