Fix disable_prepared_statements clobbering pool size

Use a fresh config when using with establish_connection

lib/gitlab/database/connection.rb

@@ -34,9 +34,12 @@ module Gitlab
         Gitlab::Runtime.max_threads + headroom
       end
 
+      def uncached_config
+        scope.connection_db_config.configuration_hash.with_indifferent_access
+      end
+
       def config
-        @config ||=
-          scope.connection_db_config.configuration_hash.with_indifferent_access
+        @config ||= uncached_config
       end
 
       def pool_size
@@ -69,7 +72,7 @@ module Gitlab
 
       # Disables prepared statements for the current database connection.
       def disable_prepared_statements
-        scope.establish_connection(config.merge(prepared_statements: false))
+        scope.establish_connection(uncached_config.merge(prepared_statements: false))
       end
 
       def read_only?

spec/lib/gitlab/database/connection_spec.rb

@@ -140,6 +140,19 @@ RSpec.describe Gitlab::Database::Connection do
 
       expect(connection.scope.connection.prepared_statements).to eq(false)
     end
+
+    context 'with dynamic connection pool size' do
+      before do
+        connection.scope.establish_connection(connection.config.merge(pool: 7))
+      end
+
+      it 'retains the set pool size' do
+        connection.disable_prepared_statements
+
+        expect(connection.scope.connection.prepared_statements).to eq(false)
+        expect(connection.scope.connection.pool.size).to eq(7)
+      end
+    end
   end
 
   describe '#read_only?' do