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.

1 files changed, 0 insertions, 2 deletions

diff --git a/sql/item_subselect.h b/sql/item_subselect.h
index 6b8b8b0b3a7..1ce3144f660 100644
--- a/sql/item_subselect.h
+++ b/sql/item_subselect.h
@@ -36,8 +36,6 @@ class Item_subselect :public Item_result_field
 protected:
   /* thread handler, will be assigned in fix_fields only */
   THD *thd;
-  /* prepared statement, or 0 */
-  Statement *stmt;
   /* substitution instead of subselect in case of optimization */
   Item *substitution;
   /* unit of subquery */