[webkit-dev] Handling flaky layout-test failures in EWS

Aakash Jain aakash_jain at apple.com
Tue Dec 3 08:40:31 PST 2019

Hi Everyone,

We have various layout-tests which are flaky (which sometimes pass and sometimes fail/crash/timeout). EWS needs to work despite these flaky tests, and need to be able to tell whether the patch being tested introduced any test failure or not.

In EWS, we have logic (same logic in both old and new EWS) on how to deal with flaky tests. The logic is roughly following:

- We apply the patch and build.

- Run layout-tests. During the test-run, we retry the failed tests. If those tests pass in retry, the run is considered to have no failing test (this retry part is same for EWS / build.webkit.org / manual-run and part of run-webkit-test script itself).

- If a test-run has some failures, EWS retry the test-run one more time (to check if those test failures were flaky). 

- Then we do one more test-run on clean-tree (without the patch), and compare the results of first run, second run, and clean tree run.

- After that we analyze the results from all three test-runs (which we call first_run, second_run and clean_tree_run). 

If there are different test failures in first and second runs (i.e.: flaky test failures), and the patch being tested did not introduce any new test failure, we retry the entire build (probably hoping that next time we will not see the flakiness). This retry can result in almost infinite loop, if someone commits a very flaky test (which is not rare), and would cause EWS to be almost stuck until the flakiness is fixed.

From https://trac.webkit.org/browser/webkit/trunk/Tools/Scripts/webkitpy/tool/bot/patchanalysistask.py#L352  ('Defer' means retrying the build).
350    # At this point we know that at least one test flaked, but no consistent failures
351    # were introduced. This is a bit of a grey-zone.
352    return False  # Defer patch

Retrying the entire build, just because of few flaky tests seems excessive and inefficient. This doesn't help identify flaky tests, and only delays the EWS result. Instead, we should mark the build as SUCCESS (since the patch did not introduce any new consistent test failure).

In other EWS test-suites like API tests and JSC tests, we do not retry the entire build on flaky test failures, we ignore the flaky tests, and only focus on tests which failed consistently. We should do the similar thing for layout-tests as well.

I am going to make that change for layout tests in https://bugs.webkit.org/show_bug.cgi?id=204769. Please let me know if anyone has a different opinion.


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