Fix for bug#4912 "mysqld crashs in case a statement is executed

a second time". The bug was caused by incompatibility of
negations elimination algorithm and PS: during first statement 
execute a subtree with negation was replaced with equivalent 
subtree without NOTs.
The problem was that although this transformation was permanent, 
items of the new subtree were created in execute-local memory.
The patch adds means to check if it is the first execute of a
prepared statement, and if this is the case, to allocate items
in memory of the prepared statement.
The implementation:
- backports Item_arena from 5.0
- adds Item_arena::is_stmt_prepare(), 
  Item_arena::is_first_stmt_execute().
- deletes THD::allocate_temporary_pool_for_ps_preparing(),
  THD::free_temporary_pool_for_ps_preparing(); they
  were redundant.
and adds a few invariants:
- thd->free_list never contains junk (= freed items)
- thd->current_arena is never null. If there is no
  prepared statement, it points at the thd. 
The rest of the patch contains mainly mechanical changes and
cleanups.


mysql-test/r/ps.result:
  Test results updated (test case for Bug#4912)
mysql-test/t/ps.test:
  A test case for Bug#4912 "mysqld crashs in case a statement is 
  executed a second time"
sql/item_cmpfunc.cc:
  current_statement -> current_arena
sql/item_subselect.cc:
  Statement -> Item_arena, current_statement -> current_arena
sql/item_subselect.h:
  Item_subselect does not need to save thd->current_statement.
sql/item_sum.cc:
  Statement -> Item_arena
sql/item_sum.h:
  Statement -> Item_arena
sql/mysql_priv.h:
  Statement -> Item_arena
sql/sql_base.cc:
  current_statement -> current_arena
sql/sql_class.cc:
  - Item_arena
  - convenient set_n_backup_statement, restore_backup_statement
  (nice idea, Sanja)
sql/sql_class.h:
  - Item_arena: backport from 5.0
  - allocate_temporary_pool_for_ps_preparing,
    free_temporary_pool_for_ps_preparing removed.
sql/sql_derived.cc:
  current_statement -> current_arena
sql/sql_lex.cc:
  current_statement -> current_arena
sql/sql_parse.cc:
  Deploy invariant that thd->free_list never contains junk items
  (backport from 5.0).
sql/sql_prepare.cc:
  - backporting Item_arena
  - no need to allocate_temporary_pool_for_ps_preparing().
sql/sql_select.cc:
  Fix for bug#4912 "mysqld crashs in case a statement is 
  executed a second time": if this is the first execute of
  a prepared statement, negation elimination is
  done in memory of the prepared statement.
sql/sql_union.cc:
  Backporting Item_arena from 5.0.

sql/sql_select.cc

@@ -4377,25 +4377,39 @@ COND *eliminate_not_funcs(THD *thd, COND *cond)
 static COND *
 optimize_cond(THD *thd, COND *conds, Item::cond_result *cond_value)
 {
+  SELECT_LEX *select= thd->lex->current_select;
   DBUG_ENTER("optimize_cond");
-  if (!conds)
+  if (conds)
+  {
+    DBUG_EXECUTE("where", print_where(conds, "original"););
+    /* Eliminate NOT operators; in case of PS/SP do it once */
+    if (thd->current_arena->is_first_stmt_execute())
+    {
+      Item_arena *arena= thd->current_arena, backup;
+      thd->set_n_backup_item_arena(arena, &backup);
+      conds= eliminate_not_funcs(thd, conds);
+      select->prep_where= conds->copy_andor_structure(thd);
+      thd->restore_backup_item_arena(arena, &backup);
+    }
+    else
+      conds= eliminate_not_funcs(thd, conds);
+    DBUG_EXECUTE("where", print_where(conds, "after negation elimination"););
+
+    /* change field = field to field = const for each found field = const */
+    propagate_cond_constants((I_List<COND_CMP> *) 0, conds, conds);
+    /*
+      Remove all instances of item == item
+      Remove all and-levels where CONST item != CONST item
+    */
+    DBUG_EXECUTE("where", print_where(conds, "after const change"););
+    conds= remove_eq_conds(thd, conds, cond_value);
+    DBUG_EXECUTE("info", print_where(conds, "after remove"););
+  }
+  else
   {
     *cond_value= Item::COND_TRUE;
-    DBUG_RETURN(conds);
+    select->prep_where= 0;
   }
-  DBUG_EXECUTE("where",print_where(conds,"original"););
-  /* eliminate NOT operators */
-  conds= eliminate_not_funcs(thd, conds);
-  DBUG_EXECUTE("where", print_where(conds, "after negation elimination"););
-  /* change field = field to field = const for each found field = const */
-  propagate_cond_constants((I_List<COND_CMP> *) 0,conds,conds);
-  /*
-    Remove all instances of item == item
-    Remove all and-levels where CONST item != CONST item
-  */
-  DBUG_EXECUTE("where",print_where(conds,"after const change"););
-  conds= remove_eq_conds(thd, conds, cond_value) ;
-  DBUG_EXECUTE("info",print_where(conds,"after remove"););
   DBUG_RETURN(conds);
 }