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

@@ -57,13 +57,11 @@ v 8.12.0 (unreleased)
 v 8.11.4 (unreleased)
   - Fix broken gitlab:backup:restore because of bad permissions on repo storage !6098 (Dirk Hörner)
   - 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
-
-v 8.11.4 (unreleased)
   - Fix issue boards leak private label names and descriptions
 
-v 8.11.3 (unreleased)
 v 8.11.3
   - Do not enforce using hash with hidden key in CI configuration. !6079
   - Allow system info page to handle case where info is unavailable

app/services/ci/process_pipeline_service.rb

@@ -10,6 +10,7 @@ module Ci
         create_builds!
       end
 
+      @pipeline.with_lock do
         new_builds =
           stage_indexes_of_created_builds.map do |index|
             process_stage(index)
@@ -18,6 +19,7 @@ module Ci
         # Return a flag if a when builds got enqueued
         new_builds.flatten.any?
       end
+    end
 
     private