[QGIS-Developer] QGIS Server plugins and thread safety

Radim Blazek radim.blazek at gmail.com
Thu Oct 25 04:02:37 PDT 2018

If we narrow down the environment to uWSGI (with multiple threads
enabled) running a Python application, which means that more instances
of the same application may run in threads but never at the same time
due to Python GIL (if I understand it well), is it still a problem? If
we ensure that there are no static variables and singletons used by
QGIS server? Is it then a problem to create/delete QObject on a non
main thread?

On Sun, Nov 19, 2017 at 5:39 PM Martin Dobias <wonder.sk at gmail.com> wrote:
> Hi Alessandro
> On Sun, Nov 19, 2017 at 10:34 AM, Alessandro Pasotti <apasotti at gmail.com> wrote:
> > Hi,
> >
> > mi recent experiments with multi-threaded python wrappers for QGIS server
> > showed a critical flaw in my original implementation of the server plugins.
> >
> > First, I want to stress that this is not a problem in FCGI server
> > implementation but only if the server is used directly from python in a
> > multi threaded server implementation.
> >
> > The problem is that the server interface that exposes the request handler is
> > a static property of the interface, that is changed on every request.
> > This means that there is a race condition in setting/accessing the handler
> > from a plugin filter, and that a plugin filter might access to the handler
> > for a wrong request.
> I think this is probably just the tip of the iceberg: most of the
> classes in qgis_core are not thread-safe, and I believe the
> implementation of server's services is not expected to be run in
> multiple threads at once either. So even though things in theory could
> work if requests are handled in multiple threads, most likely there
> will be crashes. Even though map rendering was made to run safe in
> multiple worker threads, there is still the assumption that the map
> rendering is started from the main thread where all the objects live
> (I'm talking about the QObject-based classes). Dealing with objects
> like map layers or project in non-main thread is likely to end up
> badly.
> I would say for now we should simply stick with handling only one
> request per server process at a time - just like how it is done with
> the FastCGI implementation.
> Cheers
> Martin
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