[Development] Threads and the SQL Module

[Eric Lemanisser]

Thanks for pointing this out Łukasz, I created a change to disable useless
mutexes on sqlite db connection :
https://codereview.qt-project.org/#/c/185835/
Unfortunately that would break your usage, but it seems like it is already
broken as Lorenz explained. All db intensive usage should see a perfromance
win.

Eric

Le jeu. 16 févr. 2017 à 07:37, Samuel Nevala <samuel.nevala at intopalo.com> a
écrit :

> Hi,
>
> Just recently needed to fix threading issues with SQLite on
> non-Qt-context. Here here is obsolete information that helped me:).
> http://www.sqlite.org/cvstrac/wiki?p=MultiThreading. From that doc: "By
> "threadsafe" we mean that you can use different SQLite database connections
> in different threads at the same time. It has never been safe to use the
> same database connection simultaneously in multiple threads."
>
> Samuel
>
> On 15 February 2017 at 21:24, Lorenz Haas <lykurg at gmail.com> wrote:
>
> Hi,
>
> AFAIS it does not matter if sqlite is thread safe or not. The problem is
> that QSQLiteDriver itself is not thread safe. For example if two threads
> call beginTransaction() on the same driver and the "COMMIT" fails, both
> threads will call QSqlDriver::setLastError() which does "d->error =
> error;". And here is the first problem.
>
> I am not an expert with QSqlQuery, but since it uses the driver, I bet,
> you'll also find UB and races there.
>
> Cheers
> Lorenz
>
> Am 15.02.2017 um 13:37 schrieb Łukasz Korbel:
> > According to
> > http://doc.qt.io/qt-5/threads-modules.html#threads-and-the-sql-module
> > database connection should be used within one thread *only*.
> >
> > I have checked SQLite driver and according to
> > https://www.sqlite.org/threadsafe.html it supports multithreading by
> > default. And it seems that Qt implementation does not restrict it
> > (
> http://code.qt.io/cgit/qt/qtbase.git/tree/src/plugins/sqldrivers/sqlite/qsql_sqlite.cpp#n645
> ).
> > Code I'm working on is creating QSqlQuery for the same connection in
> > different threads and it works.
> >
> > I understand it doesn't mean Qt want or need to support such use case
> > but maybe documentation
> > can be slightly more accurate? Right now it implies that database
> > connection can never be used from different threads but it seem it
> > depends more on particular driver/3rdparty lib.
> >
> >
> > *Łukasz Korbel *
> > Senior Software Developer
> > Email:        lkorbel at milosolutions.com
> > Skype:        lukasz_korbel
> > *Milo Solutions*
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