the Chromium logo

The Chromium Projects

Fixing Flaky Unit Tests


Chrome has dozens of unit-test suites. They generally have the name <component>_unittests, e.g., 'views_unittests', and the catch-all 'unit_tests'. Unit tests run in batches of 10 tests per process. If a test fails, it is retried in its own process. If the retry succeeds, the original failure is called a hidden flake. This type of flake tends to be fairly reproducible if you run the same set of tests in a single process as the failing trybot did. To determine what that set of tests is, do the following:

Up to 9 of the previous tests listed in the output run in the same process as the failing test. There’s currently no indication of which tests are in the same process in the log. However, if a test crashes, then the remaining tests in the batch of 10 are skipped, and retried singly, each in their own process. In this case, it's easy to see what the previous tests in the batch are, by counting how many tests were skipped, e.g., if 5 tests are skipped, the 4 tests before the crashing test were run in the same process.

To run the tests locally, use --test-launcher-filter-file=<file with names of up to 10 tests>, or gtest_filter=<test1>:<test2>:<test3>... If you get a reproducible case, run the test suite under the debugger, and debug the failure.

Causes of failures, and typical fixes

Global state interactions between tests

If one test leaves some global state in an unexpected, non-default state, subsequent tests in the same batch/process can fail. Working backwards from the failure to the nearest global variable used tends to uncover the root issue. Comparing a failed run to a successful run of the test in question can also point to the root cause, e.g., using a debugger or logging to find where the execution path diverges between success and failure.

If you’ve found the global variable left in an unexpected state, the next step is to find the test that’s leaving it in an unexpected state. The simplest approach is to remove earlier tests from the gtest_filter and see when the flaky test stops failing. Another technique is to add a DCHECK in the OnTestEnd function in the test executable that checks if the global variable is in the default state (see How to add OnTestEnd checks ). Running all the tests in the test executable with that DCHECK will uncover any other tests that are leaving the global state in a non-default state, e.g., 2948066. You may want to commit the DCHECK change to prevent this from happening in the future.

If you know the culprit is in the same test class, you can put the DCHECK in the TearDown() method for the test class.

There are several different ways to fix this type of issue. If possible, use a method that prevents this type of flake from recurring.

Scoped state setters, aka RAII

Declare an object whose constructor sets the global state to the desired state, and whose destructor restores it to the default. If there is a test-only method that sets the desired state, making it private will force tests to use the scoped state setter, and prevent this type of flake from recurring.

Example CLs: 2831260, 2679227, 2918613, 2919893

Force state to default in OnTestStart

If the global state is used by a lot of tests, and is critical to them working correctly, resetting the state in the executable’s OnTestStart method is reasonable. For example, we do this to force the app locale to en-US, because so many tests expect the locale to be en-US. In some cases, non-test code changes global state, which means the test infrastructure or tests themselves need to reset the global state.

Example CL: 2678132.

Have the test or test class set the required state

Sometimes it’s not feasible to keep the global state from mutating, usually because non-test code can change some global state without the test necessarily knowing about it. In cases like this, it’s simplest for the test to set or reset the global state to the expected value at the start of the test. The less frequently used the global state is, the more likely it is that you should reset the global state in the start of tests that use it, as opposed to forcing the state to its default in OnTestStart.

Example CLs : 2668507, 2668674, 2693168

Leaking global observers

If a test registers a global observer or listener and forgets to unregister it, several bad things can happen. One is that a deleted observer may be called by a subsequent test, which will usually crash, but may corrupt memory. Adding DCHECKs that global observers are cleaned up in OnTestEnd can sometimes work, but some Chrome code leaves global observers registered by design. Adding logging when the number of global observers increases between OnTestStart and OnTestEnd can help pinpoint truly leaky tests.

Example CLs: 1653905, 2618221

Data races

If the test is only flaking on Thread Sanitizer (Tsan) trybots, most likely Tsan is detecting a data race in the test. The most common cause of this error used to be initializing a ScopedFeatureList after the test has started up a thread that checks if a feature is enabled, or destroying a ScopedFeatureList while a thread that checks if a feature is enabled is still running. 3400019 fixed the general case of this but corner cases have occurred since then. Tests can avoid the issue completely by initializing the ScopedFeatureList in the test class's constructor, which means that tests that use the same features will be in the same class. If the test has its own task environment, it needs to be destroyed before the ScopedFeatureList, so the ScopedFeatureList should be declared before the task environment in the test class.

Example CL: 2773418

A similar, but much less common issue can occur in tests that call SetupCommandLine, e.g., 2775863

Race conditions can also happen when a thread is started before one of the objects it references is created, or joined after one of these objects is destroyed. These may be fixed by changing the order in which objects are declared, in order to enforce creation and destruction order.

Sometimes destruction of an object can race with another thread trying to access the object. While not ideal, making sure the BrowserTaskEnvironment runs all outstanding tasks before destroying the object can work around this kind of issue, e.g., 2907652.

Sequence Checker errors

Typically, these are caused by multiple threads unexpectedly accessing the same object. If the error comes when a ref-counted object is destroyed on a different thread than it was created on, a simple fix is to make the object inherit from RefCountedDeleteOnSequence, e.g., 2785850. Sequence Checker can also DCHECK if a global async callback is leaked by one test and a subsequent test triggers the callback. This was ameliorated by 2718665.

