MDEV-15620 Crash when using "SET @@NEW.a=expr" inside a trigger

The problem resided in this branch of the "option_value_no_option_type" rule:

| '@' '@' opt_var_ident_type internal_variable_name equal set_expr_or_default

Summary:

1. internal_variable_name initialized tmp.var to trg_new_row_fake_var (0x01).
2. The condition "if (tmp.var == NULL)" did not check
   the special case with trg_new_row_fake_var,
   so Lex->set_system_variable(&tmp, $3, $6) was
   called with tmp.var pointing to trg_new_row_fake_var,
   which created a sys_var instance pointing to 0x01 instead of
   a real system variable.
3. Later, at the trigger invocation time, this method was called:
   sys_var::do_deprecated_warning (this=0x1, thd=0x7ffe6c000a98)
   Notice, "this" is equal to trg_new_row_fake_var (0x01)

Solution:

The old implementation with separate rules
internal_variable_name (in sql_yacc.yy and sql_yacc_ora.yy) and
internal_variable_name_directly_assignable (in sql_yacc_ora.yy only)
was too complex and hard to follow.

Rewriting the code in a more straightforward way.

1. Changing LEX::set_system_variable()

from:

bool set_system_variable(struct sys_var_with_base *, enum_var_type, Item *);

to:

bool set_system_variable(enum_var_type, sys_var *, const LEX_CSTRING *, Item *);

2. Adding new methods in LEX, which operate with variable names:

bool set_trigger_field(const LEX_CSTRING *, const LEX_CSTRING *, Item *);
bool set_system_variable(enum_var_type var_type, const LEX_CSTRING *name,
                         Item *val);
bool set_system_variable(THD *thd, enum_var_type var_type,
                         const LEX_CSTRING *name1,
                         const LEX_CSTRING *name2,
                         Item *val);
bool set_default_system_variable(enum_var_type var_type,
                                 const LEX_CSTRING *name,
                                 Item *val);
bool set_variable(const LEX_CSTRING *name, Item *item);

3. Changing the grammar to call the new methods directly
   in option_value_no_option_type,
   Removing rules internal_variable_name and
   internal_variable_name_directly_assignable.

4. Removing "struct sys_var_with_base" and trg_new_row_fake_var.

Good side effect:

- The code in /sql reduced from 314 to 183 lines.
- MDEV-15615 Unexpected syntax error instead of "Unknown system variable" ...
  was also fixed automatically

1 files changed, 0 insertions, 1 deletions

diff --git a/sql/set_var.h b/sql/set_var.h
index d0143e1e524..9014a9c4936 100644
--- a/sql/set_var.h
+++ b/sql/set_var.h
@@ -393,7 +393,6 @@ SHOW_VAR* enumerate_sys_vars(THD *thd, bool sorted, enum enum_var_type type);
 int fill_sysvars(THD *thd, TABLE_LIST *tables, COND *cond);
 
 sys_var *find_sys_var(THD *thd, const char *str, size_t length=0);
-bool find_sys_var_null_base(THD *thd, struct sys_var_with_base *tmp);
 int sql_set_variables(THD *thd, List<set_var_base> *var_list, bool free);
 
 #define SYSVAR_AUTOSIZE(VAR,VAL)                        \