debian-mirror-gitlab/doc/development/testing_guide/flaky_tests.md

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# Flaky tests

## What's a flaky test?

It's a test that sometimes fails, but if you retry it enough times, it passes,
eventually.

## What are the potential cause for a test to be flaky?

### Unclean environment

**Label:** `flaky-test::unclean environment`

**Description:** The environment got dirtied by a previous test. The actual cause is probably not the flaky test here.

**Difficulty to reproduce:** Moderate. Usually, running the same spec files until the one that's failing reproduces the problem.

**Resolution:** Fix the previous tests and/or places where the environment is modified, so that
it's reset to a pristine test after each test.

**Examples:**

- [Example 1](https://gitlab.com/gitlab-org/gitlab/-/issues/378414#note_1142026988): A migration
  test might roll-back the database, perform its testing, and then roll-up the database in an
  inconsistent state, so that following tests might not know about certain columns.
- [Example 2](https://gitlab.com/gitlab-org/gitlab/-/issues/368500): A test modifies data that is
  used by a following test.
- [Example 3](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/103434#note_1172316521): A test for a database query passes in a fresh database, but in a
  CI/CD pipeline where the database is used to process previous test sequences, the test fails. This likely
    means that the query itself needs to be updated to work in a non-clean database.

### Ordering assertion

**Label:** `flaky-test::ordering assertion`

**Description:** The test is expecting a specific order in the data under test yet the data is in
a non-deterministic order.

**Difficulty to reproduce:** Easy. Usually, running the test locally several times would reproduce
the problem.

**Resolution:** Depending on the problem, you might want to:

- loosen the assertion if the test shouldn't care about ordering but only on the elements
- fix the test by specifying a deterministic ordering
- fix the app code by specifying a deterministic ordering

**Examples:**

- [Example 1](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/10148/diffs): Without
  specifying `ORDER BY`, database will not give deterministic ordering, or data race happening
  in the tests.
- [Example 2](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/106936/diffs).

### Dataset-specific

**Label:** `flaky-test::dataset-specific`

**Description:** The test assumes the dataset is in a particular (usually limited) state, which
might not be true depending on when the test run during the test suite.

**Difficulty to reproduce:** Moderate, as the amount of data needed to reproduce the issue might be
difficult to achieve locally.

**Resolution:** Fix the test to not assume that the dataset is in a particular state, don't hardcode IDs.

**Examples:**

- [Example 1](https://gitlab.com/gitlab-org/gitlab/-/issues/378381): The database is recreated when
  any table has more than 500 columns. It could pass in the merge request, but fail later in
  `master` if the order of tests changes.
- [Example 2](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/91016/diffs): A test asserts
  that trying to find a record with an nonexistent ID returns an error message. The test uses an
  hardcoded ID that's supposed to not exist (e.g. `42`). If the test is run early in the test
  suite, it might pass as not enough records were created before it, but as soon as it would run
  later in the suite, there could be a record that actually has the ID `42`, hence the test would
  start to fail.

### Random input

**Label:** `flaky-test::random input`

**Description:** The test use random values, that sometimes match the expectations, and sometimes not.

**Difficulty to reproduce:** Easy, as the test can be modified locally to use the "random value"
used at the time the test failed

**Resolution:** Once the problem is reproduced, it should be easy to debug and fix either the test
or the app.

**Examples:**

- [Example 1](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/20121): The test isn't robust enough to handle a specific data, that only appears sporadically since the data input is random.

### Unreliable DOM Selector

**Label:** `flaky-test::unreliable dom selector`

**Description:** The DOM selector used in the test is unreliable.

**Difficulty to reproduce:** Moderate to difficult. Depending on whether the DOM selector is duplicated, or appears after a delay etc.
Adding a delay in API or controller could help reproducing the issue.

**Resolution:** It really depends on the problem here. It could be to wait for requests to finish, to scroll down the page etc.

