Bisecting Python unit test errors to find test interdependencies


Many of our test runs use parallelization to run faster. Sometimes we see test
failures which we can’t reproduce locally, because locally we usually run
sequentially; and even then, the test ordering seems to be somewhat
unpredictable so it’s hard to reproduce the exact test ordering seen in our
test runner.

Most of the time these failures are due to unidentified test interdependencies:
either test A causes test B to pass (where running test B in isolation would
fail), or test A causes B to fail (where running B in isolation would pass). And we have seen more complex scenarios where C passes, A-B-C passes, but A-C fails (because A sets C up for failure, while B would set C up for success). We added some diagnostic output to our test runner so it would show exactly the list of tests each process runs. This way we can copy the list and run it locally, which usually reproduces the failure.

But we needed a tool to then determine exactly which of the tests preceding the failing one was setting up the failure conditions. So I wrote this simple bisecter script, which expects a list of test names, which must contain the faily test “A”, and of course, the name of the faily test “A”. It looks for “A” in the list and will use bisection to determine which of the tests preceding “A” is causing the failure.

As an example, I used it to find a test failure in Ubuntu SSO:



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