variable settings (rpl_sys)

Problem: under certain conditions (e.g. user variables usage in triggers)
accessing a user defined variable we may use a variables hash table that
belongs to already deleted thread. It happens if
thd= new THD;
has the same address as just deleted thd as we use
if (stored_thd == thd)
to check.
That may lead to unpredictable results, server crash etc.

Fix: use thread_id instead of thd address to distinguish threads.

Note: no simple and repeatable test case.


sql/item_func.cc:
  Fix for bug#40770: Server Crash when running with triggers including
  variable settings (rpl_sys)
    - store and use thd->thread_id to distinguish threads instead of
  thread address as it may be the same as just deleted thread had, 
  i.e. we may get (old_thd == new_thd) after
  delete old_thd;
  new_thd= new THD;
    - set entry_thread_id only when we get a real entry, clear it 
  if the hash search fails.
sql/item_func.h:
  Fix for bug#40770: Server Crash when running with triggers including
  variable settings (rpl_sys)
    - Item_func_set_user_var::entry_thread_id introduced.