**Examples:**

- [Example 1](https://gitlab.com/gitlab-org/gitlab/-/issues/338341): A non-unique CSS selector
  matching more than one element, or a non-waiting selector method that does not allow rendering
  time before throwing an `element not found` error.
- [Example 2](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/101728/diffs): A CSS selector
  only appears after a GraphQL requests has finished, and the UI has updated.

### Datetime-sensitive

**Label:** `flaky-test::datetime-sensitive`

**Description:** The test is assuming a specific date or time.

**Difficulty to reproduce:** Easy to moderate, depending on whether the test consistently fails after a certain date, or only fails at a given time or date.

**Resolution:** Freezing the time is usually a good solution.

**Examples:**

- [Example 1](https://gitlab.com/gitlab-org/gitlab/-/issues/118612): A test that breaks after some time passed.

### Unstable infrastructure

**Label:** `flaky-test::unstable infrastructure`

**Description:** The test fails from time to time due to infrastructure issues.

**Difficulty to reproduce:** Hard. It's really hard to reproduce CI infrastructure issues. It might
be possible by using containers locally.

**Resolution:** Starting a conversation with the Infrastructure department in a dedicated issue is
usually a good idea.

**Examples:**

- [Example 1](https://gitlab.com/gitlab-org/gitlab/-/issues/363214): The runner is under heavy load at this time.
- [Example 2](https://gitlab.com/gitlab-org/gitlab/-/issues/360559): The runner is having networking issues, making a job failing early

## Quarantined tests

When a test frequently fails in `master`,
create [a ~"failure::flaky-test" issue](https://about.gitlab.com/handbook/engineering/workflow/#broken-master).

If the test cannot be fixed in a timely fashion, there is an impact on the
productivity of all the developers, so it should be quarantined by
assigning the `:quarantine` metadata with the issue URL, and add the `~"quarantined test"` label to the issue.

```ruby
it 'succeeds', quarantine: 'https://gitlab.com/gitlab-org/gitlab/-/issues/12345' do
  expect(response).to have_gitlab_http_status(:ok)
end
```

This means it is skipped unless run with `--tag quarantine`:

```shell
bin/rspec --tag quarantine
```

Once a test is in quarantine, there are 3 choices:

- Fix the test (that is, get rid of its flakiness).
- Move the test to a lower level of testing.
- Remove the test entirely (for example, because there's already a
  lower-level test, or it's duplicating another same-level test, or it's testing
  too much etc.).

## Automatic retries and flaky tests detection

On our CI, we use [RSpec::Retry](https://github.com/NoRedInk/rspec-retry) to automatically retry a failing example a few
times (see [`spec/spec_helper.rb`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/spec_helper.rb) for the precise retries count).

We also use a custom [`RspecFlaky::Listener`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/tooling/rspec_flaky/listener.rb).
This listener runs in the `update-tests-metadata` job in `maintenance` scheduled pipelines
on the `master` branch, and saves flaky examples to `rspec/flaky/report-suite.json`.
The report file is then retrieved by the `retrieve-tests-metadata` job in all pipelines.

This was originally implemented in: <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/13021>.

If you want to enable retries locally, you can use the `RETRIES` environment variable.
For instance `RETRIES=1 bin/rspec ...` would retry the failing examples once.

To generate the reports locally, use the `FLAKY_RSPEC_GENERATE_REPORT` environment variable.
For example, `FLAKY_RSPEC_GENERATE_REPORT=1 bin/rspec ...`.

### Usage of the `rspec/flaky/report-suite.json` report

The `rspec/flaky/report-suite.json` report is:

- Used for [automatically skipping known flaky tests](../pipelines/index.md#automatic-skipping-of-flaky-tests).
- [Imported into Snowflake](https://gitlab.com/gitlab-data/analytics/-/blob/master/extract/gitlab_flaky_tests/upload.py)
  once per day, for monitoring with the [internal dashboard](https://app.periscopedata.com/app/gitlab/888968/EP---Flaky-tests).

