Define a namespace traversal cache

1. Add a traversal_ids array column to the namespaces table.
2. Define Traversal::Hierarchy to sync traversal_ids using recursive
traversal methods.

app/models/namespace/traversal_hierarchy.rb

+# frozen_string_literal: true
+#
+# A Namespace::TraversalHierarchy is the collection of namespaces that descend
+# from a root Namespace as defined by the Namespace#traversal_ids attributes.
+#
+# This class provides operations to be performed on the hierarchy itself,
+# rather than individual namespaces.
+#
+# This includes methods for synchronizing traversal_ids attributes to a correct
+# state. We use recursive methods to determine the correct state so we don't
+# have to depend on the integrity of the traversal_ids attribute values
+# themselves.
+#
+class Namespace
+  class TraversalHierarchy
+    attr_accessor :root
+
+    def self.for_namespace(namespace)
+      new(recursive_root_ancestor(namespace))
+    end
+
+    def initialize(root)
+      raise StandardError.new('Must specify a root node') if root.parent_id
+
+      @root = root
+    end
+
+    # Update all traversal_ids in the current namespace hierarchy.
+    def sync_traversal_ids!
+      # An issue in Rails since 2013 prevents this kind of join based update in
+      # ActiveRecord. https://github.com/rails/rails/issues/13496
+      # Ideally it would be:
+      #   `incorrect_traversal_ids.update_all('traversal_ids = cte.traversal_ids')`
+      sql = """
+            UPDATE namespaces
+            SET traversal_ids = cte.traversal_ids
+            FROM (#{recursive_traversal_ids}) as cte
+            WHERE namespaces.id = cte.id
+              AND namespaces.traversal_ids <> cte.traversal_ids
+            """
+      Namespace.connection.exec_query(sql)
+    end
+
+    # Identify all incorrect traversal_ids in the current namespace hierarchy.
+    def incorrect_traversal_ids
+      Namespace
+        .joins("INNER JOIN (#{recursive_traversal_ids}) as cte ON namespaces.id = cte.id")
+        .where('namespaces.traversal_ids <> cte.traversal_ids')
+    end
+
+    private
+
+    # Determine traversal_ids for the namespace hierarchy using recursive methods.
+    # Generate a collection of [id, traversal_ids] rows.
+    #
+    # Note that the traversal_ids represent a calculated traversal path for the
+    # namespace and not the value stored within the traversal_ids attribute.
+    def recursive_traversal_ids
+      root_id = Integer(@root.id)
+
+      """
+      WITH RECURSIVE cte(id, traversal_ids, cycle) AS (
+        VALUES(#{root_id}, ARRAY[#{root_id}], false)
+      UNION ALL
+        SELECT n.id, cte.traversal_ids || n.id, n.id = ANY(cte.traversal_ids)
+        FROM namespaces n, cte
+        WHERE n.parent_id = cte.id AND NOT cycle
+      )
+      SELECT id, traversal_ids FROM cte
+      """
+    end
+
+    # This is essentially Namespace#root_ancestor which will soon be rewritten
+    # to use traversal_ids. We replicate here as a reliable way to find the
+    # root using recursive methods.
+    def self.recursive_root_ancestor(namespace)
+      Gitlab::ObjectHierarchy
+        .new(Namespace.where(id: namespace))
+        .base_and_ancestors
+        .reorder(nil)
+        .find_by(parent_id: nil)
+    end
+  end
+end

changelogs/unreleased/traversal-hierarchy.yml

+---
+title: Define a namespace traversal cache
+merge_request: 35713
+author:
+type: performance

db/migrate/20200609012539_add_traversal_ids_to_namespaces.rb

+# frozen_string_literal: true
+
+class AddTraversalIdsToNamespaces < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    with_lock_retries do
+      add_column :namespaces, :traversal_ids, :integer, array: true, default: [], null: false
+    end
+  end
+
+  def down
+    with_lock_retries do
+      remove_column :namespaces, :traversal_ids
+    end
+  end
+end

db/structure.sql

@@ -13099,7 +13099,8 @@ CREATE TABLE public.namespaces (
     max_personal_access_token_lifetime integer,
     push_rule_id bigint,
     shared_runners_enabled boolean DEFAULT true NOT NULL,
-    allow_descendants_override_disabled_shared_runners boolean DEFAULT false NOT NULL
+    allow_descendants_override_disabled_shared_runners boolean DEFAULT false NOT NULL,
+    traversal_ids integer[] DEFAULT '{}'::integer[] NOT NULL
 );
 
