[QGIS-Developer] QGIS Server plugins and thread safety

[Alessandro Pasotti]

I would go for multiprocess and stick to a single thread per-process.

But David may have a better advice.

Btw, it's nice to see a use case for Python Vector data providers :)




On Thu, Oct 25, 2018 at 1:02 PM Radim Blazek <radim.blazek at gmail.com> wrote:

> 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?
>
> Radim
> 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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-- 
Alessandro Pasotti
w3:   www.itopen.it
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