Rewrote location query for load-balancer to use single query

Instead of relying on one query, catching and exception and then another
 fallback query, we now use a case statement which always work.

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

@@ -135,8 +135,6 @@ module Gitlab
         def primary_write_location
           location = read_write do |connection|
             ::Gitlab::Database.get_write_location(connection)
-          rescue ActiveRecord::StatementInvalid
-            ::Gitlab::Database.get_replay_write_location(connection)
           end
 
           return location if location

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
@@ -255,15 +255,6 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer, :request_store do
 
       expect { lb.primary_write_location }.to raise_error(RuntimeError)
     end
-
-    it 'fallbacks to #get_replay_write_location when #get_write_location raises error' do
-      connection = double(:connection)
-      allow(lb).to receive(:read_write).and_yield(connection)
-      allow(::Gitlab::Database).to receive(:get_write_location).and_raise(ActiveRecord::StatementInvalid)
-      expect(::Gitlab::Database).to receive(:get_replay_write_location).and_return('0/C73A0D88')
-
-      lb.primary_write_location
-    end
   end
 
   describe '#all_caught_up?' do

lib/gitlab/database.rb

@@ -260,17 +260,7 @@ module Gitlab
     # @return [String]
     def self.get_write_location(ar_connection)
       row = ar_connection
-        .select_all("SELECT pg_current_wal_insert_lsn()::text AS location")
-        .first
-
-      row['location'] if row
-    end
-
-    # @param [ActiveRecord::Connection] ar_connection
-    # @return [String]
-    def self.get_replay_write_location(ar_connection)
-      row = ar_connection
-              .select_all("SELECT pg_last_wal_replay_lsn()::text AS location")
+              .select_all('SELECT CASE WHEN pg_is_in_recovery() = true THEN pg_last_wal_replay_lsn()::text ELSE pg_current_wal_insert_lsn()::text END AS location;')
               .first
 
       row['location'] if row