Admin should be able to turn shared runners into specific ones:

The regression was introduced by:

https://gitlab.com/gitlab-org/gitlab-ce/commit/1b8f52d9206bdf19c0dde04505c4c0b1cf46cfbe

I did that because there's a test specifying that a shared runner cannot
be enabled, in the API. So I assume that is the case for non-admin, but
admins should be able to do so anyway.

Also added a test to make sure this won't regress again.

Closes #19039

app/controllers/admin/runner_projects_controller.rb

@@ -4,7 +4,7 @@ class Admin::RunnerProjectsController < Admin::ApplicationController
   def create
     @runner = Ci::Runner.find(params[:runner_project][:runner_id])
 
-    return head(403) if @runner.is_shared? || @runner.locked?
+    return head(403) if @runner.locked?
 
     runner_project = @runner.assign_to(@project, current_user)
 

spec/features/admin/admin_runners_spec.rb

@@ -62,19 +62,35 @@ describe "Admin Runners" do
     end
 
     describe 'enable/create' do
-      before do
-        @project1.runners << runner
-        visit admin_runner_path(runner)
+      shared_examples 'enable runners' do
+        it 'enables a runner for a project' do
+          within '.unassigned-projects' do
+            click_on 'Enable'
+          end
+
+          assigned_project = page.find('.assigned-projects')
+
+          expect(assigned_project).to have_content(@project2.path)
+        end
       end
 
-      it 'enables specific runner for project' do
-        within '.unassigned-projects' do
-          click_on 'Enable'
+      context 'with specific runner' do
+        before do
+          @project1.runners << runner
+          visit admin_runner_path(runner)
         end
 
-        assigned_project = page.find('.assigned-projects')
+        it_behaves_like 'enable runners'
+      end
+
+      context 'with shared runner' do
+        before do
+          @project1.destroy
+          runner.update(is_shared: true)
+          visit admin_runner_path(runner)
+        end
 
-        expect(assigned_project).to have_content(@project2.path)
+        it_behaves_like 'enable runners'
       end
     end