## Problems we had in the past at GitLab

- [`rspec-retry` is biting us when some API specs fail](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/29242): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/9825>
- [Sporadic RSpec failures due to `PG::UniqueViolation`](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/28307#note_24958837): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/9846>
  - Follow-up: <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/10688>
  - [Capybara.reset_session! should be called before requests are blocked](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/33779): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/12224>
- ffaker generates funky data that tests are not ready to handle (and tests should be predictable so that's bad!):
  - [Make `spec/mailers/notify_spec.rb` more robust](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/20121): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/10015>
  - [Transient failure in `spec/requests/api/commits_spec.rb`](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/27988#note_25342521): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/9944>
  - [Replace ffaker factory data with sequences](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/29643): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/10184>
  - [Transient failure in spec/finders/issues_finder_spec.rb](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/30211#note_26707685): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/10404>

### Order-dependent flaky tests

These flaky tests can fail depending on the order they run with other tests. For example:

- <https://gitlab.com/gitlab-org/gitlab/-/issues/327668>

To identify the tests that lead to such failure, we can use `scripts/rspec_bisect_flaky`,
which would give us the minimal test combination to reproduce the failure:

1. First obtain the list of specs that ran before the flaky test. You can search
   for the list under `Knapsack node specs:` in the CI job output log.
1. Save the list of specs as a file, and run:

    ```shell
    cat knapsack_specs.txt | xargs scripts/rspec_bisect_flaky
    ```

If there is an order-dependency issue, the script above will print the minimal
reproduction.

### Time-sensitive flaky tests

- <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/10046>
- <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/10306>

### Array order expectation

- <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/10148>

### Feature tests

- [Be sure to create all the data the test need before starting exercise](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/32622#note_31128195): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/12059>
- [Bis](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/34609#note_34048715): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/12604>
- [Bis](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/34698#note_34276286): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/12664>
- [Assert against the underlying database state instead of against a page's content](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/31437): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/10934>
- In JS tests, shifting elements can cause Capybara to mis-click when the element moves at the exact time Capybara sends the click
  - [Dropdowns rendering upward or downward due to window size and scroll position](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/17660)
  - [Lazy loaded images can cause Capybara to mis-click](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/18713)
- [Triggering JS events before the event handlers are set up](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/18742)
- [Wait for the image to be lazy-loaded when asserting on a Markdown image's `src` attribute](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/25408)
- [Avoid asserting against flash notice banners](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/79432)

#### Capybara viewport size related issues

- [Transient failure of spec/features/issues/filtered_search/filter_issues_spec.rb](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/29241#note_26743936): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/10411>

#### Capybara JS driver related issues

- [Don't wait for AJAX when no AJAX request is fired](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/30461): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/10454>
- [Bis](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/34647): <https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/12626>

#### Capybara expectation times out

- [Test imports a project (via Sidekiq) that is growing over time, leading to timeouts when the import takes longer than 60 seconds](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/22599)

### Hanging specs

If a spec hangs, it might be caused by a [bug in Rails](https://github.com/rails/rails/issues/45994):

- <https://gitlab.com/gitlab-org/gitlab/-/merge_requests/81112>
- <https://gitlab.com/gitlab-org/gitlab/-/issues/337039>

## Resources

- [Flaky Tests: Are You Sure You Want to Rerun Them?](https://semaphoreci.com/blog/2017/04/20/flaky-tests.html)
- [How to Deal With and Eliminate Flaky Tests](https://semaphoreci.com/community/tutorials/how-to-deal-with-and-eliminate-flaky-tests)
- [Tips on Treating Flakiness in your Rails Test Suite](https://semaphoreci.com/blog/2017/08/03/tips-on-treating-flakiness-in-your-test-suite.html)
- ['Flaky' tests: a short story](https://www.ombulabs.com/blog/rspec/continuous-integration/how-to-track-down-a-flaky-test.html)
- [Using Insights to Discover Flaky, Slow, and Failed Tests](https://circleci.com/blog/using-insights-to-discover-flaky-slow-and-failed-tests/)

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