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Submitting code best practices

Before You Begins

Looping In Your Reviewers

Before you start coding, select a lead reviewer (likely this will be your TL) and run your plan by them. This will prime them for your review, as well as surface any major concerns early on.

Draft a Design Doc (If Applicable)

If your CL will take more than 2-3 days to implement, create at least a one-pager that describes your approach. The larger the CL or set of CL's, the greater the need for a design doc. At least your TL should review, but it's also a great idea to socialize with your immediate team and/or wider team (either via email or presenting in weekly tech demos) depending on how large/complete the problem is.

How to write a design:

Familiarize Yourself with Coding Best Practices

Review style guides outlined in Writing Code -- this is more of an ongoing process in order to review these resources and improving your skills.


Testing is an important part of the coding process to ensure code we add doesn't break anything, and to future-proof our code from others breaking it. Manual testing is to make sure that code we add/change can behave well with a DUT and does what we expect it to do. Unit tests are to test all edge cases for each file we add in and to protect our code from unexpected changes in the future.

Manual tests

Before your CL is ready for review, you need to manually test your changes on your DUT. Describe your manual testing in your commit message under Tested:. For example,

Tested: verified that subsequent, guest, and initial discovery
notifications and associate account notifications auto dismiss after
12 seconds on DUT using JBL 225TWS

Unit tests

Every implementation file in your CL is expected to have corresponding unit test file. If a file does not need unit tests, make sure to include why in the commit message.

Low coverage reason: is not used fully in unit tests,
however this is a minor file with enums that are triggered in manual testing
and are not essential to implementation. Because it's an action enum, testing
one enum tests all enums.

Every unit test is expected to contribute to a file's >90% test coverage. Before you add reviewers to your CL, run CQ Dry Run on Gerrit, and once completed, there will be a summary of code coverage. Example of Gerrit code coverage summary showing the file's test coverage

In addition, within the implementation file, you are able to see which lines are missing unit test coverage highlighted in orange. Add unit tests that trigger these lines of code to increase code coverage of a file. Example of Gerrit missing code coverage lines highlighted in orange

Note: If you are adding metrics, you do not need to add a unit test for coverage; unit tests have historically not caught bugs in UMA metric collection implementations, because of complex interactions in the systems that emit metrics. Using chrome://histograms for manual testing and the UMA dashboard for canary/dev channel is a more meaningful integration test that can find bugs in metric collection implementations. However, metrics can be useful in unit tests to test isolated code paths where a metric emitted is the best way to ensure that is is working as expected.

If you are not adding a unit test for a metric, add the following Tested: bit. For example,

Tested:Verified on chrome://histograms that the metrics in this CL are logged
when the xyz event happens.

Choosing Code Reviewers

Who to include as a reviewer

Make sure you always add your TL as a reviewer for the CLs. If you don't have a TL related to your CL, at least someone from your team needs to be added to your CL. If external reviewers are needed for your CL, please ask someone from your team to review and +1 your CL before sending your CL out for review by the external team, and save the external reviewer time by specifying precisely which files you need them to review.

Local Owners

Add the most local code owners for the files you are changing, not the owners with most coverage. Local owners will have the most context for the changes that you are making. Beware, Gerrit will prompt you to pick the ones with the most coverage, and discourage too many code reviewers.

For example, if you wanted to change a file for the Bluetooth Settings page (chrome/browser/resources/settings/chromeos/os_bluetooth_page/) anyone that is an OWNER of chrome, chrome/browser, chrome/browser/resources, chrome/browser/resources/settings, or chrome/browser/resources/settings/chromeos will be able to review your code. However, they may not have the most context. Like mentioned above, Gerrit may not prompt you with the most local OWNERS. To find the most local OWNERS, go to the closest level directory with an OWNERS file and add one of these OWNERS to your CL.

Example of OWNERS local in the Bluetooth directory

Submitting Code Flow

  1. Upload code (git cl upload).
  2. Run CQ Dry Run and optionally include other debug try bots.
  3. Code-Review: choose reviewers from OWNERS files to review code. Gerrit also offers a “Find Owners” button.
  4. Address reviewer comments, upload changes.
  5. Once all reviewers have approved, click “SUBMIT TO CQ” (CQ: commit-queue) at the top of the page under the header to submit.


As mentioned above, running CQ Dry Run will provide a summary of code coverage. More importantly, CQ Dry Run will run the integration tests that are required to submit code. Run CQ Dry Run before you request a review and make sure all tests pass; if not, resolve all errors before requesting a review.

By default, any Chromium CL submitted via the Commit Queue must pass all tests for release builds. Tests on debug builds are skipped on the CQ because (1) they take a very long time to run and would slow down CL submission and (2) usually they will produce the same pass/fail result as release builds.

However, in some cases (e.g., debugging memory issues or working on WebUI code with optimize_webui = false), it may be valuable to run your code through debug trybots. These are the trybots worth testing on:

To run these tests on your CL, you have two options:

  1. On the command line, run:

    $ git cl try -B chromium/try \
               -b win_chromium_dbg_ng \
               -b linux_chromium_dbg_ng \
               -b mac_chromium_dbg_ng \
               -b linux-chromeos-dbg
  2. Go to the Gerrit UI and click "Choose Tryjobs", then add the trybots you'd like to test.

Getting Help

  1. When a tryjob fails for the reasons that are unrelated to the change you are trying to submit, open a bug (there is a "File bug" link in failure results).

  2. If you need help from CrOS sheriffs, contact them via Chat at go/cros-oncall-weekly (you can also auto-join weekly chats - see go/cros-oncall-chat-tool#auto-joining-gocros-oncall-weekly).

  3. Chrome sheriffs are on Slack -- channels #sheriffing and #ops.

Bug fix iteration and verification

Some additional advice applies when fixing bugs:

Write regression tests when fixing regressions

Try to reproduce the bug in a unit test before you implement a fix. Then you can verify that your fix actually fixes the original bug via unit testing. This is called Test Driven Development (TDD): a style of code development in which you first write the test, then write the code to be tested. To learn more about TDD, check out C++ Unit Test Code Lab and this presentation which discuss the importance of TDD.

In addition, in the CL that fixes the bug, please add a comment above your unit test as follows: “// Regression test for b/xxxxx”. For example,

// Regression test for b/261041950.
      FastPairPairingEventCalledDuringBluetoothAdapterPairingEvent) {

Note: see Debugging tips and tricks to learn some methods available to debug a regression.

Don’t close bugs whose outcome is uncertain

When you fix a bug, ask yourself if there is some method to quantify/verify your fix when it hits public, and create a plan to check that method if so.

The Smart Lock team provides the following example of verifying your fix, well after your fix lands:

The Smart Lock team landed a super impactful fix, b/216829464, that dramatically improved feature reliability, but the bug was just marked closed as soon as the CL landed -- so when we got a huge positive metrics spike a couple months later (b/237892911) we didn't know why, and spent a ton of time tracking down the original fix.

In this case, we could have used bugjuggler (see go/bugjuggler) to track follow up verification on the bug fix. In this case, we could have set a bugjuggler reminder on b/216829464 for 2/4/8 weeks to check if the expected metrics improved in dev/beta/stable channel. Your particular bug might require verifying that crash stats have fallen, etc., and bugjuggler is the best way to circle back and verify.