Implement the logic for locking runner

app/models/ci/build.rb

@@ -291,9 +291,7 @@ module Ci
     end
     def can_be_served?(runner)
-      return false unless has_tags? || runner.run_untagged?
-
-      (tag_list - runner.tag_list).empty?
+      runner.can_serve?(self)
     end
     def has_tags?

app/models/ci/runner.rb

@@ -91,6 +91,12 @@ module Ci
       !shared?
     end
+    def can_serve?(build)
+      not_locked_or_locked_to?(build.project) &&
+        run_untagged_or_has_tags?(build) &&
+        accepting_tags?(build.tag_list)
+    end
+
     def only_for?(project)
       projects == [project]
     end
@@ -111,5 +117,17 @@ module Ci
           'can not be empty when runner is not allowed to pick untagged jobs')
       end
     end
+
+    def not_locked_or_locked_to?(project)
+      !locked? || projects.exists?(id: project.id)
+    end
+
+    def run_untagged_or_has_tags?(build)
+      run_untagged? || build.has_tags?
+    end
+
+    def accepting_tags?(target_tags)
+      (target_tags - tag_list).empty?
+    end
   end
 end

spec/models/build_spec.rb

@@ -306,6 +306,57 @@ describe Ci::Build, models: true do
         it_behaves_like 'tagged build picker'
       end
     end
+
+    context 'when runner is locked' do
+      before { runner.locked = true }
+
+      shared_examples 'locked build picker' do |serve_matching_tags|
+        context 'when runner cannot pick untagged jobs' do
+          before { runner.run_untagged = false }
+
+          it 'cannot handle builds without tags' do
+            expect(build.can_be_served?(runner)).to be_falsey
+          end
+        end
+
+        context 'when having runner tags' do
+          before { runner.tag_list = ['bb', 'cc'] }
+
+          it "#{serve_matching_tags} handle it for matching tags" do
+            build.tag_list = ['bb']
+            expected = if serve_matching_tags
+                         be_truthy
+                       else
+                         be_falsey
+                       end
+            expect(build.can_be_served?(runner)).to expected
+          end
+
+          it 'cannot handle it for builds without matching tags' do
+            build.tag_list = ['aa']
+            expect(build.can_be_served?(runner)).to be_falsey
+          end
+        end
+      end
+
+      context 'when serving the same project' do
+        it 'can handle it' do
+          expect(build.can_be_served?(runner)).to be_truthy
+        end
+
+        it_behaves_like 'locked build picker', true
+      end
+
+      context 'serving a different project' do
+        before { runner.runner_projects.destroy_all }
+
+        it 'cannot handle it' do
+          expect(build.can_be_served?(runner)).to be_falsey
+        end
+
+        it_behaves_like 'locked build picker', false
+      end
+    end
   end
   describe '#has_tags?' do