Fix for bug#40770: Server Crash when running with triggers including

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

@@ -3810,11 +3810,14 @@ static user_var_entry *get_variable(HASH *hash, LEX_STRING &name,
 
 bool Item_func_set_user_var::set_entry(THD *thd, bool create_if_not_exists)
 {
-  if (thd == entry_thd && entry)
+  if (entry && thd->thread_id == entry_thread_id)
     goto end; // update entry->update_query_id for PS
-  entry_thd= thd;
   if (!(entry= get_variable(&thd->user_vars, name, create_if_not_exists)))
+  {
+    entry_thread_id= 0;
     return TRUE;
+  }
+  entry_thread_id= thd->thread_id;
   /* 
      Remember the last query which updated it, this way a query can later know
      if this variable is a constant item in the query (it is if update_query_id

sql/item_func.h

@@ -1295,16 +1295,16 @@ class Item_func_set_user_var :public Item_func
   enum Item_result cached_result_type;
   user_var_entry *entry;
   /*
-    The entry_thd variable is used:
+    The entry_thread_id variable is used:
     1) to skip unnecessary updates of the entry field (see above);
     2) to reset the entry field that was initialized in the other thread
        (for example, an item tree of a trigger that updates user variables
-       may be shared between several connections, and the entry_thd field
+       may be shared between several connections, and the entry_thread_id field
        prevents updates of one connection user variables from a concurrent
        connection calling the same trigger that initially updated some
        user variable it the first connection context).
   */
-  THD *entry_thd;
+  my_thread_id entry_thread_id;
   char buffer[MAX_FIELD_WIDTH];
   String value;
   my_decimal decimal_buff;
@@ -1321,7 +1321,7 @@ class Item_func_set_user_var :public Item_func
   LEX_STRING name; // keep it public
   Item_func_set_user_var(LEX_STRING a,Item *b)
     :Item_func(b), cached_result_type(INT_RESULT),
-     entry(NULL), entry_thd(NULL), name(a)
+     entry(NULL), entry_thread_id(0), name(a)
   {}
   enum Functype functype() const { return SUSERVAR_FUNC; }
   double val_real();