Update READ_QUERY regex

* The READ_QUERY regex would consider reads to be writes if they started with
    spaces or parens. For example, a UNION query might have parens around each
    SELECT - (SELECT ...) UNION (SELECT ...).
  * It will now correctly treat these queries as reads.

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -102,7 +102,7 @@ module ActiveRecord
       end
 
       def self.build_read_query_regexp(*parts) # :nodoc:
-        parts = parts.map { |part| /\A\s*#{part}/i }
+        parts = parts.map { |part| /\A[\(\s]*#{part}/i }
         Regexp.union(*parts)
       end
 

activerecord/test/cases/adapters/mysql2/mysql2_adapter_test.rb

@@ -204,6 +204,14 @@ class Mysql2AdapterTest < ActiveRecord::Mysql2TestCase
     end
   end
 
+  def test_doesnt_error_when_a_read_query_with_leading_chars_is_called_while_preventing_writes
+    @conn.execute("INSERT INTO `engines` (`car_id`) VALUES ('138853948594')")
+
+    @conn.while_preventing_writes do
+      assert_equal 1, @conn.execute("(\n( SELECT `engines`.* FROM `engines` WHERE `engines`.`car_id` = '138853948594' ) )").entries.count
+    end
+  end
+
   private
 
     def with_example_table(definition = "id int auto_increment primary key, number int, data varchar(255)", &block)

activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb

@@ -432,6 +432,16 @@ module ActiveRecord
         end
       end
 
+      def test_doesnt_error_when_a_read_query_with_leading_chars_is_called_while_preventing_writes
+        with_example_table do
+          @connection.execute("INSERT INTO ex (data) VALUES ('138853948594')")
+
+          @connection.while_preventing_writes do
+            assert_equal 1, @connection.execute("(\n( SELECT * FROM ex WHERE data = '138853948594' ) )").entries.count
+          end
+        end
+      end
+
       private
 
         def with_example_table(definition = "id serial primary key, number integer, data character varying(255)", &block)

activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb

@@ -625,6 +625,16 @@ module ActiveRecord
         end
       end
 
+      def test_doesnt_error_when_a_read_query_with_leading_chars_is_called_while_preventing_writes
+        with_example_table "id int, data string" do
+          @conn.execute("INSERT INTO ex (data) VALUES ('138853948594')")
+
+          @conn.while_preventing_writes do
+            assert_equal 1, @conn.execute("  SELECT data from ex WHERE data = '138853948594'").count
+          end
+        end
+      end
+
       private
 
         def assert_logged(logs)