Deprecate `LoadBalancer#select_caught_up_hosts`

Deprecate redudnant host search implementation with the simpler method
we already have in the class.

config/feature_flags/development/load_balancing_refine_load_balancer_methods.yml

+---
+name: load_balancing_refine_load_balancer_methods
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/65356
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/335109
+milestone: '14.1'
+type: development
+group: group::memory
+default_enabled: false

lib/gitlab/database/load_balancing/load_balancer.rb

@@ -165,6 +165,7 @@ module Gitlab
         # while we only need a single host: https://gitlab.com/gitlab-org/gitlab/-/issues/326125#note_615271604
         # Also, shuffling the list afterwards doesn't seem to be necessary.
         # This may be improved by merging this method with `select_up_to_date_host`.
+        # Could be removed when `:load_balancing_refine_load_balancer_methods` FF is rolled out
         def select_caught_up_hosts(location)
           all_hosts = @host_list.hosts
           valid_hosts = all_hosts.select { |host| host.caught_up?(location) }
@@ -201,6 +202,7 @@ module Gitlab
           true
         end
 
+        # Could be removed when `:load_balancing_refine_load_balancer_methods` FF is rolled out
         def set_consistent_hosts_for_request(hosts)
           RequestStore[VALID_HOSTS_CACHE_KEY] = hosts
         end

lib/gitlab/database/load_balancing/sticking.rb

@@ -53,8 +53,14 @@ module Gitlab
           # write location. If no such location exists, err on the side of caution.
           return false unless location
 
-          load_balancer.select_caught_up_hosts(location).tap do |selected|
-            unstick(namespace, id) if selected
+          if ::Feature.enabled?(:load_balancing_refine_load_balancer_methods)
+            load_balancer.select_up_to_date_host(location).tap do |selected|
+              unstick(namespace, id) if selected
+            end
+          else
+            load_balancer.select_caught_up_hosts(location).tap do |selected|
+              unstick(namespace, id) if selected
+            end
           end
         end
 

spec/lib/gitlab/database/load_balancing/sticking_spec.rb

@@ -325,10 +325,22 @@ RSpec.describe Gitlab::Database::LoadBalancing::Sticking, :redis do
       end
 
       it 'returns true, selects hosts, and unsticks if any secondary has caught up' do
-        expect(lb).to receive(:select_caught_up_hosts).and_return(true)
+        expect(lb).to receive(:select_up_to_date_host).and_return(true)
         expect(described_class).to receive(:unstick).with(:project, 42)
         expect(described_class.select_caught_up_replicas(:project, 42)).to be true
       end
+
+      context 'when :load_balancing_refine_load_balancer_methods FF is disabled' do
+        before do
+          stub_feature_flags(load_balancing_refine_load_balancer_methods: false)
+        end
+
+        it 'returns true, selects hosts, and unsticks if any secondary has caught up' do
+          expect(lb).to receive(:select_caught_up_hosts).and_return(true)
+          expect(described_class).to receive(:unstick).with(:project, 42)
+          expect(described_class.select_caught_up_replicas(:project, 42)).to be true
+        end
+      end
     end
   end
 end