assertion .\filesort.cc, line 797

A query with the "ORDER BY @@some_system_variable" clause,
where @@some_system_variable is NULL, causes assertion
failure in the filesort procedures.

The reason of the failure is in the value of
Item_func_get_system_var::maybe_null: it was unconditionally
set to false even if the value of a variable was NULL.


mysql-test/r/variables.result:
  Added test case for bug #42778.
mysql-test/suite/sys_vars/r/innodb_data_home_dir_basic.result:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/r/innodb_flush_method_basic.result:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/r/rpl_init_slave_func.result:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/r/ssl_capath_basic.result:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/r/ssl_cipher_basic.result:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/t/innodb_data_home_dir_basic.test:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/t/innodb_flush_method_basic.test:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/t/ssl_capath_basic.test:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/suite/sys_vars/t/ssl_cipher_basic.test:
  Updated test case for bug #42778:
  system variables were NOT NULL, now they are nullable.
mysql-test/t/variables.test:
  Added test case for bug #42778.
sql/item.cc:
  Bug #42778: delete order by null global variable causes
              assertion .\filesort.cc, line 797
  
  The longlong_from_string_with_check function has been modified
  to skip unwanted warnings: now it uses the THD::no_errors
  flag to suppress warnings.
  The Item_func_get_system_var::update_null_value method
  sets the no_error flag.
sql/item_func.cc:
  Bug #42778: delete order by null global variable causes
              assertion .\filesort.cc, line 797
  
  1. The Item_func_get_system_var::fix_length_and_dec method
     has been modified to make system variables truly nullable.
  
  2. The Item_func_get_system_var::update_null_value method
     method has been overloaded with a simple wrapper (like
     Item_field::update_null_value) to suppress unwanted warnings
     from Item_func_get_system_var::val_int() calls on non-numeric
     variable values: the Item_func_get_system_var::update_null_value
     method sets and restores THD::no_errors flag for a nested
     call of the longlong_from_string_with_check function.
sql/item_func.h:
  Bug #42778: delete order by null global variable causes
              assertion .\filesort.cc, line 797
  
  The Item_func_get_system_var::update_null_value method
  method has been overloaded.