Fix concurrency issue with migration for user_interacted_projects table.

The concurrency issue originates from inserts on
`user_interacted_projects` from the app while running the post-deploy
migration.

This change comes with a strategy to lock the table while removing
duplicates and creating the unique index (and similar for FK
constraints).

Also, we'll have a non-unique index until the post-deploy migration is
finished to speed up queries during that time.

Closes #44205.

1 files changed, 13 insertions, 0 deletions

diff --git a/lib/gitlab/database/migration_helpers.rb b/lib/gitlab/database/migration_helpers.rb
index dbe6259fce7..21287a8efd0 100644
--- a/lib/gitlab/database/migration_helpers.rb
+++ b/lib/gitlab/database/migration_helpers.rb
@@ -859,6 +859,19 @@ into similar problems in the future (e.g. when new tables are created).
           BackgroundMigrationWorker.perform_in(delay_interval * index, job_class_name, [start_id, end_id])
         end
       end
+
+      def foreign_key_exists?(table, column)
+        foreign_keys(table).any? do |key|
+          key.options[:column] == column.to_s
+        end
+      end
+
+      # Rails' index_exists? doesn't work when you only give it a table and index
+      # name. As such we have to use some extra code to check if an index exists for
+      # a given name.
+      def index_exists_by_name?(table, index)
+        indexes(table).map(&:name).include?(index)
+      end
     end
   end
 end