Don't allow ALTER TABLE ... ORDER BY on SEQUENCE objects

MDEV-19320 Sequence gets corrupted and produces ER_KEY_NOT_FOUND
           (Can't find record) after ALTER .. ORDER BY

mysql-test/suite/sql_sequence/alter.result

@@ -242,3 +242,10 @@ CREATE SEQUENCE t1 engine=innodb;
 ALTER IGNORE TABLE t1 ADD CHECK (start_value < minimum_value);
 ERROR HY000: Sequence 'test.t1' table structure is invalid (Sequence tables cannot have any constraints)
 DROP SEQUENCE t1;
+CREATE SEQUENCE s;
+ALTER TABLE s ORDER BY cache_size;
+ERROR HY000: Sequence 'test.s' table structure is invalid (ORDER BY)
+SELECT NEXTVAL(s);
+NEXTVAL(s)
+1
+DROP SEQUENCE s;

mysql-test/suite/sql_sequence/alter.test

@@ -149,3 +149,14 @@ CREATE SEQUENCE t1 engine=innodb;
 --error ER_SEQUENCE_INVALID_TABLE_STRUCTURE
 ALTER IGNORE TABLE t1 ADD CHECK (start_value < minimum_value);
 DROP SEQUENCE t1;
+
+#
+# MDEV-19320 Sequence gets corrupted and produces ER_KEY_NOT_FOUND (Can't
+# find record) after ALTER .. ORDER BY
+#
+
+CREATE SEQUENCE s;
+--error ER_SEQUENCE_INVALID_TABLE_STRUCTURE
+ALTER TABLE s ORDER BY cache_size;
+SELECT NEXTVAL(s);
+DROP SEQUENCE s;

sql/sql_sequence.cc

@@ -208,6 +208,11 @@ bool check_sequence_fields(LEX *lex, List<Create_field> *fields)
     reason= "Sequence tables cannot have any constraints";
     goto err;
   }
+  if (lex->alter_info.flags & ALTER_ORDER)
+  {
+    reason= "ORDER BY";
+    goto err;
+  }
 
   for (field_no= 0; (field= it++); field_no++)
   {