src/docs/testing/web_test_expectations.md

# Web Test Expectations and Baselines

The primary function of the web tests is as a regression test suite; this
means that, while we care about whether a page is being rendered correctly, we
care more about whether the page is being rendered the way we expect it to. In
other words, we look more for changes in behavior than we do for correctness.

[TOC]

All web tests have "expected results", or "baselines", which may be one of
several forms. The test may produce one or more of:

* A text file containing JavaScript log messages.
* A text rendering of the Render Tree.
* A screen capture of the rendered page as a PNG file.
* WAV files of the audio output, for WebAudio tests.

For any of these types of tests, baselines are checked into the web_tests
directory. The filename of a baseline is the same as that of the corresponding
test, but the extension is replaced with `-expected.{txt,png,wav}` (depending on
the type of test output). Baselines usually live alongside tests, with the
exception when baselines vary by platforms; read
[Web Test Baseline Fallback](web_test_baseline_fallback.md) for more
details.

Lastly, we also support the concept of "reference tests", which check that two
pages are rendered identically (pixel-by-pixel). As long as the two tests'
output match, the tests pass. For more on reference tests, see
[Writing ref tests](https://trac.webkit.org/wiki/Writing%20Reftests).

## Failing tests

When the output doesn't match, there are two potential reasons for it:

* The port is performing "correctly", but the output simply won't match the
  generic version. The usual reason for this is for things like form controls,
  which are rendered differently on each platform.
* The port is performing "incorrectly" (i.e., the test is failing).

In both cases, the convention is to check in a new baseline (aka rebaseline),
even though that file may be codifying errors. This helps us maintain test
coverage for all the other things the test is testing while we resolve the bug.

*** promo
If a test can be rebaselined, it should always be rebaselined instead of adding
lines to TestExpectations.
***

Bugs at [crbug.com](https://crbug.com) should track fixing incorrect behavior,
not lines in
[TestExpectations](../../third_party/blink/web_tests/TestExpectations). If a
test is never supposed to pass (e.g. it's testing Windows-specific behavior, so
can't ever pass on Linux/Mac), move it to the
[NeverFixTests](../../third_party/blink/web_tests/NeverFixTests) file. That
gets it out of the way of the rest of the project.

There are some cases where you can't rebaseline and, unfortunately, we don't
have a better solution than either:

1. Reverting the patch that caused the failure, or
2. Adding a line to TestExpectations and fixing the bug later.

In this case, **reverting the patch is strongly preferred**.

These are the cases where you can't rebaseline:

* The test is a reference test.
* The test gives different output in release and debug; in this case, generate a
  baseline with the release build, and mark the debug build as expected to fail.
* The test is flaky, crashes or times out.
* The test is for a feature that hasn't yet shipped on some platforms yet, but
  will shortly.

## Handling flaky tests

<!-- TODO(crbug.com/40262793): Describe the current flakiness dashboard and
    LUCI test history. -->

Once you decide that a test is truly flaky, you can suppress it using the
TestExpectations file, as [described below](#updating-the-expectations-files).
We do not generally expect Chromium sheriffs to spend time trying to address
flakiness, though.

## How to rebaseline

Since baselines themselves are often platform-specific, updating baselines in
general requires fetching new test results after running the test on multiple
platforms.

### Rebaselining using try jobs

The recommended way to rebaseline for a currently-in-progress CL is to use
results from try jobs, by using the command-tool
`third_party/blink/tools/blink_tool.py rebaseline-cl`:

1. First, upload a CL.
2. Trigger try jobs by running `blink_tool.py rebaseline-cl`. This should
   trigger jobs on
   [tryserver.blink](https://ci.chromium.org/p/chromium/g/tryserver.blink/builders).
3. Wait for all try jobs to finish.
4. Run `blink_tool.py rebaseline-cl` again to fetch new baselines.
5. Commit the new baselines and upload a new patch.

This way, the new baselines can be reviewed along with the changes, which helps
the reviewer verify that the new baselines are correct. It also means that there
is no period of time when the web test results are ignored.

