Commit Checklist for Chromium Workflow

Here is a helpful checklist to go through before uploading change lists (CLs) on Gerrit and during the code review process. Gerrit is the code review platform for the Chromium project. This checklist is designed to be streamlined. See contributing to Chromium for a more thorough reference. The intended audience is software engineers who are unfamiliar with contributing to the Chromium project. Feel free to skip steps that are not applicable to the patchset you're currently uploading.

According to the Checklist Manifesto by Atul Gawande, checklists are a marvelous tool for ensuring consistent quality in the work you produce. Checklists also help you work more efficiently by ensuring you never skip a step or waste brain power figuring out the next step to take.

1. Create a new branch or switch to the correct branch

You should create a new branch before starting any development work. It's helpful to branch early and to branch often in Git. Use the command git new-branch <branch_name>. This is equivalent to git checkout -b <branch_name> --track origin/main.

You may also want to set another local branch as the upstream branch. You can do that with git checkout -b <branch_name> --track <upstream_branch>. Do this if you want to split your work across multiple CLs, but some CLs have dependencies on others. Use git new-branch --upstream_current <new_branch_name> to create a new branch while setting the current branch as the upstream.

Mark the associated crbug as “started” so that other people know that you have started working on the bug. Taking this step can avoid duplicated work.

If you have already created a branch, don‘t forget to git checkout <branch_name> to the correct branch before resuming development work. There’s few things more frustrating than to finish implementing your ideas or feedback, and to spend hours debugging some mysterious bug, only to discover that the bug was caused by working on the wrong branch this whole time.

2. If there's a local upstream branch, rebase the upstream changes

Suppose you have a downstream branch chained to an upstream branch. If you commit changes to the upstream branch, and you want the changes to appear in your downstream branch, you need to:

  • git checkout <branch_name> to the downstream branch.
  • Run git rebase -i @{u} to pull the upstream changes into the current branch.
  • Run git rebase -i @{u} again to rebase the downstream changes onto the upstream branch.

Expect to fix numerous merge conflicts. Use git rebase --continue once you're done.

3. Make your changes

Do your thing. There's no further advice here about how to write or fix code.

4. Make sure the code builds correctly

After making your changes, check that common targets build correctly:

  • chrome (for Linux, ChromeOS, etc.)
  • unit_tests
  • browser_tests

You can find instructions here for building various targets.

It‘s easy to inadvertently break one of the other builds you’re not currently working on without realizing it. Even though the Commit Queue should catch any build errors, checking locally first can save you some time since the CQ Dry Run can take a while to run, on the order of a few hours sometimes.

5. Test your changes

Test your changes manually by running the Chrome binary or deploying your changes to a test device. If you're testing Chrome for ChromeOS, follow the Simple Chrome instructions to deploy your changes to a test device. Make sure you hit every code path you changed.

Some testing tips:

  • Use LOG(ERROR) << "debug print statement" for debugging. You can find the logs in /var/logs/chrome/ on the ChromeOS device. You can add a keyword to your print statement to help find your log statements more quickly.
  • Use GDB for setting breakpoints while debugging.

Think about testing any edge cases that could break your code. Some common edge cases to consider:

  • Guest mode
  • Enterprise/EDU/Supervised users
  • Accessibility
  • Official Chrome-branded build (for Googlers)

6. Write unit or browser tests for any new code

Consider automating any manual testing you did in the previous step.

7. Ensure the code is formatted nicely

Run git cl format --js. The --js option also formats JavaScript changes.

8. Review your changes

Use git diff to review all of the changes you‘ve made from the previous commit. Use git upstream-diff to review all of the changes you’ve made from the upstream branch. The output from git upstream-diff is what will be uploaded to Gerrit.

9. Stage relevant files for commit

Run git add <path_to_file> for all of the files you've modified that you want to include in the CL. Unlike other version-control systems such as svn, you have to specifically git add the files you want to commit before calling git commit.

10. Commit your changes

Run git commit. Be sure to write a useful commit message. Here are some tips for writing good commit messages. A shortcut for combining the previous step and this one is git commit -a -m <commit_message>.

11. Squash your commits

If you have many commits on your current branch, and you want to avoid some nasty commit-by-commit merge conflicts in the next step, consider collecting all your changes into one commit. Run git rebase -i @{u}. The @{u} is a short-hand pointer for the upstream branch, which is usually origin/main, but can also be one of your local branches. After running the git rebase command, you should see a list of commits, with each commit starting with the word “pick”. Make sure the first commit says “pick” and change the rest from “pick” to “squash”. This will squash each commit into the previous commit, which will continue until each commit is squashed into the first commit.

An alternative way to squash your commits into a single commit is to do git commit --amend in the previous step.

Alternatively you can also run git squash-branch.

12. Rebase your local repository

Rebasing is a neat way to sync changes from the remote repository and resolve any merge conflict errors on your CL. Run git rebase-update. This command updates all of your local branches with remote changes that have landed since you started development work, which could've been a while ago. It also deletes any branches that match the remote repository, such as after the CL associated with that branch has been merged. In summary, git rebase-update cleans up your local branches.

You may run into rebase conflicts. Fix them manually before proceeding with git rebase --continue.

Note that rebasing has the potential to break your build, so you might want to try re-building afterwards. You need to run gclient sync -D before trying to build again after a rebase-update, to update third-party dependencies.

