Skip retrying for reads on connection errors if primary only

Retrying is pointless, given we only have the one host, and causes
an infinite loop

Changelog: fixed

lib/gitlab/database/load_balancing/load_balancer.rb

@@ -54,7 +54,10 @@ module Gitlab
               connection = host.connection
               return yield connection
             rescue StandardError => error
-              if serialization_failure?(error)
+              if primary_only?
+                # If we only have primary configured, retrying is pointless
+                raise error
+              elsif serialization_failure?(error)
                 # This error can occur when a query conflicts. See
                 # https://www.postgresql.org/docs/current/static/hot-standby.html#HOT-STANDBY-CONFLICT
                 # for more information.

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

@@ -141,6 +141,24 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer, :request_store do
       lb.read { raise conflict_error }
     end
 
+    context 'only primary is configured' do
+      let(:lb) do
+        config = Gitlab::Database::LoadBalancing::Configuration.new(ActiveRecord::Base)
+        allow(config).to receive(:load_balancing_enabled?).and_return(false)
+
+        described_class.new(config)
+      end
+
+      it 'does not retry a query on connection error if only the primary is configured' do
+        host = double(:host, query_cache_enabled: true)
+
+        allow(lb).to receive(:host).and_return(host)
+        allow(host).to receive(:connection).and_raise(PG::UnableToSend)
+
+        expect { lb.read }.to raise_error(PG::UnableToSend)
+      end
+    end
+
     it 'uses the primary if no secondaries are available' do
       allow(lb).to receive(:connection_error?).and_return(true)