Allow CTEs in read-only queries.

Common Table Expressions in PostgreSQL allow a different way to define
derived tables. Here's an example from the pg docs:

	WITH regional_sales AS (
	        SELECT region, SUM(amount) AS total_sales
	        FROM orders
	        GROUP BY region
	     ), top_regions AS (
	        SELECT region
	        FROM regional_sales
	        WHERE total_sales > (SELECT SUM(total_sales)/10 FROM regional_sales)
	     )
	SELECT region,
	       product,
	       SUM(quantity) AS product_units,
	       SUM(amount) AS product_sales
	FROM orders
	WHERE region IN (SELECT region FROM top_regions)
	GROUP BY region, product;

https://www.postgresql.org/docs/current/queries-with.html

This commit adds the :with keyword to the set of keywords allowed to
begin a query against a `prevent_writes` PostgreSQL connection.

Thx to @kamipo, this also adds the same support for sqlite3 and mysql2

https://www.sqlite.org/lang_with.html
https://dev.mysql.com/doc/refman/8.0/en/with.html

activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb

@@ -20,7 +20,7 @@ def query(sql, name = nil) # :nodoc:
         end
 
         READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(
-          :begin, :commit, :explain, :select, :set, :show, :release, :savepoint, :rollback, :describe, :desc
+          :begin, :commit, :explain, :select, :set, :show, :release, :savepoint, :rollback, :describe, :desc, :with
         ) # :nodoc:
         private_constant :READ_QUERY
 

activerecord/lib/active_record/connection_adapters/postgresql/database_statements.rb

@@ -67,7 +67,7 @@ def query(sql, name = nil) #:nodoc:
           end
         end
 
-        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :commit, :explain, :select, :set, :show, :release, :savepoint, :rollback) # :nodoc:
+        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :commit, :explain, :select, :set, :show, :release, :savepoint, :rollback, :with) # :nodoc:
         private_constant :READ_QUERY
 
         def write_query?(sql) # :nodoc:

activerecord/lib/active_record/connection_adapters/sqlite3/database_statements.rb

@@ -4,7 +4,7 @@ module ActiveRecord
   module ConnectionAdapters
     module SQLite3
       module DatabaseStatements
-        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :commit, :explain, :select, :pragma, :release, :savepoint, :rollback) # :nodoc:
+        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :commit, :explain, :select, :pragma, :release, :savepoint, :rollback, :with) # :nodoc:
         private_constant :READ_QUERY
 
         def write_query?(sql) # :nodoc:

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

@@ -232,6 +232,15 @@ def test_doesnt_error_when_a_read_query_with_leading_chars_is_called_while_preve
     end
   end
 
+  def test_doesnt_error_when_a_read_query_with_a_cte_is_called_while_preventing_writes
+    @conn.execute("INSERT INTO `engines` (`car_id`) VALUES ('138853948594')")
+
+    @connection_handler.while_preventing_writes do
+      sql = "WITH matching_cars AS (SELECT `engines`.* FROM `engines` WHERE `engines`.`car_id` = '138853948594') SELECT * FROM matching_cars"
+      assert_equal 1, @conn.execute(sql).entries.count
+    end
+  end
+
   def test_read_timeout_exception
     ActiveRecord::Base.establish_connection(
       ActiveRecord::Base.configurations[:arunit].merge("read_timeout" => 1)

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

@@ -457,6 +457,17 @@ def test_doesnt_error_when_a_read_query_with_leading_chars_is_called_while_preve
         end
       end
 
+      def test_doesnt_error_when_a_read_query_with_a_cte_is_called_while_preventing_writes
+        with_example_table do
+          @connection.execute("INSERT INTO ex (data) VALUES ('138853948594')")
+
+          @connection_handler.while_preventing_writes do
+            sql = "WITH matching_ex_values AS (SELECT * FROM ex WHERE data = '138853948594') SELECT * FROM matching_ex_values"
+            assert_equal 1, @connection.execute(sql).entries.count
+          end
+        end
+      end
+
       private
         def with_example_table(definition = "id serial primary key, number integer, data character varying(255)", &block)
           super(@connection, "ex", definition, &block)

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

@@ -652,6 +652,17 @@ def test_doesnt_error_when_a_read_query_with_leading_chars_is_called_while_preve
         end
       end
 
+      def test_doesnt_error_when_a_read_query_with_a_cte_is_called_while_preventing_writes
+        with_example_table "id int, data string" do
+          @conn.execute("INSERT INTO ex (data) VALUES ('138853948594')")
+
+          @connection_handler.while_preventing_writes do
+            sql = "WITH matching_ex_values AS (SELECT * FROM ex WHERE data = '138853948594') SELECT * FROM matching_ex_values"
+            assert_equal 1, @conn.execute(sql).entries.count
+          end
+        end
+      end
+
       private
         def assert_logged(logs)
           subscriber = SQLSubscriber.new