Process skipped jobs in the pipeline when retrying it

app/services/ci/retry_pipeline_service.rb

 module Ci
   class RetryPipelineService < ::BaseService
+    include Gitlab::OptimisticLocking
+
     def execute(pipeline)
       unless can?(current_user, :update_pipeline, pipeline)
         raise Gitlab::Access::AccessDeniedError
       end
-      pipeline.builds.failed_or_canceled.tap do |builds|
-        stage_idx = builds.order('stage_idx ASC')
-          .pluck('DISTINCT stage_idx').first
-
-        pipeline.mark_as_processable_after_stage(stage_idx)
-        builds.find_each do |build|
-          next unless build.retryable?
-          Ci::RetryBuildService.new(project, current_user)
-            .reprocess(build)
-        end
+      pipeline.builds.failed_or_canceled.find_each do |build|
+        next unless build.retryable?
+        Ci::RetryBuildService.new(project, current_user)
+          .reprocess(build)
+      end
+      pipeline.builds.skipped.find_each do |skipped|
+        retry_optimistic_lock(skipped) { |build| build.process }
       end
       MergeRequests::AddTodoWhenBuildFailsService

lib/gitlab/optimistic_locking.rb

 module Gitlab
   module OptimisticLocking
-    extend self
+    module_function
     def retry_lock(subject, retries = 100, &block)
       loop do
@@ -15,5 +15,7 @@ module Gitlab
         end
       end
     end
+
+    alias :retry_optimistic_lock :retry_lock
   end
 end