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, 1 insertions, 3 deletions

diff --git a/sql/item_cmpfunc.cc b/sql/item_cmpfunc.cc
index 23bdad1aae5..75dbbecf187 100644
--- a/sql/item_cmpfunc.cc
+++ b/sql/item_cmpfunc.cc
@@ -589,10 +589,8 @@ bool Item_in_optimizer::fix_left(THD *thd,
   /*
     If it is preparation PS only then we do not know values of parameters =>
     cant't get there values and do not need that values.
-
-    TODO: during merge with 5.0 it should be changed on !thd->only_prepare()
   */
-  if (!thd->current_statement)
+  if (! thd->current_arena->is_stmt_prepare())
     cache->store(args[0]);
   if (cache->cols() == 1)
   {