Service dependency issues

The dependency manager is global, and is not cleared between tests running in the same process. This can lead to differences in the dependency graph between one test and the next, which can cause objects to get initialized in an unexpected order. If a test fails because a service is null, or service initialization fails unexpectedly, it may be related to changes made to the dependency graph by a previous test. Often it's best to hand this kind of problem off to a test owner. One common cause of this is use of AtExitManager. When unit tests use AtExitManager, all singletons are reset. This removes them from the dependency graph, which gets the dependency graph into a broken state for the next test. Removing the use of AtExitManger should fix this, e.g., 3293747. Or, tests can ensure that the state they need exists, e.g., 2937452.

Global maps not cleared between tests

Some code keeps a global map between an object and some other state, and does not clear it between tests. E.g., one test can cause a Profile* to get added to the map, and a subsequent test in the same process can create a Profile object with the same address, causing state to leak between tests.

Example CLs: 2970728, 2904474

Contention for global system resources

Generally, unit tests run in multiple shards on a machine, so several tests can be running at the same time on a machine. Also, other tests suites can be running on the same machine. Tests can contend for things like the file system and the clipboard. For example, if two tests run in separate processes, but both modify the same file, flakiness can ensue. This can be fixed by overriding Chrome’s path to the file, using ScopedFilePath. For clipboard issues, tests should use ui::TestClipboard instead of the system clipboard.

Example CLs: 2986782, 1625942

[Please feel free to add other types of issues and sample CLs here]

Techniques for reproducing flakes

Run with --single-process-tests in debugger

Run the whole suite (or a subset) in the debugger. If a test crashes, you will be dropped into the debugger and you can investigate what caused the crash.

Build tests with gn args is_debug=true, is_component_build=true.

Debug builds initialize allocated and freed memory to known states, which makes it easier to determine if an object is used after it’s deleted, or is uninitialized. E.g., with the Windows allocator, used in component builds, 0xcdcdcdcdcd means the memory is uninitialized, and 0xdddddddd means it is freed. If a flake doesn’t reproduce, build with gn args from the “Lookup GN Args” step on the failing trybot.

Run with --test-launcher-filter-file= where file contains a list of tests

Run tests starting with the first letter of the flaky test

E.g., --single-process-tests --gtest_filter=S*.*. This is just a crude way of sharding a test suite, so you can shake out global state bugs without having to run the whole suite.

Run with--gtest_shuffle

Repeatedly run with --gtest_shuffle, and then, use --gtest_random_seed to reproduce the failures found with a particular --gtest_shuffle run.

Increase the batch size with --test-launcher-batch-limit

Increase the number of tests run in each process via --test-launcher-batch-limit=<desired value>. This will increase the likelihood of global state interactions between tests. Fixing those will help prevent future flakiness.

Use --gtest_flagfile

Run with --gtest_flagfile=<file> where file contains --gtest_filter=<test1>:<test2>:<test3>... This allows you to completely control the tests that run, and their order.

Add OnTestEnd checks to find tests which violate constraints

Each unit test executable has a small .cc module with a main() function that runs the tests. It is typically called In order to add OnTestEnd checks, the test suite needs to subclass TestSuite if it does not already do so, add a listener to the test suite, and override the OnTestEnd method in its listener. base/test/ has a relatively simple example of this. Also see

Parameterize flaky test with a repeat count and run on trybots

Create a CL that runs the test many times, and adds any useful additional logging, e.g.:

 TEST_F(ExtensionServiceTest, InstallTheme)


class ExtensionServiceFlakyTest : public ExtensionServiceTest,
                                  public testing::WithParamInterface<int> {};

#if defined(MEMORY_SANITIZER) || defined(THREAD_SANITIZER) || \
constexpr int kInstantiationCount = 500;
#elif !defined(NDEBUG)
constexpr int kInstantiationCount = 1000;
constexpr int kInstantiationCount = 3000;

                         testing::Range(0, kInstantiationCount));

TEST_P(ExtensionServiceFlakyTest, InstallTheme) {

Upload CL and run on trybots.

Windows App Verifier (AppVerif.exe)

This app can catch a lot of different kinds of errors, including Windows handle issues and AddressSanitizer (ASan)-like things like memory use after free (as long as PartitionAlloc is disabled, which it is in component builds). To run it on a unit test, run AppVerif.exe, pick File | Add Application, and browse to the unit test you wish to test. Then, run that unit test under the debugger, with --single-process-tests. If App Verifier detects an error, it will drop you into the debugger.


gtest-parallel can be useful for reproducing race conditions, resource contention, and simulating trybot behavior under heavy load.

It can be found on GitHub here. Example Windows command:

python "c:\src\utils\gtest-parallel-master\" out\Default\unit_tests.exe --gtest_filter=ExtensionServiceSyncTest.ProcessSyncDataUninstall --repeat=500 --workers=80 > par_run.txt

The higher the number of workers, the more likely test timeouts are, so if there are a lot of test timeouts, reduce the number of workers.