MDEV-37243: SP memory root protection disappears after a metadata change

Problem:
  When a stored routine involes a cursor and metadata of table on which
  the cursor is defined changes, the SP instruction has to be reparsed.
  For ex:
  CREATE OR REPLACE TABLE t1 (a INT);

  CREATE OR REPLACE FUNCTION f1() RETURNS INT
  BEGIN
    DECLARE vc INT DEFAULT 0;
    DECLARE cur CURSOR FOR SELECT a FROM t1;
    OPEN cur;
    FETCH cur INTO vc;
    CLOSE cur;
    RETURN vc;
  END;

  SELECT f1(); - first execution, sp-mem_root marked read-only on exec
  SELECT f1(); - read-only sp-mem_root
  ALTER TABLE t1 MODIFY a TEXT; - metadta change
  SELECT f1(); - reparse, rerun instr and mark new mem_root read-only

  sp_lex_instr is re-parsed after the metadata change, which sets up a
  new mem_root for reparsing. Once the instruction is re-parsed and
  re-executed(via reset_lex_and_exec_core), the new memory root assigned
  to the instruction being reparsed remains writable. This violates the
  invariant of SP memory root protection.

Fix:
  Mark the new memory root created for reparsing with read-only flag,
  after the first execution of the SP instruction.

Author: raghunandanbhat

sql/sp_instr.cc

@@ -534,7 +534,19 @@ int sp_lex_keeper::validate_lex_and_exec_core(THD *thd, uint *nextp,
     m_first_execution= false;
 
     if (!rc)
+    {
+      /*
+       sp_lex_instr is re-parsed after the metadata change, which sets up a new
+       mem_root for reparsing. Once the sp_lex_instr is reparsed and re-executed
+       (via reset_lex_and_exec_core) it should be marked as read-only to enforce
+       sp memory root protection.
+       */
+#ifdef PROTECT_STATEMENT_MEMROOT
+      if (rerun_the_same_instr && instr->mem_root)
+        instr->mem_root->flags |= ROOT_FLAG_READ_ONLY;
+#endif
       break;
+    }
 
     /*
       Raise the error upper level in case: