Merge branch 'block-concurrent-pipeline-processings' into 'master'

Block concurrent pipeline processings

## What does this MR do?

It's possible that two builds that succeed at the same will try to
concurrently process pipeline. This can lead to scenario when it will
fail to trigger next stages.

This MR makes sure that process of updating is blocking.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/21263

See merge request !6090

CHANGELOG

@@ -9,6 +9,11 @@ v 8.11.4
   - Do not enforce using hash with hidden key in CI configuration. !6079
   - Fix hover leading space bug in pipeline graph !5980
   - Do not enforce using hash with hidden key in CI configuration. !6079
+  - Creating an issue through our API now emails label subscribers !5720
+  - Block concurrent updates for Pipeline
+  - Fix resolving conflicts on forks
+  - Fix diff commenting on merge requests created prior to 8.10
+  - Fix issue boards leak private label names and descriptions
 
 v 8.11.3
   - Don't show resolve conflicts link before MR status is updated

app/services/ci/process_pipeline_service.rb

@@ -10,13 +10,15 @@ module Ci
         create_builds!
       end
 
-      new_builds =
-        stage_indexes_of_created_builds.map do |index|
-          process_stage(index)
-        end
+      @pipeline.with_lock do
+        new_builds =
+          stage_indexes_of_created_builds.map do |index|
+            process_stage(index)
+          end
 
-      # Return a flag if a when builds got enqueued
-      new_builds.flatten.any?
+        # Return a flag if a when builds got enqueued
+        new_builds.flatten.any?
+      end
     end
 
     private