Eagerly close Mysql2 prepared statements

Ref: brianmario/mysql2#1383

If we eagerly close uncached statements, we drastically reduce the
risk of one of them being GCed between `#query` and `#affected_rows`.

The bug is still there thouhg, and can happen when `#execute` is
used.

activerecord/lib/active_record/connection_adapters/mysql2/database_statements.rb

@@ -71,6 +71,12 @@ def perform_query(raw_connection, sql, binds, type_casted_binds, prepare:, notif
                 ActiveSupport::Dependencies.interlock.permit_concurrent_loads do
                   result = stmt.execute(*type_casted_binds)
                   @affected_rows_before_warnings = stmt.affected_rows
+
+                  # Ref: https://github.com/brianmario/mysql2/pull/1383
+                  # by eagerly closing uncached prepared statements, we also reduce the chances of
+                  # that bug happening. It can still happen if `#execute` is used as we have no callback
+                  # to eagerly close the statement.
+                  result.instance_variable_set(:@_ar_stmt_to_close, stmt) if result && !prepare
                   result
                 end
               rescue ::Mysql2::Error
@@ -102,17 +108,34 @@ def perform_query(raw_connection, sql, binds, type_casted_binds, prepare:, notif
             end
           end
 
-          def cast_result(result)
-            if result.nil? || result.fields.empty?
+          def cast_result(raw_result)
+            return ActiveRecord::Result.empty if raw_result.nil?
+
+            fields = raw_result.fields
+
+            result = if fields.empty?
               ActiveRecord::Result.empty
             else
-              ActiveRecord::Result.new(result.fields, result.to_a)
+              ActiveRecord::Result.new(fields, raw_result.to_a)
             end
+
+            free_raw_result(raw_result)
+
+            result
           end
 
-          def affected_rows(result)
+          def affected_rows(raw_result)
+            free_raw_result(raw_result) if raw_result
+
             @affected_rows_before_warnings
           end
+
+          def free_raw_result(raw_result)
+            raw_result.free
+            if stmt = raw_result.instance_variable_get(:@_ar_stmt_to_close)
+              stmt.close
+            end
+          end
       end
     end
   end