#### Handle bot timeouts

When a change will cause many tests to fail, the try jobs may exit early because
the number of failures exceeds the limit, or the try jobs may timeout because
more time is needed for the retries. Rebaseline based on such results are not
suggested. The solution is to temporarily increase the number of shards in
[`test_suite_exceptions.pyl`](/testing/buildbot/test_suite_exceptions.pyl) in your CL.
Change the values back to its original value before sending the CL to CQ.

#### Options

The tests which `blink_tool.py rebaseline-cl` tries to download new baselines for
depends on its arguments.

* By default, it tries to download all baselines for tests that failed in the
  try jobs.
* If you pass `--only-changed-tests`, then only tests modified in the CL will be
  considered.
* You can also explicitly pass a list of test names, and then just those tests
  will be rebaselined.
* By default, it finds the try jobs by looking at the latest patchset. If you
  have finished try jobs that are associated with an earlier patchset and you
  want to use them instead of scheduling new try jobs, you can add the flag
  `--patchset=n` to specify the patchset. This is very useful when the CL has
  'trivial' patchsets that are created e.g. by editing the CL descrpition.

### Rebaseline script in results.html

Web test results.html linked from bot job result page provides an alternative
way to rebaseline tests for a particular platform.

* In the bot job result page, find the web test results.html link and click it.
* Choose "Rebaseline script" from the dropdown list after "Test shown ... in format".
* Click "Copy report" (or manually copy part of the script for the tests you want
  to rebaseline).
* In local console, change directory into `third_party/blink/web_tests/platform/<platform>`.
* Paste.
* Add files into git and commit.

The generated command includes `blink_tool.py optimize-baselines <tests>` which
removes redundant baselines.

### Local manual rebaselining

```bash
third_party/blink/tools/run_web_tests.py --reset-results foo/bar/test.html
```

If there are current expectation files for `web_tests/foo/bar/test.html`,
the above command will overwrite the current baselines at their original
locations with the actual results. The current baseline means the `-expected.*`
file used to compare the actual result when the test is run locally, i.e. the
first file found in the [baseline search path](https://cs.chromium.org/search/?q=port/base.py+baseline_search_path).

If there are no current baselines, the above command will create new baselines
in the platform-independent directory, e.g.
`web_tests/foo/bar/test-expected.{txt,png}`.

When you rebaseline a test, make sure your commit description explains why the
test is being re-baselined.

### Rebaselining flag-specific expectations

See [Testing Runtime Flags](./web_tests.md#Testing-Runtime-Flags) for details
about flag-specific expectations.

The [Rebaseline Tool](#How-to-rebaseline) supports all flag-specific suites that
[run in CQ/CI](/third_party/blink/tools/blinkpy/common/config/builders.json).
You may also rebaseline flag-specific results locally with:

```bash
third_party/blink/tools/run_web_tests.py --flag-specific=config --reset-results foo/bar/test.html
```

New baselines will be created in the flag-specific baselines directory, e.g.
`web_tests/flag-specific/config/foo/bar/test-expected.{txt,png}`

Then you can commit the new baselines and upload the patch for review.

Sometimes it's difficult for reviewers to review the patch containing only new
files. You can follow the steps below for easier review.

1. Copy existing baselines to the flag-specific baselines directory for the
   tests to be rebaselined:
   ```bash
   third_party/blink/tools/run_web_tests.py --flag-specific=config --copy-baselines foo/bar/test.html
   ```
   Then add the newly created baseline files, commit and upload the patch.
   Note that the above command won't copy baselines for passing tests.

2. Rebaseline the test locally:
   ```bash
   third_party/blink/tools/run_web_tests.py --flag-specific=config --reset-results foo/bar/test.html
   ```
   Commit the changes and upload the patch.

3. Request review of the CL and tell the reviewer to compare the patch sets that
   were uploaded in step 1 and step 2 to see the differences of the rebaselines.