13. Upload the CL to Gerrit

Run git cl upload. Some useful options include:

  • --cq-dry-run (or -d) will set the patchset to do a CQ Dry Run. It is a good idea to run try jobs for each new patchset with significant changes.
  • -r <chromium_username> will add reviewers.
  • -b <bug_number> automatically populates the bug reference line of the commit message. Use -b None if there is no relevant crbug.
  • -x <bug_number> automatically populates the bug reference line of the commit message and will automatically mark the bug as closed when the CL is submitted and merged.
  • --edit-description will let you update the commit message. Using square brackets in the commit message title, like [hashtag], will add a hashtag to your CL. This feature is useful for grouping related CLs together.

Check git cl issue to ensure that you are uploading to the correct Gerrit CL. If you are uploading a new CL, then the issue number will be none. Uploading will automatically create a new CL. Use git cl issue <issue_number> to target an existing CL for uploading new patches.

To help guide your reviewers, it is also recommended to provide a title for each patchset summarizing the changes and indicating whose comments the patchset addresses. Running git cl upload will upload a new patchset and prompt you for a brief patchset title. The title defaults to your most recent commit summary (the -T option will use this without prompting). If you tend to squash all your commits into one, try to enter a new summary each time you upload. You can also modify the patchset title directly in Gerrit.

14. Check the CL again in Gerrit

Run git cl web to go to the Gerrit URL associated with the current branch. Open the latest patchset and verify that all of the uploaded files are correct. Click Expand All to check over all of the individual line-by-line changes again. Basically do a self-review before asking your reviewers for a review.

15. Make sure all auto-regression tests pass

Click CQ Dry Run. Fix any errors because otherwise the CL won't pass the commit queue (CQ) checks. Consider waiting for the CQ Dry Run to pass before notifying your reviewers, in case the results require major changes in your CL.

Alternatively you can run git cl try.

16. Add reviewers to review your code

Click Find Owners or run git cl owners to find file owners to review your code and instruct them about which parts you want them to focus on. Prefer owners who are more specific to files you are modifying, as they usually have the best domain knowledge (i.e. prefer //chrome/foo/bar/OWNERS over //chrome/OWNERS). Next, add anyone else you think should review your code. The blame functionality in Code Search is a good way to identify reviewers who may be familiar with the parts of code your CL touches. For your CL to land, you need an approval from an owner for each file you‘ve changed, unless you are an owner of some files, in which case you don’t need separate owner approval for those files.

You are expected to wait for all actively participating reviewers to CR+1 the change before submitting (CQ+2), even if your CL already has all required owners reviews. Other than preventing confusion and mistakes, this expectation exists because:

  1. Participating reviewers are helping you write sustainable code, and letting them sign off is respectful of their efforts.
  2. The owners system is not perfect, and sometimes you will need an owner who can approve the whole change, but will delegate approval of pieces to other, more knowledgeable owners.

If this expectation needs to be broken, then the reason should be justified in a comment, and appropriate extra care may be appropriate (e.g. getting a post-submit review, monitoring for failing or flaky tests, reverting if any problems occur, etc).

17. Start Your Review

Click on the Start Review button to begin the actual review process. Until you press this button, nobody will look at your change. Once pressed, you'll have the opportunity to include an additional message in the notification sent to your reviewers.

18. Implement feedback from your reviewers

Then go through this commit checklist again. Reply to all comments from the reviewers on Gerrit and mark all resolved issues as resolved. To see all unresolved comments, click on the “Comments” tab in Gerrit. Other than freeform interaction on the comments (using Reply or Quote), here are common conventions:

  • Clicking Done on the comment will comment “Done” and resolve this comment. This usually is used in response to a requested change by the reviewer, and tells the reviewer that you have made the change that they requested.
  • Clicking Ack on the comment will comment “Ack” (short for “Acknowledged”) and resolve this comment. This usually is used in response to a non-actionable comment by the reviewer, and tells the reviewer that you understand.

Finally, click Reply on the CL to ensure that your reviewers receive a notification. Doing this signals that your CL is ready for review again, since the assumption is that your CL is not ready for review until you hit reply.

To ensure a fast, productive, and respectful review, please follow the guidelines in Respectful Changes.

If your change is simple and you feel confident that your reviewer will approve your CL on the next iteration, you can set Auto-Submit +1. The CL will proceed to the next step automatically after approval. This feature is useful if your reviewer is in a different time zone and you want to land the CL sooner. Setting this flag also puts the onus on your reviewer to land the CL.

19. Land your CL

Once you have obtained a Looks Good To Me (LGTM), which is reflected by a Code-Review+1 in Gerrit, from at least one owner for each file, then you have the minimum prerequisite to land your changes. As mentioned above, you are generally expected to wait for all of your reviewers to approve your changes as well, even if you already have OWNERS approval. Don't use chrome/OWNERS as a blanket stamp if your CL makes significant changes to subsystems. Click Submit to CQ (Commit-Queue +2) to both try your change in the commit queue (CQ) and automatically land it if successful.

Alternatively you can run git cl set-commit.

Just because your CL made it through the CQ doesn‘t mean you’re in the clear yet. There might be internal non-public try job failures, or bugs that went unnoticed during the code review process. Consider monitoring the Chromium tree for about a day after your CL lands. If the Sheriff or anyone else brings any failures to your attention, revert the CL first and ask questions later. Gerrit can automatically generate revert CLs.

20. Cleanup

After your CL is landed, you can use git rebase-update or git cl archive to clean up your local branches. These commands will automatically delete merged branches. Please mark the associated crbug as “fixed”.