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Contributing to Chromium

This page assumes a working Chromium checkout and build. Note that a full Chromium checkout includes external repositories with their own workflows for contributing, such as v8 and Skia. Similarly, ChromiumOS, which includes Chromium as a subrepository, has its own development workflow.

[TOC]

Related resources

Communicate

When writing a new feature or fixing an existing bug, get a second opinion before going too far. If it's a new feature idea, propose it to the appropriate discussion group. If it's in the existing code base, talk to some of the folks in the "OWNERS" file (see code review policies for more) for the code being changed.

  • If a change needs further context outside the CL, it should be tracked in the bug system. Bugs are the right place for long histories, discussion and debate, attaching screenshots, and linking to other associated bugs. Bugs are unnecessary for changes isolated enough to need none of these.
  • If there isn't a bug and there should be one, please file a new bug.
  • Just because there is a bug in the bug system doesn't necessarily mean that a patch will be accepted.

Design Documents

Any nontrivial technical effort that will significantly impact Chromium should have a design doc (template). Specifically, we require design docs in the following cases:

  • When writing code that will have a large impact on Chromium as a whole, e.g. when you are changing code in Chromium's critical path (page loading, rendering).
  • When beginning a large technical undertaking that should be documented for historical reasons (>1 person-month of work can be used as a general guideline).

Send public design docs to [email protected]. Google internal Chrome design docs should follow the process at go/chrome-dd-review-process.

Legal stuff

All contributors must have valid Gerrit/Google accounts (which means you must be old enough to manage your own account) and complete the contributor license agreement.

For individual contributors, please complete the Individual Contributor License Agreement online. Corporate contributors must fill out the Corporate Contributor License Agreement and send it to us as described on that page.

First-time contributors

Add your (or your organization's) name and contact info to the AUTHORS file for Chromium or Chromium OS. Please include this as part of your first patch and not as a separate standalone patch.

External contributor checklist for reviewers

Before LGTMing a change from a non-chromium.org address, ensure that the contribution can be accepted:

  • Definition: The "author" is the email address that owns the code review request on https://chromium-review.googlesource.com
  • Ensure the author is already listed in AUTHORS. In some cases, the author's company might have a wildcard rule (e.g. *@google.com).
  • If the author or their company is not listed, the CL should include a new AUTHORS entry.
    • Ensure the new entry is reviewed by a reviewer who works for Google.
    • Contributor License Agreement can be verified by Googlers at http://go/cla.
    • If there is a corporate CLA for the author's company, it must list the person explicitly (or the list of authorized contributors must say something like "All employees"). If the author is not on their company's roster, do not accept the change.

Initial git setup

  1. Visit https://chromium-review.googlesource.com/new-password and follow the on-screen instructions to get credentials for uploading changes.
  2. Tell git about your name, email and some other settings. 
    git config --global user.name "My Name"
git config --global user.email "[email protected]"
git config --global core.autocrlf false
git config --global core.filemode false
git config --local gerrit.host true
# Uncomment this if you want your pull commands to always rebase.
# git config --global branch.autosetuprebase always
# Uncomment if you want new branches to track the current branch.
# git config --global branch.autosetupmerge always
  3. Visit https://chromium-review.googlesource.com/settings/ to ensure that your preferred email is set to the same one you use in your git configuration.

Creating a change

First, create a new branch for your change in git. Here, we create a branch called mychange (use whatever name you want here), with origin/main as the upstream branch.

git checkout -b mychange -t origin/main

Write and test your change.

  • Conform to the style guide.
  • Include tests.
  • Patches should be a reasonable size to review. Review time often increases exponentially with patch size.

Commit your change locally in git:

git commit -a

If you are not familiar with git, GitHub's resources to learn git is useful for the basics. However, keep in mind that the Chromium workflow is not the same as the GitHub pull request workflow.

Uploading a change for review

Note: go through the commit checklist for Chromium before uploading a change for review.

Chromium uses a Gerrit instance hosted at https://chromium-review.googlesource.com for code reviews. In order to upload your local change to Gerrit, use git-cl from depot_tools to create a new Gerrit change, based on the diff between the current branch and its upstream branch:

git cl upload

This will open a text editor to create a description for the new change. This description will be used as the commit message when the change is landed in the Chromium tree. Descriptions should be formatted as follows:

Summary of change (one line)

Longer description of change addressing as appropriate: why the change
is made, context if it is part of many changes, description of previous
behavior and newly introduced differences, etc.

Long lines should be wrapped to 72 columns for easier log message
viewing in terminals.

Bug: 123456

A short subject and a blank line after the subject are crucial: git uses this as a heuristic for tools like git log --oneline. Use the bug number from the issue tracker (see more on CL footer syntax). Also see How to Write a Git Commit Message, which has more in-depth tips for writing a good commit description.

Chromium-specific description tips

  • Links to previous CLs should be formatted as https://crrev.com/c/NUMBER, which is slightly shorter than https://chromium-review.googlesource.com.

  • If there are instructions for testers to verify the change is correct, include them with the Test: tag:

    Test: Load example.com/page.html and click the foo-button; see
crbug.com/123456 for more details.

After saving the change description, git-cl runs some presubmit scripts to check for common errors. If everything passes, git-cl will print something like this:

remote: SUCCESS
remote:
remote: New Changes:
remote:   https://chromium-review.googlesource.com/c/chromium/src/+/1485699 Use base::TimeDelta::FromTimeSpec helper in more places. [WIP]

Additional flags can be used to specify reviewers, bugs fixed by the change, et cetera:

git cl upload -r [email protected],[email protected] -b 123456

See git cl help upload for a full list of flags.

