Fix picking CI builds

The conflict happens when we try to update a build,
but fail to do so due to fact that we update the same build concurrently for two different runners.

app/services/ci/register_build_service.rb

@@ -29,10 +29,10 @@ module Ci
         build.run!
       end
 
-      build
+      [build, true]
 
     rescue StateMachines::InvalidTransition, ActiveRecord::StaleObjectError
-      nil
+      [nil, false]
     end
 
     private

lib/ci/api/builds.rb

@@ -23,19 +23,25 @@ module Ci
 
           new_update = current_runner.ensure_runner_queue_value
 
-          build = Ci::RegisterBuildService.new(current_runner).execute
+          build, valid = Ci::RegisterBuildService.new(current_runner).execute
 
-          if build
-            Gitlab::Metrics.add_event(:build_found,
-                                      project: build.project.path_with_namespace)
+          if valid
+            if build
+              Gitlab::Metrics.add_event(:build_found,
+                                        project: build.project.path_with_namespace)
 
-            present build, with: Entities::BuildDetails
-          else
-            Gitlab::Metrics.add_event(:build_not_found)
+              present build, with: Entities::BuildDetails
+            else
+              Gitlab::Metrics.add_event(:build_not_found)
 
-            header 'X-GitLab-Last-Update', new_update
+              header 'X-GitLab-Last-Update', new_update
 
-            build_not_found!
+              build_not_found!
+            end
+          else
+            # We received build that is invalid due to concurrency conflict
+            Gitlab::Metrics.add_event(:build_invalid)
+            conflict!
           end
         end