Give 409 Conflict whenever the runner was already enabled

CHANGELOG

@@ -40,6 +40,7 @@ v 8.9.0 (unreleased)
   - Added artifacts:when to .gitlab-ci.yml - this requires GitLab Runner 1.3
   - Todos will display target state if issuable target is 'Closed' or 'Merged'
   - Fix bug when sorting issues by milestone due date and filtering by two or more labels
+  - POST to API /projects/:id/runners/:runner_id would give 409 if the runner was already enabled for this project
   - Add support for using Yubikeys (U2F) for two-factor authentication
   - Link to blank group icon doesn't throw a 404 anymore
   - Remove 'main language' feature

app/models/ci/runner.rb

@@ -56,7 +56,7 @@ module Ci
     def assign_to(project, current_user = nil)
       self.is_shared = false if shared?
       self.save
-      project.runner_projects.create(runner_id: self.id)
+      project.runner_projects.create(runner_id: self.id).persisted?
     end
 
     def display_name

lib/api/runners.rb

@@ -96,9 +96,12 @@ module API
 
         runner = get_runner(params[:runner_id])
         authenticate_enable_runner!(runner)
-        runner.assign_to(user_project)
 
+        if runner.assign_to(user_project)
           present runner, with: Entities::Runner
+        else
+          conflict!("Runner was already enabled for this project")
+        end
       end
 
       # Disable project's runner

spec/requests/api/runners_spec.rb

@@ -375,7 +375,7 @@ describe API::Runners, api: true  do
         expect do
           post api("/projects/#{project.id}/runners", user), runner_id: specific_runner.id
         end.to change{ project.runners.count }.by(0)
-        expect(response.status).to eq(201)
+        expect(response.status).to eq(409)
       end
 
       it 'should not enable shared runner' do