Geo: Exclude tables that start with pg_ from FDW check

PostgreSQL-specific tables, such as `pg_stat_statements` and `pg_buffercache`,
aren't necessary for our FDW check to work and cause unnecessary confusion.

ee/changelogs/unreleased/sh-exclude-pg-tables-from-fdw-check.yml

+---
+title: 'Geo: Exclude tables that start with pg_ from FDW check'
+merge_request:
+author:
+type: fixed

ee/lib/gitlab/geo/fdw.rb

@@ -61,6 +61,7 @@ module Gitlab
               FROM information_schema.tables
              WHERE table_schema = '#{FDW_SCHEMA}'
                AND table_type = 'FOREIGN TABLE'
+               AND table_name NOT LIKE 'pg_%'
           SQL
 
           ::Geo::TrackingBase.connection.execute(sql).first.fetch('count').to_i
@@ -72,18 +73,26 @@ module Gitlab
       # @return [Boolean] whether schemas match and are not empty
       def self.foreign_schema_tables_match?
         Gitlab::Geo.cache_value(:geo_fdw_schema_tables_match) do
-          secondary = retrieve_schema_tables(ActiveRecord::Base, ActiveRecord::Base.connection_config[:database], DEFAULT_SCHEMA)
-          fdw = retrieve_schema_tables(::Geo::TrackingBase, Rails.configuration.geo_database['database'], FDW_SCHEMA)
+          schema = gitlab_schema
 
-          s = secondary.to_a
-          f = fdw.to_a
-
-          s.present? && s == f
+          schema.present? && schema == fdw_schema
         end
       end
 
       def self.count_tables_match?
-        ActiveRecord::Schema.tables.count == count_tables
+        gitlab_tables.count == count_tables
+      end
+
+      def self.gitlab_tables
+        ActiveRecord::Schema.tables.reject { |table| table.start_with?('pg_') }
+      end
+
+      def self.gitlab_schema
+        retrieve_schema_tables(ActiveRecord::Base, ActiveRecord::Base.connection_config[:database], DEFAULT_SCHEMA).to_a
+      end
+
+      def self.fdw_schema
+        retrieve_schema_tables(::Geo::TrackingBase, Rails.configuration.geo_database['database'], FDW_SCHEMA).to_a
       end
 
       def self.retrieve_schema_tables(adapter, database, schema)
@@ -92,10 +101,11 @@ module Gitlab
               FROM information_schema.columns
              WHERE table_catalog = '#{database}'
                AND table_schema = '#{schema}'
+               AND table_name NOT LIKE 'pg_%'
           ORDER BY table_name, column_name, data_type
         SQL
 
-        adapter.connection.execute(sql)
+        adapter.connection.select_all(sql)
       end
       private_class_method :retrieve_schema_tables
     end

ee/lib/gitlab/geo/health_check.rb

@@ -21,7 +21,7 @@ module Gitlab
 
         unless Gitlab::Geo::Fdw.fdw_up_to_date?
           return "The Geo database has an outdated FDW remote schema.".tap do |output|
-            output << " It contains #{Gitlab::Geo::Fdw.count_tables} of #{ActiveRecord::Schema.tables.count} expected tables." unless Gitlab::Geo::Fdw.count_tables_match?
+            output << " It contains #{Gitlab::Geo::Fdw.count_tables} of #{Gitlab::Geo::Fdw.gitlab_tables.count} expected tables." unless Gitlab::Geo::Fdw.count_tables_match?
           end
         end
 

ee/spec/lib/gitlab/geo/fdw_spec.rb

@@ -36,6 +36,18 @@ describe Gitlab::Geo::Fdw, :geo do
     end
   end
 
+  describe '.gitlab_tables' do
+    it 'excludes pg_ tables' do
+      tables = described_class.gitlab_tables
+
+      ActiveRecord::Base.connection.create_table(:pg_gitlab_test)
+
+      expect(described_class.gitlab_tables).to eq(tables)
+
+      ActiveRecord::Base.connection.drop_table(:pg_gitlab_test)
+    end
+  end
+
   describe 'fdw_capable?' do
     context 'with mocked FDW environment' do
       it 'returns true when PostgreSQL FDW is enabled' do
@@ -70,6 +82,20 @@ describe Gitlab::Geo::Fdw, :geo do
       it 'returns true' do
         expect(described_class.fdw_capable?).to be_truthy
       end
+
+      context 'with a pg_ table' do
+        before do
+          ActiveRecord::Base.connection.create_table(:pg_gitlab_test)
+        end
+
+        after do
+          ActiveRecord::Base.connection.drop_table(:pg_gitlab_test)
+        end
+
+        it 'returns true' do
+          expect(described_class.fdw_capable?).to be_truthy
+        end
+      end
     end
   end