FreeTDS Development Group

["Frediano Ziglio" <freddy77 AT gmail.com>]

2008/11/5 Frediano Ziglio <freddy77 AT gmail.com>

> Il giorno mar, 04/11/2008 alle 20.48 -0800, Man Min Yan ha scritto:
> > Hi Frediano,
> >
> > You're right, tds_free_dynamic doesn't seem to be right. It's not
> removing the items from the tds->dyns list properly. Also, my previous patch
> is not right as it would throw away any previously prepared statement.
> Here's my 2nd go at fixing this problem.
> >
> > I've also tried to identify where the prepares were happening, and it
> seems to be a freetds problem. Here's the call stack:
> >     main()
> >     perl_run()
> >     Perl_runops_standard()
> >     Perl_pp_entersub()
> >     XS_DBI_dispatch()
> >     XS_DBD__ODBC__st_execute()
> >     odbc_st_execute()
> >     SQLExecute()    from libodbc.so.1
> >     SQLExecute() at odbc.c:3343
> >     _SQLExecute() at odbc.c:3142
> >     tds_submit_prepare() at query.c:1007
> >
> > The code in _SQLExecute is actually un-preparing the statement,
> re-preparing it, then executing it every time it is called.
> >
> > Rgds,
> > Man Min
> >
>
> Mmmm... currently there are 3 issues:
> - generated ids can create duplication leading to dynamic reuse, perhaps
> we should throw error on same id and generate betters ids. TODO: test in
> libTDS to check no duplication on generated ids


Perhaps the best way is to allow NULL id calling tds_alloc_dynamic and
generate id in tds_alloc_dynamic. Also I don't understand why
tds_alloc_dynamic should return same dynamic, perhaps is better to return
NULL... id duplicated should not be allowed, even by CTLib.(ct_dynamic state
that id are univoque)


>
> - on reused dynamic I had cur_dyn set but no dynamic on list... that is
> a huge memory problem! TODO: test in odbc to remove the problem


Mmmm... this is due to tds_alloc_dynamic returning same dynamic, two
statements have stmt->dyn set on same dynamic so on second
tds_submit_unprepare you have dynamic not in list, you set tds->cur_dyn and
you get the error, avoiding duplicate return should fix the problem. Perhaps
adding reference counting to TDSDYNAMIC would help.


>
> - odbc prepare too much, in this case due to rebinding. TODO: cache
> argument and query in libTDS to avoid this case.
>


This is a TODO, we should add a test on freeclose test which already could
round_trips and inserts (but with another SQLFreeStmt option).

freddy77