 CREATE SEQUENCE public.namespaces_id_seq
@@ -23531,6 +23532,7 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200608075553
 20200608214008
 20200609002841
+20200609012539
 20200609142506
 20200609142507
 20200609142508

spec/factories/namespaces.rb

@@ -29,5 +29,35 @@ FactoryBot.define do
     trait :with_root_storage_statistics do
       association :root_storage_statistics, factory: :namespace_root_storage_statistics
     end
+
+    # Construct a hierarchy underneath the namespace.
+    # Each namespace will have `children` amount of children,
+    # and `depth` levels of descendants.
+    trait :with_hierarchy do
+      transient do
+        children { 4 }
+        depth    { 4 }
+      end
+
+      after(:create) do |namespace, evaluator|
+        def create_graph(parent: nil, children: 4, depth: 4)
+          return unless depth > 1
+
+          children.times do
+            factory_name = parent.model_name.singular
+            child = FactoryBot.create(factory_name, parent: parent)
+            create_graph(parent: child, children: children, depth: depth - 1)
+          end
+
+          parent
+        end
+
+        create_graph(
+          parent:   namespace,
+          children: evaluator.children,
+          depth:    evaluator.depth
+        )
+      end
+    end
   end
 end

spec/models/namespace/traversal_hierarchy_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Namespace::TraversalHierarchy, type: :model do
+  let_it_be(:root, reload: true) { create(:namespace, :with_hierarchy) }
+
+  describe '.for_namespace' do
+    let(:hierarchy) { described_class.for_namespace(namespace) }
+
+    context 'with root group' do
+      let(:namespace) { root }
+
+      it { expect(hierarchy.root).to eq root }
+    end
+
+    context 'with child group' do
+      let(:namespace) { root.children.first.children.first }
+
+      it { expect(hierarchy.root).to eq root }
+    end
+
+    context 'with group outside of hierarchy' do
+      let(:namespace) { create(:namespace) }
+
+      it { expect(hierarchy.root).not_to eq root }
+    end
+  end
+
+  describe '.new' do
+    let(:hierarchy) { described_class.new(namespace) }
+
+    context 'with root group' do
+      let(:namespace) { root }
+
+      it { expect(hierarchy.root).to eq root }
+    end
+
+    context 'with child group' do
+      let(:namespace) { root.children.first }
+
+      it { expect { hierarchy }.to raise_error(StandardError, 'Must specify a root node') }
+    end
+  end
+
+  describe '#incorrect_traversal_ids' do
+    subject { described_class.new(root).incorrect_traversal_ids }
+
+    it { is_expected.to match_array Namespace.all }
+  end
+
+  describe '#sync_traversal_ids!' do
+    let(:hierarchy) { described_class.new(root) }
+
+    before do
+      hierarchy.sync_traversal_ids!
+      root.reload
+    end
+
+    it_behaves_like 'hierarchy with traversal_ids'
+    it { expect(hierarchy.incorrect_traversal_ids).to be_empty }
+  end
+end

spec/support/shared_examples/namespaces/hierarchy_examples.rb

+# frozen_string_literal: true
+
+RSpec.shared_examples 'hierarchy with traversal_ids' do
+  # A convenient null node to represent the parent of root.
+  let(:null_node) { double(traversal_ids: []) }
+
+  # Walk the tree to assert that the current_node's traversal_id is always
+  # present and equal to it's parent's traversal_ids plus it's own ID.
+  def validate_traversal_ids(current_node, parent = null_node)
+    expect(current_node.traversal_ids).to be_present
+    expect(current_node.traversal_ids).to eq parent.traversal_ids + [current_node.id]
+
+    current_node.children.each do |child|
+      validate_traversal_ids(child, current_node)
+    end
+  end
+
+  it 'will be valid' do
+    validate_traversal_ids(root)
+  end
+end