Merge branch '283971-fix-geo-database-load-balancer' into 'master'

Geo: adds database load balancing support to work on standby clusters

See merge request gitlab-org/gitlab!53147

ee/changelogs/unreleased/283971-fix-geo-database-load-balancer.yml

+---
+title: 'Geo: adds database load balancing support to work on standby clusters'
+merge_request: 53147
+author:
+type: added

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

@@ -243,8 +243,8 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer, :request_store do
   end
 
   describe '#primary_write_location' do
-    it 'returns a String' do
-      expect(lb.primary_write_location).to be_an_instance_of(String)
+    it 'returns a String in the right format' do
+      expect(lb.primary_write_location).to match(/[A-E0-9]{1,8}\/[A-E0-9]{1,8}/)
     end
 
     it 'raises an error if the write location could not be retrieved' do

lib/gitlab/database.rb

@@ -256,11 +256,28 @@ module Gitlab
       row['system_identifier']
     end
 
+    # @param [ActiveRecord::Connection] ar_connection
+    # @return [String]
     def self.get_write_location(ar_connection)
-      row = ar_connection
-        .select_all("SELECT pg_current_wal_insert_lsn()::text AS location")
-        .first
-
+      use_new_load_balancer_query = Gitlab::Utils.to_boolean(ENV['USE_NEW_LOAD_BALANCER_QUERY'], default: false)
+
+      sql = if use_new_load_balancer_query
+              <<~NEWSQL
+                SELECT CASE
+                    WHEN pg_is_in_recovery() = true AND EXISTS (SELECT 1 FROM pg_stat_get_wal_senders())
+                      THEN pg_last_wal_replay_lsn()::text
+                    WHEN pg_is_in_recovery() = false
+                      THEN pg_current_wal_insert_lsn()::text
+                      ELSE NULL
+                    END AS location;
+              NEWSQL
+            else
+              <<~SQL
+                SELECT pg_current_wal_insert_lsn()::text AS location
+              SQL
+            end
+
+      row = ar_connection.select_all(sql).first
       row['location'] if row
     end