Fixing the redislite atexit bug

Found a bug in redislite, researched the cause, and bombarded it with solutions . A story about contributing whatever you can.
Source Code
https://github.com/bionikspoon/isolate_redislite_tox_psutil
Issue
https://github.com/yahoo/redislite/issues/93

Pairing problems with solutions

Summary

I came across a bug while rebuilding cache_requests.  To summarize, python has a built-in atexit hook used to register clean-up routines.  One of my dependencies — redislite — was using this hook in a way that conflicted with setuptools.  This was causing my tests to incorrectly fail on Tox and Travis.

Not understanding the problem initially, I dumped all my logs and raised an issue with yahoo’s redislite team.

Then, I went on to solve it!

Researching the problem

First I isolated the issue.  I used my custom cookiecutter to scaffold an empty project where I could recreate the bug with the least amount of code.

I did some research and found the exact source of the problem. I found a blog post, where someone ran into the same obscure problem. This author tracked the problem down to setuptools: between test runs setuptools deletes all the loaded modules THEN atexit runs. If your atexit cleanup function requires modules to run, they will not be there.  Really, this seems like a problem with setuptools, but that set of tools is WAY too complicated for me to get involved with.  That blog post even suggested a fix.  It was bit convoluted, so I suggested an easier fix and posted a gist to illustrate what my fix would look like.  I also demonstrated my solution works in that empty repo I built before.

I reported my new findings to the redislite repo.  At this point, my personal obligation to pair problems with solutions is fulfilled.

Creating solutions

I went on to experiment with a few solutions.  The solution I landed on, conceptually goes like this:

  1. when registering your cleanup function, also pass in a copy of the current modules;
  2. when your cleanup function runs, the first thing to do is restore loaded modules using that copy.

This is actually really simple!  In python, everything is a dict — everything, including loaded modules!  You access this dict as sys.modules.  So you pass a copy — sys.modules.copy() — in with your cleanup function, your cleanup function restores the modules.

Mission Accomplished!

Conclusion

Several months later, they accepted my PR!  I found a problem.  I solved. I solved it again. And again.  Each time more effective and  more permanent than the last.  It feels good.

In retrospect, 2 things:

  1. At the time, I thought of some ways to write tests for this, I should have written those out, and
  2. A context manager would have been a better way to swap out `sys.modules`  — ideally this context manager would swap the copy of modules in to run the function and out when its done.

Next time I revisit those projects, these points will be on my mind.

Over all, this was a fun adventure into the open source community!