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, 1 deletions

diff --git a/sql/sql_derived.cc b/sql/sql_derived.cc
index 01459d3fc7a..30b06e91082 100644
--- a/sql/sql_derived.cc
+++ b/sql/sql_derived.cc
@@ -151,7 +151,7 @@ static int mysql_derived(THD *thd, LEX *lex, SELECT_LEX_UNIT *unit,
     if it is preparation PS only then we do not need real data and we
     can skip execution (and parameters is not defined, too)
   */
-  if (!thd->current_statement)
+  if (! thd->current_arena->is_stmt_prepare())
   {
     if (is_union)
     {