Correctly dump check constraints for MySQL 8.0.16+

If you're using MySQL 8.0.16+ and your database contains a table with
a check constraint, the first and last characters of the constraint
will be stripped when dumping the schema; this makes it impossible
to use check constraints in a MySQL 8.0 database with the `:ruby`
schema format; once dumped, they cannot be re-imported.

This is because, up until MySQL 8.0.16, all check constraints were
surrounded by an extra set of parentheses; this behaviour differed
from that presented by MariaDB, which handled them correctly thanks
to an engine check within the AbstractMySqlAdapter.

With this change, we only strip the parentheses if they are present,
which prevents the corruption of the constraint.

It is also possible that the exported check constraint will contain
invalid and unexpected whitespace characters; we handle that here too,
stripping out sequences of `\\` and `\n`, as well as any other
whitespace.

Author: stevehill1981

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -526,7 +526,8 @@ def check_constraints(table_name)
               name: row["name"]
             }
             expression = row["expression"]
-            expression = expression[1..-2] unless mariadb? # remove parentheses added by mysql
+            expression = expression[1..-2] if expression.start_with?("(") && expression.end_with?(")")
+            expression = strip_whitespace_characters(expression)
             CheckConstraintDefinition.new(table_name, expression, options)
           end
         else
@@ -714,6 +715,12 @@ def extract_precision(sql_type)
       EMULATE_BOOLEANS_TRUE = { emulate_booleans: true }.freeze
 
       private
+        def strip_whitespace_characters(expression)
+          expression = expression.gsub(/\\n|\\\\/, "")
+          expression = expression.gsub(/\s{2,}/, " ")
+          expression
+        end
+
         def text_type?(type)
           TYPE_MAP.lookup(type).is_a?(Type::String) || TYPE_MAP.lookup(type).is_a?(Type::Text)
         end

activerecord/test/cases/migration/check_constraint_test.rb

@@ -23,11 +23,21 @@ class Trade < ActiveRecord::Base
             t.integer :price
             t.integer :quantity
           end
+
+          if current_adapter?(:Mysql2Adapter)
+            @connection.create_table "constraint_test", force: true do |t|
+              t.json :options, default: nil
+            end
+          end
         end
 
         teardown do
           @connection.drop_table "trades", if_exists: true rescue nil
           @connection.drop_table "purchases", if_exists: true rescue nil
+
+          if current_adapter?(:Mysql2Adapter)
+            @connection.drop_table "constraint_test", if_exists: true rescue nil
+          end
         end
 
         def test_check_constraints
@@ -44,6 +54,28 @@ def test_check_constraints
             assert_equal "price > discounted_price", constraint.expression
           end
 
+          if current_adapter?(:Mysql2Adapter)
+            begin
+              @connection.add_check_constraint(:constraint_test, <<~SQL,
+                json_contains('
+                  {
+                    "a": 1,
+                    "b": 2,
+                    "c": {
+                      "d": 4
+                    }
+                  }
+                ', options)
+              SQL
+              name: "non_empty_test_array")
+
+              constraint = @connection.check_constraints("constraint_test").find { |c| c.name == "non_empty_test_array" }
+              assert_includes constraint.expression, "json_contains"
+            ensure
+              @connection.remove_check_constraint(:constraint_test, name: "non_empty_test_array", if_exists: true)
+            end
+          end
+
           if current_adapter?(:PostgreSQLAdapter)
             begin
               # Test that complex expression is correctly parsed from the database