MDEV-31578 DECLARE CURSOR: "Memory not freed: 280 bytes lost" on syntax error

When CURSOR parameters get parsed, their sp_assignment_lex instances
(one instance per parameter) get collected to List<sp_assignment_lex>.

These instances get linked to sphead only in the end of the list.
If a syntax error happened in the middle of the parameter list,
these instances were not deleted, which caused memory leaks.

Fix:

using a Bison %destructor to free rules of the <sp_assignment_lex_list>
type (on syntax errors).

Afte the fix these sp_assignment_lex instances from CURSOR parameters
deleted as follows:

- If the CURSOR statement was fully parsed, then these instances
  get properly linked to sp_head structures, so they are deleted
  during ~sp_head (this did not change)

- If the CURSOR statement failed on a syntax error, then by Bison's
  %destructor (this is being added in the current patch).

mysql-test/main/sp-memory-leak.result

@@ -19,3 +19,24 @@ END
 $$
 ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'expected_END_here;
 END' at line 2
+#
+# MDEV-31578 DECLARE CURSOR: "Memory not freed: 280 bytes lost" on syntax error
+#
+BEGIN NOT ATOMIC
+DECLARE cur CURSOR (a INT) FOR SELECT a+1;
+OPEN cur(sp_followed_by_syntax_error();
+CLOSE cur;
+END;
+$$
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ';
+CLOSE cur;
+END' at line 3
+BEGIN NOT ATOMIC
+DECLARE cur CURSOR (a INT) FOR SELECT a+1;
+OPEN cur(1,sp_followed_by_syntax_error();
+CLOSE cur;
+END;
+$$
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ';
+CLOSE cur;
+END' at line 3

mysql-test/main/sp-memory-leak.test

@@ -27,3 +27,28 @@ BEGIN NOT ATOMIC
 END
 $$
 DELIMITER ;$$
+
+
+--echo #
+--echo # MDEV-31578 DECLARE CURSOR: "Memory not freed: 280 bytes lost" on syntax error
+--echo #
+
+DELIMITER $$;
+--error ER_PARSE_ERROR
+BEGIN NOT ATOMIC
+  DECLARE cur CURSOR (a INT) FOR SELECT a+1;
+  OPEN cur(sp_followed_by_syntax_error();
+  CLOSE cur;
+END;
+$$
+DELIMITER ;$$
+
+DELIMITER $$;
+--error ER_PARSE_ERROR
+BEGIN NOT ATOMIC
+  DECLARE cur CURSOR (a INT) FOR SELECT a+1;
+  OPEN cur(1,sp_followed_by_syntax_error();
+  CLOSE cur;
+END;
+$$
+DELIMITER ;$$

sql/sql_lex.h

@@ -3429,7 +3429,11 @@ public:
   sp_head *sphead;
   sp_name *spname;
   bool sp_lex_in_use;   // Keep track on lex usage in SPs for error handling
-
+  void delete_if_not_sp_lex_in_use()
+  {
+    if (!sp_lex_in_use)
+      delete this;
+  }
   sp_pcontext *spcont;
 
   st_sp_chistics sp_chistics;

sql/sql_yacc.yy

@@ -1535,6 +1535,21 @@ bool my_yyoverflow(short **a, YYSTYPE **b, size_t *yystacksize);
         cursor_actual_parameters
         opt_parenthesized_cursor_actual_parameters
 
+%destructor
+{
+  if ($$)
+  {
+    sp_assignment_lex *elem;
+    List_iterator<sp_assignment_lex> li(*$$);
+    while ((elem= li++))
+    {
+      if (!elem->sp_lex_in_use)
+        delete elem;
+    }
+  }
+} <sp_assignment_lex_list>
+
+
 %type <var_type>
         option_type opt_var_type opt_var_ident_type