[Development] Threads and the SQL Module

[Samuel Nevala]

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Here here is obsolete information that helped me:).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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