Fixes #262: Missing or duplicated results for .filter() when threading is enabled

[weisdd]

As mentioned in #262, we sometimes see missing or duplicated results when calling nb.dcim.devices.filter() with threading enabled.
Troubleshooting showed that the requests themselves are scheduled with correct offsets, though when the actual calls happen, params seem to be overwritten, further causing incorrect results. Not always, but often enough, and especially visible with low MAX_PAGE_SIZE values.

In the implementation of query.py, I've noticed that concurrent sessions seem to use the same copy of the attrubute "self.filters" (that params = self.filters). That's why params.update(add_params) would sometimes cause collisions depending on the time when a particular request is called.

        params = {}
         if not url_override:
             if self.filters:
                 params = self.filters
             if add_params:
                 params.update(add_params)

An example of this dict:

{'role': ['switch', 'router'], 'has_primary_ip': True, 'limit': 20, 'offset': 60}

I think changing it to:

        params = {}
        if not url_override:
            if self.filters:
                params.update(self.filters)
            if add_params:
                params.update(add_params)

will help to mitigate the issue. At least, in my tests it seems to work.

UPD: a bit rewritten my text to make it clearer.

[weisdd]

@zachmoody :)

[zachmoody]

Makes sense, nice catch.

[tyler-8]

Sorry for the headache from my introduction of threading, great find!

[weisdd]

@tyler-8 No worries, after working in TAC, I have a soft spot for troubleshooting activities :)
And thanks for the threads, in our use case they make a huge difference!

[MattIPv4]

Hey, @weisdd - Thanks a ton for this sweet PR! 😄

Would you please shoot me an email when you get a chance?
mcowley at digitalocean dot com 🎉