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, 7 insertions, 0 deletions

diff --git a/mysql-test/r/ps.result b/mysql-test/r/ps.result
index 27f4ce7f815..0523143f91d 100644
--- a/mysql-test/r/ps.result
+++ b/mysql-test/r/ps.result
@@ -219,3 +219,10 @@ Name	Engine	Version	Row_format	Rows	Avg_row_length	Data_length	Max_data_length	I
 t1	MyISAM	9	Dynamic	0	0	0	4294967295	1024	0	NULL	#	#	#	latin1_swedish_ci	NULL		
 deallocate prepare stmt1 ;
 drop table t1;
+create table t1(a varchar(2), b varchar(3));
+prepare stmt1 from "select a, b from t1 where (not (a='aa' and b < 'zzz'))";
+execute stmt1;
+a	b
+execute stmt1;
+a	b
+deallocate prepare stmt1;