Merge branch...

Merge branch '18131-pressing-merge-when-build-succeeds-at-the-same-exact-time-that-a-build-is-failing-will-merge-the-failed-build' into 'master'

Fix race condition on auto merge

Resolves #18131

See merge request !4443

CHANGELOG

@@ -42,6 +42,7 @@ v 8.9.0 (unreleased)
   - Add DB index on users.state
   - Add rake task 'gitlab:db:configure' for conditionally seeding or migrating the database
   - Changed the Slack build message to use the singular duration if necessary (Aran Koning)
+  - Fix race condition on merge when build succeeds
   - Links from a wiki page to other wiki pages should be rewritten as expected
   - Add option to project to only allow merge requests to be merged if the build succeeds (Rui Santos)
   - Fix issues filter when ordering by milestone

app/controllers/projects/merge_requests_controller.rb

@@ -204,10 +204,19 @@ class Projects::MergeRequestsController < Projects::ApplicationController
 
     @merge_request.update(merge_error: nil)
 
-    if params[:merge_when_build_succeeds].present? && @merge_request.pipeline && @merge_request.pipeline.active?
-      MergeRequests::MergeWhenBuildSucceedsService.new(@project, current_user, merge_params)
-        .execute(@merge_request)
-      @status = :merge_when_build_succeeds
+    if params[:merge_when_build_succeeds].present? 
+      if @merge_request.pipeline && @merge_request.pipeline.active?
+        MergeRequests::MergeWhenBuildSucceedsService.new(@project, current_user, merge_params)
+                                                        .execute(@merge_request)
+        @status = :merge_when_build_succeeds
+      elsif @merge_request.pipeline.success?
+        # This can be triggered when a user clicks the auto merge button while
+        # the tests finish at about the same time
+        MergeWorker.perform_async(@merge_request.id, current_user.id, params)
+        @status = :success
+      else
+        @status = :failed
+      end
     else
       MergeWorker.perform_async(@merge_request.id, current_user.id, params)
       @status = :success