Merge branch 'sh-fix-schema-migrations-seq-scans' into 'master'

Avoid sequential scans loading schema_migrations table when loading application settings

See merge request gitlab-org/gitlab-ce!19541

lib/gitlab/current_settings.rb

@@ -24,7 +24,22 @@ module Gitlab
       private
 
       def ensure_application_settings!
+        cached_application_settings || uncached_application_settings
+      end
+
+      def cached_application_settings
         return in_memory_application_settings if ENV['IN_MEMORY_APPLICATION_SETTINGS'] == 'true'
+
+        begin
+          ::ApplicationSetting.cached
+        rescue
+          # In case Redis isn't running
+          # or the Redis UNIX socket file is not available
+          # or the DB is not running (we use migrations in the cache key)
+        end
+      end
+
+      def uncached_application_settings
         return fake_application_settings unless connect_to_db?
 
         current_settings = ::ApplicationSetting.current

spec/lib/gitlab/current_settings_spec.rb

@@ -5,6 +5,13 @@ describe Gitlab::CurrentSettings do
     stub_env('IN_MEMORY_APPLICATION_SETTINGS', 'false')
   end
 
+  shared_context 'with settings in cache' do
+    before do
+      create(:application_setting)
+      described_class.current_application_settings # warm the cache
+    end
+  end
+
   describe '#current_application_settings', :use_clean_rails_memory_store_caching do
     it 'allows keys to be called directly' do
       db_settings = create(:application_setting,
@@ -31,17 +38,30 @@ describe Gitlab::CurrentSettings do
     end
 
     context 'with DB unavailable' do
+      context 'and settings in cache' do
+        include_context 'with settings in cache'
+
+        it 'fetches the settings from cache without issuing any query' do
+          expect(ActiveRecord::QueryRecorder.new { described_class.current_application_settings }.count).to eq(0)
+        end
+      end
+
+      context 'and no settings in cache' do
         before do
           # For some reason, `allow(described_class).to receive(:connect_to_db?).and_return(false)` causes issues
           # during the initialization phase of the test suite, so instead let's mock the internals of it
           allow(ActiveRecord::Base.connection).to receive(:active?).and_return(false)
+          expect(ApplicationSetting).not_to receive(:current)
         end
 
         it 'returns an in-memory ApplicationSetting object' do
-        expect(ApplicationSetting).not_to receive(:current)
-
           expect(described_class.current_application_settings).to be_a(Gitlab::FakeApplicationSettings)
         end
+
+        it 'does not issue any query' do
+          expect(ActiveRecord::QueryRecorder.new { described_class.current_application_settings }.count).to eq(0)
+        end
+      end
     end
 
     context 'with DB available' do
@@ -52,9 +72,21 @@ describe Gitlab::CurrentSettings do
         ar_wrapped_defaults.slice(*::ApplicationSetting.defaults.keys)
       end
 
-      before do
+      context 'and settings in cache' do
+        include_context 'with settings in cache'
+
+        it 'fetches the settings from cache' do
           # For some reason, `allow(described_class).to receive(:connect_to_db?).and_return(true)` causes issues
           # during the initialization phase of the test suite, so instead let's mock the internals of it
+          expect(ActiveRecord::Base.connection).not_to receive(:active?)
+          expect(ActiveRecord::Base.connection).not_to receive(:cached_table_exists?)
+          expect(ActiveRecord::Migrator).not_to receive(:needs_migration?)
+          expect(ActiveRecord::QueryRecorder.new { described_class.current_application_settings }.count).to eq(0)
+        end
+      end
+
+      context 'and no settings in cache' do
+        before do
           allow(ActiveRecord::Base.connection).to receive(:active?).and_return(true)
           allow(ActiveRecord::Base.connection).to receive(:cached_table_exists?).with('application_settings').and_return(true)
         end
@@ -123,4 +155,5 @@ describe Gitlab::CurrentSettings do
         end
       end
     end
+  end
 end