Merge branch 'stomlinson/feature-flags-skip-load-balancing' into 'master'

Bypass load balancer during feature flag checks

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/137319



Merged-by: Dylan Griffith <dyl.griffith@gmail.com>
Approved-by: Dylan Griffith <dyl.griffith@gmail.com>
Reviewed-by: Simon Tomlinson <stomlinson@gitlab.com>
Co-authored-by: Simon Tomlinson <stomlinson@gitlab.com>

lib/feature.rb

@@ -4,6 +4,33 @@
 require 'flipper/adapters/active_support_cache_store'
 
 module Feature
+  module BypassLoadBalancer
+    FLAG = 'FEATURE_FLAGS_BYPASS_LOAD_BALANCER'
+    class FlipperRecord < ActiveRecord::Base # rubocop:disable Rails/ApplicationRecord -- This class perfectly replaces
+      # Flipper::Adapters::ActiveRecord::Model, which inherits ActiveRecord::Base
+      include DatabaseReflection
+      self.abstract_class = true
+
+      # Bypass the load balancer by restoring the default behavior of `connection`
+      # before the load balancer patches ActiveRecord::Base
+      def self.connection
+        retrieve_connection
+      end
+    end
+
+    class FlipperFeature < FlipperRecord
+      self.table_name = 'features'
+    end
+
+    class FlipperGate < FlipperRecord
+      self.table_name = 'feature_gates'
+    end
+
+    def self.enabled?
+      Gitlab::Utils.to_boolean(ENV[FLAG], default: false)
+    end
+  end
+
   # Classes to override flipper table names
   class FlipperFeature < Flipper::Adapters::ActiveRecord::Feature
     include DatabaseReflection
@@ -70,7 +97,8 @@ def get(key)
     end
 
     def persisted_names
-      return [] unless ApplicationRecord.database.exists?
+      model = BypassLoadBalancer.enabled? ? BypassLoadBalancer::FlipperRecord : ApplicationRecord
+      return [] unless model.database.exists?
 
       # This loads names of all stored feature flags
       # and returns a stable Set in the following order:
@@ -306,7 +334,9 @@ def with_feature(key)
     def unsafe_get(key)
       # During setup the database does not exist yet. So we haven't stored a value
       # for the feature yet and return the default.
-      return unless ApplicationRecord.database.exists?
+
+      model = BypassLoadBalancer.enabled? ? BypassLoadBalancer::FlipperRecord : ApplicationRecord
+      return unless model.database.exists?
 
       flag_stack = ::Thread.current[:feature_flag_recursion_check] || []
       Thread.current[:feature_flag_recursion_check] = flag_stack
@@ -340,10 +370,15 @@ def flipper
     end
 
     def build_flipper_instance(memoize: false)
-      active_record_adapter = Flipper::Adapters::ActiveRecord.new(
-        feature_class: FlipperFeature,
-        gate_class: FlipperGate)
-
+      active_record_adapter = if BypassLoadBalancer.enabled?
+                                Flipper::Adapters::ActiveRecord.new(
+                                  feature_class: BypassLoadBalancer::FlipperFeature,
+                                  gate_class: BypassLoadBalancer::FlipperGate)
+                              else
+                                Flipper::Adapters::ActiveRecord.new(
+                                  feature_class: FlipperFeature,
+                                  gate_class: FlipperGate)
+                              end
       # Redis L2 cache
       redis_cache_adapter =
         ActiveSupportCacheStoreAdapter.new(