1 files changed, 76 insertions, 0 deletions
diff --git a/lib/gitlab/background_migration/backfill_namespace_traversal_ids_children.rb b/lib/gitlab/background_migration/backfill_namespace_traversal_ids_children.rb
new file mode 100644
index 00000000000..79e7a2f2279
--- /dev/null
+++ b/lib/gitlab/background_migration/backfill_namespace_traversal_ids_children.rb
@@ -0,0 +1,76 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    # A job to set namespaces.traversal_ids in sub-batches, of all namespaces with
+    # a parent and not already set.
+    # rubocop:disable Style/Documentation
+    class BackfillNamespaceTraversalIdsChildren
+      class Namespace < ActiveRecord::Base
+        include ::EachBatch
+
+        self.table_name = 'namespaces'
+
+        scope :base_query, -> { where.not(parent_id: nil) }
+      end
+
+      PAUSE_SECONDS = 0.1
+
+      def perform(start_id, end_id, sub_batch_size)
+        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
+            UPDATE namespaces
+            SET traversal_ids = calculated_ids.traversal_ids
+            FROM #{calculated_traversal_ids(ranged_query)} calculated_ids
+            WHERE namespaces.id = calculated_ids.id
+              AND namespaces.traversal_ids = '{}'
+          SQL
+          ActiveRecord::Base.connection.execute(update_sql)
+
+          sleep PAUSE_SECONDS
+        end
+
+        # We have to add all arguments when marking a job as succeeded as they
+        #  are all used to track the job by `queue_background_migration_jobs_by_range_at_intervals`
+        mark_job_as_succeeded(start_id, end_id, sub_batch_size)
+      end
+
+      private
+
+      # Calculate the ancestor path for a given set of namespaces.
+      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 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(
+          'BackfillNamespaceTraversalIdsChildren',
+          arguments
+        )
+      end
+    end
+  end
+end