Fix race condition in namespace directory creation

changelogs/unreleased/namespace-race-condition.yml

+---
+title: Fix project creation failure due to race condition in namespace directory creation
+merge_request: 10268
+author: Robin Bobbitt

lib/gitlab/shell.rb

@@ -174,7 +174,10 @@ module Gitlab
     #   add_namespace("/path/to/storage", "gitlab")
     #
     def add_namespace(storage, name)
-      FileUtils.mkdir_p(full_path(storage, name), mode: 0770) unless exists?(storage, name)
+      path = full_path(storage, name)
+      FileUtils.mkdir_p(path, mode: 0770) unless exists?(storage, name)
+    rescue Errno::EEXIST => e
+      Rails.logger.warn("Directory exists as a file: #{e} at: #{path}")
     end
 
     # Remove directory from repositories storage