Addressing Flaky GTests

Understanding builder results

LUCI Analysis lists the top flake clusters of tests along with any associated bug and failure counts in different contexts.

Reproducing the flaky test

If debugging via bot is too slow or you otherwise need to drill further into the cause of the flake, you can try to reproduce the flake locally. Reproducing the flake can be difficult, so it can help to try and replicate the test environment as closely as possible.

Copy the gn args from one of the bots where the flake occurs, and try to choose a bot close to your system, i.e. linux-rel if you're building on linux. To get the gn args, you can again click on the timestamp in the flake portal to view the bot run details, and search for the “lookup GN args” build step to copy the args.

Build and run the test locally. Depending on the frequency of the flake, it may take some time to reproduce. Some helpful flags:

• --gtest_repeat=100
• --gtest_also_run_disabled_tests (if the flaky test(s) you're looking at have been disabled)

If you're unable to reproduce the flake locally, you can also try uploading your patch with the debug logging and flaky test enabled to try running the bot to reproduce the flake with more information.

Another good solution is to use Swarming -- which will let you mimic bot conditions to better reproduce flakes that actually occur on CQ bots.

Swarming

For a more detailed dive into swarming you can follow this link.

As an example, suppose we have built Chrome using the GN args from above into a directory out/linux-rel, then we can simply run this command within the chromium/src directory:

tools/run-swarmed.py out/linux-rel browser_tests -- --gtest_filter="*<YOUR_TEST_NAME_HERE>*" --gtest_repeat=20 --gtest_also_run_disabled_tests

This allows us to quickly iterate over errors using logs to reproduce flakes and even fix them!

TODO: Add more tips for reproducing flaky tests

Debugging the flaky test

If the test is flakily timing out, consider any asynchronous code that may cause race conditions, where the test subject may early exit and miss a callback, or return faster than the test can start waiting for it (i.e. make sure event listeners are spawned before invoking the event). Make sure event listeners are for the proper event instead of a proxy (e.g. Wait for the correct event in test).

Consider possible bugs in the system or test infrastructure (e.g. races in glibc).

For browsertest flakes, consider possible inter-process issues, such as the renderer taking too long or returning something unexpected (e.g. flaky RenderFrameHostImplBrowserTest).

For browsertest flakes that check EvalJs results, make sure test objects are not destroyed before JS may read their values (e.g. flaky PaymentAppBrowserTest).

For browsertest flakes that involve dialogs or widgets, make sure that test objects are not destroyed because focus is lost on the dialog (e.g flaky AccessCodeCastHandlerBrowserTest).

Preventing similar flakes

Once you understand the problem and have a fix for the test, think about how the fix may apply to other tests, or if documentation can be improved either in the relevant code or this flaky test documentation.