Ensure Ci::Runner#assign_to errors for group runners

app/models/ci/runner.rb

@@ -111,6 +111,8 @@ module Ci
       if shared?
         self.is_shared = false if shared?
         self.runner_type = :project_type
+      elsif group_type?
+        raise ArgumentError, 'Transitioning a group runner to a project runner is not supported'
       end
 
       self.save

spec/models/ci/runner_spec.rb

@@ -200,16 +200,29 @@ describe Ci::Runner do
 
   describe '#assign_to' do
     let!(:project) { FactoryBot.create(:project) }
-    let!(:shared_runner) { FactoryBot.create(:ci_runner, :shared) }
 
-    before do
-      shared_runner.assign_to(project)
+    subject { runner.assign_to(project) }
+
+    context 'with shared_runner' do
+      let!(:runner) { FactoryBot.create(:ci_runner, :shared) }
+
+      it 'transitions shared runner to project runner and assigns project' do
+        subject
+        expect(runner).to be_specific
+        expect(runner).to be_project_type
+        expect(runner.projects).to eq([project])
+        expect(runner.only_for?(project)).to be_truthy
+      end
     end
 
-    it { expect(shared_runner).to be_specific }
-    it { expect(shared_runner).to be_project_type }
-    it { expect(shared_runner.projects).to eq([project]) }
-    it { expect(shared_runner.only_for?(project)).to be_truthy }
+    context 'with group runner' do
+      let!(:runner) { FactoryBot.create(:ci_runner, runner_type: :group_type) }
+
+      it 'raises an error' do
+        expect { subject }
+          .to raise_error(ArgumentError, 'Transitioning a group runner to a project runner is not supported')
+      end
+    end
   end
 
   describe '.online' do