Remove feature flag query_cache_for_load_balancing

- Fix lint error

ee/lib/gitlab/database/load_balancing/host.rb

@@ -7,7 +7,7 @@ module Gitlab
       class Host
         attr_reader :pool, :last_checked_at, :intervals, :load_balancer, :host, :port
 
-        delegate :connection, :release_connection, :enable_query_cache!, :disable_query_cache!, to: :pool
+        delegate :connection, :release_connection, :enable_query_cache!, :disable_query_cache!, :query_cache_enabled, to: :pool
 
         CONNECTION_ERRORS =
           if defined?(PG)

ee/lib/gitlab/database/load_balancing/load_balancer.rb

@@ -12,7 +12,6 @@ module Gitlab
       # always returns a connection to the primary.
       class LoadBalancer
         CACHE_KEY = :gitlab_load_balancer_host
-        ENSURE_CACHING_KEY = 'ensure_caching'
 
         attr_reader :host_list
 
@@ -103,7 +102,6 @@ module Gitlab
             host.release_connection
           end
 
-          RequestStore.delete(ENSURE_CACHING_KEY)
           RequestStore.delete(CACHE_KEY)
         end
 
@@ -180,19 +178,8 @@ module Gitlab
 
         private
 
-        # TODO:
-        # Move enable_query_cache! to ConnectionPool (https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/database.rb#L223)
-        # when the feature flag is removed in https://gitlab.com/gitlab-org/gitlab/-/issues/276203.
         def ensure_caching!
-          # Feature (Flipper gem) reads the data from the database, and it would cause the infinite loop here.
-          # We need to ensure that the code below is executed only once, until the feature flag is removed.
-          return if RequestStore[ENSURE_CACHING_KEY]
-
-          RequestStore[ENSURE_CACHING_KEY] = true
-
-          if Feature.enabled?(:query_cache_for_load_balancing, default_enabled: true)
-            host.enable_query_cache!
-          end
+          host.enable_query_cache! unless host.query_cache_enabled
         end
       end
     end

ee/spec/lib/gitlab/database/load_balancing/load_balancer_spec.rb

@@ -50,29 +50,24 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer, :request_store do
       host = double(:host)
 
       allow(lb).to receive(:host).and_return(host)
+      allow(host).to receive(:query_cache_enabled).and_return(true)
+
       expect(host).to receive(:connection).and_return(connection)
-      expect(host).to receive(:enable_query_cache!).once
 
       expect { |b| lb.read(&b) }.to yield_with_args(connection)
-
-      expect(RequestStore[described_class::ENSURE_CACHING_KEY]).to be true
     end
 
-    context 'when :query_cache_for_load_balancing feature flag is disabled' do
-      before do
-        stub_feature_flags(query_cache_for_load_balancing: false)
-      end
+    it 'ensures that query cache is enabled' do
+      connection = double(:connection)
+      host = double(:host)
 
-      it 'yields a connection for a read without enabling query cache' do
-        connection = double(:connection)
-        host = double(:host)
+      allow(lb).to receive(:host).and_return(host)
+      allow(host).to receive(:query_cache_enabled).and_return(false)
+      allow(host).to receive(:connection).and_return(connection)
 
-        allow(lb).to receive(:host).and_return(host)
-        expect(host).to receive(:connection).and_return(connection)
-        expect(host).not_to receive(:enable_query_cache!)
+      expect(host).to receive(:enable_query_cache!).once
 
-        expect { |b| lb.read(&b) }.to yield_with_args(connection)
-      end
+      lb.read { 10 }
     end
 
     it 'marks hosts that are offline' do
@@ -168,7 +163,6 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer, :request_store do
       lb.release_host
 
       expect(RequestStore[described_class::CACHE_KEY]).to be_nil
-      expect(RequestStore[described_class::ENSURE_CACHING_KEY]).to be_nil
     end
   end