1 files changed, 77 insertions, 0 deletions
diff --git a/lib/gitlab/background_migration/backfill_ci_namespace_mirrors.rb b/lib/gitlab/background_migration/backfill_ci_namespace_mirrors.rb
new file mode 100644
index 00000000000..2247747ba08
--- /dev/null
+++ b/lib/gitlab/background_migration/backfill_ci_namespace_mirrors.rb
@@ -0,0 +1,77 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    # A job to create ci_namespace_mirrors entries in batches
+    class BackfillCiNamespaceMirrors
+      class Namespace < ActiveRecord::Base # rubocop:disable Style/Documentation
+        include ::EachBatch
+
+        self.table_name = 'namespaces'
+        self.inheritance_column = nil
+
+        scope :base_query, -> do
+          select(:id, :parent_id)
+        end
+      end
+
+      PAUSE_SECONDS = 0.1
+      SUB_BATCH_SIZE = 500
+
+      def perform(start_id, end_id)
+        batch_query = Namespace.base_query.where(id: start_id..end_id)
+        batch_query.each_batch(of: SUB_BATCH_SIZE) do |sub_batch|
+          first, last = sub_batch.pluck(Arel.sql('MIN(id), MAX(id)')).first
+          ranged_query = Namespace.unscoped.base_query.where(id: first..last)
+
+          update_sql = <<~SQL
+            INSERT INTO ci_namespace_mirrors (namespace_id, traversal_ids)
+            #{insert_values(ranged_query)}
+            ON CONFLICT (namespace_id) DO NOTHING
+          SQL
+          # We do nothing on conflict because we consider they were already filled.
+
+          Namespace.connection.execute(update_sql)
+
+          sleep PAUSE_SECONDS
+        end
+
+        mark_job_as_succeeded(start_id, end_id)
+      end
+
+      private
+
+      def insert_values(batch)
+        calculated_traversal_ids(
+          batch.allow_cross_joins_across_databases(url: 'https://gitlab.com/gitlab-org/gitlab/-/issues/336433')
+        )
+      end
+
+      # Copied from lib/gitlab/background_migration/backfill_namespace_traversal_ids_children.rb
+      def calculated_traversal_ids(batch)
+        <<~SQL
+          WITH RECURSIVE cte(source_id, namespace_id, parent_id, height) AS (
+            (
+              SELECT batch.id, batch.id, batch.parent_id, 1
+              FROM (#{batch.to_sql}) AS batch
+            )
+            UNION ALL
+            (
+              SELECT cte.source_id, n.id, n.parent_id, cte.height+1
+              FROM namespaces n, cte
+              WHERE n.id = cte.parent_id
+            )
+          )
+          SELECT flat_hierarchy.source_id as namespace_id,
+                 array_agg(flat_hierarchy.namespace_id ORDER BY flat_hierarchy.height DESC) as traversal_ids
+          FROM (SELECT * FROM cte FOR UPDATE) flat_hierarchy
+          GROUP BY flat_hierarchy.source_id
+        SQL
+      end
+
+      def mark_job_as_succeeded(*arguments)
+        Gitlab::Database::BackgroundMigrationJob.mark_all_as_succeeded('BackfillCiNamespaceMirrors', arguments)
+      end
+    end
+  end
+end