## Kinds of expectations files

* [TestExpectations](../../third_party/blink/web_tests/TestExpectations): The
  main test failure suppression file. In theory, this should be used for
  temporarily marking tests as flaky.
  See [the `run_wpt_tests.py` doc](run_web_platform_tests.md) for information
  about WPT coverage for Chrome.
* [ASANExpectations](../../third_party/blink/web_tests/ASANExpectations):
  Tests that fail under ASAN.
* [CfTTestExpecations](../../third_party/blink/web_tests/CfTTestExpecations):
  Tests that fail under Chrome for Testing
* [LeakExpectations](../../third_party/blink/web_tests/LeakExpectations):
  Tests that have memory leaks under the leak checker.
* [MobileTestExpectations](../../third_party/blink/web_tests/MobileTestExpectations)
  Tests that fails under Chrome Android and Chrome WebView platform.
* [MSANExpectations](../../third_party/blink/web_tests/MSANExpectations):
  Tests that fail under MSAN.
* [NeverFixTests](../../third_party/blink/web_tests/NeverFixTests): Tests
  that we never intend to fix (e.g. a test for Windows-specific behavior will
  never be fixed on Linux/Mac). Tests that will never pass on any platform
  should just be deleted, though.
* [SlowTests](../../third_party/blink/web_tests/SlowTests): Tests that take
  longer than the usual timeout to run. Slow tests are given 5x the usual
  timeout.
* [StaleTestExpectations](../../third_party/blink/web_tests/StaleTestExpectations):
  Platform-specific lines that have been in TestExpectations for many months.
  They're moved here to get them out of the way of people doing rebaselines
  since they're clearly not getting fixed anytime soon.
* [W3CImportExpectations](../../third_party/blink/web_tests/W3CImportExpectations):
  A record of which W3C tests should be imported or skipped.

### Flag-specific expectations files

It is possible to handle tests that only fail when run with a particular flag
being passed to `content_shell`. See
[web_tests/FlagExpectations/README.txt](../../third_party/blink/web_tests/FlagExpectations/README.txt)
for more.

## Updating the expectations files

### Ordering

The file is not ordered. If you put new changes somewhere in the middle of the
file, this will reduce the chance of merge conflicts when landing your patch.

### Syntax

*** promo
Please see [The Chromium Test List Format](http://bit.ly/chromium-test-list-format)
for a more complete and up-to-date description of the syntax.
***

The syntax of the file is roughly one expectation per line. An expectation can
apply to either a directory of tests, or a specific tests. Lines prefixed with
`# ` are treated as comments, and blank lines are allowed as well.

The syntax of a line is roughly:

```
[ bugs ] [ "[" modifiers "]" ] test_name_or_directory [ "[" expectations "]" ]
```

* Tokens are separated by whitespace.
* **The brackets delimiting the modifiers and expectations from the bugs and the
  test_name_or_directory are not optional**; however the modifiers component is optional. In
  other words, if you want to specify modifiers or expectations, you must
  enclose them in brackets.
* If test_name_or_directory is a directory, it should be ended with `/*`, and all
  tests under the directory will have the expectations, unless overridden by
  more specific expectation lines. **The wildcard is intentionally only allowed at the
  end of test_name_or_directory, so that it will be easy to reason about
  which test(s) a test expectation will apply to.**
* Lines are expected to have one or more bug identifiers, and the linter will
  complain about lines missing them. Bug identifiers are of the form
  `crbug.com/12345`, `code.google.com/p/v8/issues/detail?id=12345` or
  `Bug(username)`.
* If no modifiers are specified, the test applies to all of the configurations
  applicable to that file.
* If specified, modifiers can be one of `Fuchsia`, `Mac`, `Mac11`,
  `Mac11-arm64`, `Mac12`, `Mac12-arm64`, `Mac13`, `Mac13-arm64`, `Mac14`,
  `Mac14-arm64`, `Mac15`, `Mac15-arm64`, `Linux`, `Chrome`, `Win`, `Win10.20h2`,
  `Win11`, `iOS17-Simulator`, and, optionally, `Release`, or `Debug`.
  Check the `# tags: ...` comments [at the top of each
  file](/third_party/blink/web_tests/TestExpectations#1) to see which modifiers
  that file supports.
* Some modifiers are meta keywords, e.g. `Win` represents `Win10.20h2` and `Win11`.
  See the `CONFIGURATION_SPECIFIER_MACROS` dictionary in
  [third_party/blink/tools/blinkpy/web_tests/port/base.py](../../third_party/blink/tools/blinkpy/web_tests/port/base.py)
  for the meta keywords and which modifiers they represent.
* Expectations can be one or more of `Crash`, `Failure`, `Pass`, `Slow`, or
  `Skip`, `Timeout`.
  Some results don't make sense for some files; check the `# results: ...`
  comment at the top of each file to see what results that file supports.
  If multiple expectations are listed, the test is considered "flaky" and any
  of those results will be considered as expected.

