[QGIS-Developer] QGIS Server plugins and thread safety

[Alessandro Pasotti]

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.

The solution is to pass the request handler (or the request and/or response
objects depending on the filter) to the plugin filters and leave the
interface for static properties only.

This would be a big API change for the server interface and filters and I'm
not sure we have time/resources to do that now.

As an alternative we could simply document the issue and change the testing
code to create a new server instance to server every request (to be
verified, and with consitent performance degradation).

What's your opinion about this issue?

BTW,  tomorrow I'll file a ticket for this problem.

-- 
Alessandro Pasotti
w3:   www.itopen.it
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