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, 2 insertions, 2 deletions

diff --git a/sql/item_sum.h b/sql/item_sum.h
index fcace9e322a..5081d592654 100644
--- a/sql/item_sum.h
+++ b/sql/item_sum.h
@@ -92,8 +92,8 @@ public:
   virtual bool setup(THD *thd) {return 0;}
   virtual void make_unique() {}
   Item *get_tmp_table_item(THD *thd);
-  bool save_args_for_prepared_statements(THD *);
-  bool save_args(Statement* stmt);
+  bool save_args_for_prepared_statement(THD *);
+  bool save_args(Item_arena *arena);
 
   bool walk (Item_processor processor, byte *argument);
 };