For example:

```
crbug.com/12345 [ Win Debug ] fast/html/keygen.html [ Crash ]
```

which indicates that the "fast/html/keygen.html" test file is expected to crash
when run in the Debug configuration on Windows, and the tracking bug for this
crash is bug \#12345 in the [Chromium issue tracker](https://crbug.com). Note
that the test will still be run, so that we can notice if it doesn't actually
crash.

Assuming you're running a debug build on Mac 10.9, the following lines are
equivalent (in terms of whether the test is performed and its expected outcome):

```
fast/html/keygen.html [ Skip ]
Bug(darin) [ Mac10.9 Debug ] fast/html/keygen.html [ Skip ]
```

### Semantics

`Slow` causes the test runner to give the test 5x the usual time limit to run.
`Slow` lines go in the
[`SlowTests` file](../../third_party/blink/web_tests/SlowTests).
A given line cannot have both Slow and Timeout.

Also, when parsing the file, we use two rules to figure out if an expectation
line applies to the current run:

1. If the configuration parameters don't match the configuration of the current
   run, the expectation is ignored.
2. Expectations that match more of a test name are used before expectations that
   match less of a test name.

If a [virtual test] has no explicit expectations (following the rules above),
it inherits its expectations from the base (nonvirtual) test.

[virtual test]: /docs/testing/web_tests.md#Virtual-test-suites

For example, if you had the following lines in your file, and you were running a
debug build on `Mac10.10`:

```
crbug.com/12345 [ Mac10.10 ] fast/html [ Failure ]
crbug.com/12345 [ Mac10.10 ] fast/html/keygen.html [ Pass ]
crbug.com/12345 [ Win11 ] fast/forms/submit.html [ Failure ]
crbug.com/12345 fast/html/section-element.html [ Failure Crash ]
```

You would expect:

* `fast/html/article-element.html` to fail with a text diff (since it is in the
  fast/html directory).
* `fast/html/keygen.html` to pass (since the exact match on the test name).
* `fast/forms/submit.html` to pass (since the configuration parameters don't
  match).
* `fast/html/section-element.html` to either crash or produce a text (or image
  and text) failure, but not time out or pass.
* `virtual/foo/fast/html/article-element.html` to fail with a text diff. The
  virtual test inherits its expectation from the first line.

Test expectation can also apply to all tests under a directory (specified with a
name ending with `/*`). A more specific expectation can override a less
specific expectation. For example:
```
crbug.com/12345 virtual/composite-after-paint/* [ Skip ]
crbug.com/12345 virtual/composite-after-paint/compositing/backface-visibility/* [ Pass ]
crbug.com/12345 virtual/composite-after-paint/compositing/backface-visibility/test.html [ Failure ]
```

*** promo
Duplicate expectations are not allowed within the file and will generate
warnings.
***

You can verify that any changes you've made to an expectations file are correct
by running:

```bash
third_party/blink/tools/lint_test_expectations.py
```

which will cycle through all of the possible combinations of configurations
looking for problems.