Prevent infinite loops in ObjectHierarchy

ee/db/migrate/20190327085945_remove_cyclic_hierarchies_in_epics.rb

@@ -30,6 +30,8 @@ class RemoveCyclicHierarchiesInEpics < ActiveRecord::Migration[5.0]
     # We only need to update the first epic of the loop to break the cycle
     epic_ids_to_update = epics_grouped_by_loop.map { |path, epics| epics.first['id'] }
 
+    # rubocop:disable Lint/UnneededCopDisableDirective
+    # rubocop:disable Migration/UpdateColumnInBatches
     update_column_in_batches(:epics, :parent_id, nil) do |table, query|
       query.where(
         table[:id].in(epic_ids_to_update)

ee/spec/migrations/remove_cyclic_hierarchies_in_epics_spec.rb

@@ -11,7 +11,15 @@ describe RemoveCyclicHierarchiesInEpics, :migration, :postgresql do
   def create_epic_with_defaults!(attributes = {})
     @last_iid += 1
 
-    epics.create!({iid: @last_iid, group_id: group.id, author_id: user.id, title: "Epic", title_html: "Epic"}.merge(attributes))
+    epics.create!(
+      attributes.reverse_merge(
+        iid: @last_iid,
+        group_id: group.id,
+        author_id: user.id,
+        title: "Epic",
+        title_html: "Epic"
+      )
+    )
   end
 
   let!(:epic_self_loop) { create_epic_with_defaults! }
@@ -53,5 +61,3 @@ describe RemoveCyclicHierarchiesInEpics, :migration, :postgresql do
     expect(epic_not_in_loop.reload.parent_id).not_to be_nil
   end
 end
-
-

lib/gitlab/object_hierarchy.rb

@@ -144,7 +144,7 @@ module Gitlab
       cte = SQL::RecursiveCTE.new(:base_and_ancestors)
 
       base_query = ancestors_base.except(:order)
-      base_query = base_query.select("1 as #{DEPTH_COLUMN}", objects_table[Arel.star]) if hierarchy_order
+      base_query = base_query.select("1 as #{DEPTH_COLUMN}", "ARRAY[id] AS tree_path", "false AS tree_cycle", objects_table[Arel.star]) if hierarchy_order
 
       cte << base_query
 
@@ -154,7 +154,17 @@ module Gitlab
         .where(objects_table[:id].eq(cte.table[:parent_id]))
         .except(:order)
 
-      parent_query = parent_query.select(cte.table[DEPTH_COLUMN] + 1, objects_table[Arel.star]) if hierarchy_order
+      if hierarchy_order
+        quoted_objects_table_name = model.connection.quote_table_name(objects_table.name)
+
+        parent_query = parent_query.select(
+          cte.table[DEPTH_COLUMN] + 1,
+          "tree_path || #{quoted_objects_table_name}.id",
+          "#{quoted_objects_table_name}.id = ANY(tree_path)",
+          objects_table[Arel.star]
+        ).where(cte.table[:tree_cycle].eq(false))
+      end
+
       parent_query = parent_query.where(cte.table[:parent_id].not_eq(stop_id)) if stop_id
 
       cte << parent_query
@@ -167,7 +177,7 @@ module Gitlab
       cte = SQL::RecursiveCTE.new(:base_and_descendants)
 
       base_query = descendants_base.except(:order)
-      base_query = base_query.select("1 as #{DEPTH_COLUMN}", objects_table[Arel.star]) if with_depth
+      base_query = base_query.select("1 AS #{DEPTH_COLUMN}", "ARRAY[id] AS tree_path", "false AS tree_cycle", objects_table[Arel.star]) if with_depth
 
       cte << base_query
 
@@ -177,7 +187,16 @@ module Gitlab
         .where(objects_table[:parent_id].eq(cte.table[:id]))
         .except(:order)
 
-      descendants_query = descendants_query.select(cte.table[DEPTH_COLUMN] + 1, objects_table[Arel.star]) if with_depth
+      if with_depth
+        quoted_objects_table_name = model.connection.quote_table_name(objects_table.name)
+
+        descendants_query = descendants_query.select(
+          cte.table[DEPTH_COLUMN] + 1,
+          "tree_path || #{quoted_objects_table_name}.id",
+          "#{quoted_objects_table_name}.id = ANY(tree_path)",
+          objects_table[Arel.star]
+        ).where(cte.table[:tree_cycle].eq(false))
+      end
 
       cte << descendants_query
       cte