Revert 'Enable Query Cache for Load balancing'

A quick fix for https://gitlab.com/gitlab-org/gitlab/-/issues/290733

Revert "Merge branch '276188-enable-query-cache-for-load-balancer' into
'master'"

This reverts commit 54f0e919, reversing
changes made to 0098407c.

ee/config/feature_flags/development/query_cache_for_load_balancing.yml

----
-name: query_cache_for_load_balancing
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46765
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/276203
-milestone: '13.7'
-type: development
-group: group::memory
-default_enabled: false

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, 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
 
@@ -29,8 +28,6 @@ module Gitlab
           conflict_retried = 0
 
           while host
-            ensure_caching!
-
             begin
               return yield host.connection
             rescue => error
@@ -98,12 +95,7 @@ module Gitlab
 
         # Releases the host and connection for the current thread.
         def release_host
-          if host = RequestStore[CACHE_KEY]
-            host.disable_query_cache!
-            host.release_connection
-          end
-
-          RequestStore.delete(ENSURE_CACHING_KEY)
+          RequestStore[CACHE_KEY]&.release_connection
           RequestStore.delete(CACHE_KEY)
         end
 
@@ -177,23 +169,6 @@ module Gitlab
             error.is_a?(PG::TRSerializationFailure)
           end
         end
-
-        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)
-            host.enable_query_cache!
-          end
-        end
       end
     end
   end

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

@@ -12,7 +12,6 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer do
 
   after do
     RequestStore.delete(described_class::CACHE_KEY)
-    RequestStore.delete(described_class::ENSURE_CACHING_KEY)
   end
 
   def raise_and_wrap(wrapper, original)
@@ -53,28 +52,8 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer do
 
       allow(lb).to receive(:host).and_return(host)
       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 '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)
-        expect(host).to receive(:connection).and_return(connection)
-        expect(host).not_to receive(:enable_query_cache!)
-
-        expect { |b| lb.read(&b) }.to yield_with_args(connection)
-      end
     end
 
     it 'marks hosts that are offline' do
@@ -163,14 +142,10 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer do
 
   describe '#release_host' do
     it 'releases the host and its connection' do
-      host = lb.host
-
-      expect(host).to receive(:disable_query_cache!)
-
+      lb.host
       lb.release_host
 
       expect(RequestStore[described_class::CACHE_KEY]).to be_nil
-      expect(RequestStore[described_class::ENSURE_CACHING_KEY]).to be_nil
     end
   end