Code review

Code reviews are covered in more detail on the code review policies page.

Finding a reviewer

Ideally, the reviewer is someone who is familiar with the area of code in question. If you're not sure who that should be, check with anyone in the nearest ancestor OWNERS file.

  • Anybody can review code, but there must be at least one owner for each affected directory.
  • If there are multiple reviewers, make it clear what each reviewer is expected to review.
  • git cl owners automatically suggests reviewers based on the OWNERS files.

Note: By default, please only select one reviewer for each file (that is, a single reviewer may review multiple files, but typically each file only needs to be reviewed by one person). It can be tempting to add multiple reviewers so that "whoever gets to it first" can review, but this has two common failure modes:

  • Reviewer Alpha and Beta both review the CL, resulting in duplicate effort.
  • Out of fear of the above failure case, neither reviewer Alpha nor Beta review the CL.

There are times when requesting multiple reviewers for the same file may be desirable - such as when the code is particularly complicated, or when the file uses multiple systems and a perspective from each is valuable. In this case, please make it explicit that you would like both reviewers to review.

Requesting review

Open the change on the web. If you can't find the link, running git cl issue will display the review URL for the current branch. Alternatively, visit https://chromium-review.googlesource.com and look in the "Outgoing Reviews" section.

Reviewers expect to review code that compiles and passes tests. If you have access, now is a good time to run your change through the automated tests.

Click Add Reviewers in the left column (if you don't see this link, make sure you are logged in). In the Reviewers field, enter a comma-separated list of the reviewers you picked.

In the same dialog, you can include an optional message to your reviewers. This space can be used for specific questions or instructions. Once you're done, make sure to click Start Review, which notifies the requested reviewers that they should review your change.

IMPORTANT: UNTIL YOU SEND THE REVIEW REQUEST, NO ONE WILL LOOK AT THE REVIEW

Review process

All changes must be reviewed (see code review policies).

You should get a response within one business day; re-ping your reviewers if you do not.

To upload new patch sets that address comments from the reviewers, simply commit more changes to your local branch and run git cl upload again.

Approval

When the reviewer is happy with the change, they will set the "Code-Review +1" label. Owners of all affected files must approve before a change can be committed. See: code review policies: owners.

Running automated tests

Before being submitted, a change must pass the commit queue (CQ). The commit queue is an automated system which sends a patch to multiple try bots running different platforms: each try bot compiles Chromium with the patch and ensures the tests still pass on that platform.

To trigger this process, click CQ Dry Run in the upper right corner of the code review tool. Note that this is equivalent to setting the "Commit-Queue +1" label. Anyone can set this label; however, the CQ will not process the patch unless the person setting the label has try job access.

If you don't have try job access and:

  • you have an @chromium.org email address, request access for yourself.
  • you have contributed a few patches, ask a reviewer to nominate you for access.
  • neither of the above is true, request that a reviewer run try jobs for you in the code review request message.

The status of the latest try job for a given patchset is visible just below the list of changed files. Each bot has its own bubble, using one of the following colors to indicate its status:

  • Gray: the bot has not started processing the patch yet.
  • Yellow: the run is in progress. Check back later!
  • Purple: the trybot encountered an exception while processing the patch. Usually, this is not the fault of the patch. Try clicking CQ Dry Run again.
  • Red: tests failed. Click on the failed bot to see what tests failed and why.
  • Green: the run passed!

Committing

Changes should generally be committed via the commit queue. This is done by clicking Submit to CQ in the upper right corner, or setting the "Commit-Queue +2" label on the change. The commit queue will then send the patch to the try bots. If all try bots return green, the change will automatically be committed. Yay!

Sometimes a test might be flaky. If you have an isolated failure that appears unrelated to your change, try sending the change to the commit queue again.

Alternatively, a developer with commit access can directly commit a change, bypassing the commit queue. This should only be used in emergencies because it will bypass all the safety nets.

Relanding a change

Occasionally changes that pass the commit queue and get submitted into Chromium will later be reverted. If this happens to your change, don't be discouraged! This can be a common part of the Chromium development cycle and happens for a variety of reasons, including a conflict with an unanticipated change or tests not covered on the commit queue.

If this happens to your change, you're encouraged to pursue a reland. When doing so, following these basic steps can streamline the re-review process:

  • Create the reland: Click the CREATE RELAND button on the original change in Gerrit. This will create a new change whose diff is identical to the original, but has a small paper-trail in the commit message that leads back to the original. This can be useful for sheriffs when debugging regressions.
  • Append the fix: If the reland requires file modifications not present in the original change, simply upload these fixes in a subsequent patchset to the reland change. By comparing the first patchset with the latest, this gives reviewers the ability to see the diff of just the reland fix.
  • Describe the fix: In the commit message of the reland change, briefly summarize what's changed that makes relanding again safe. Explanations can include: "included needed fix", "disabled failing tests", "crash was fixed elsewhere". Specifically for that last case: if the reland change is identical to the original and the reland fix was handled separately in a preceding change, make sure to link to that change in the commit message of the reland.

Code guidelines

In addition to the adhering to the styleguide, the following general rules of thumb can be helpful in navigating how to structure changes:

  • Code in the Chromium project should be in service of other code in the Chromium project. This is important so developers can understand the constraints informing a design decision. Those constraints should be apparent from the scope of code within the boundary of the project and its various repositories. In general, for each line of code, you should be able to find a product in the Chromium repositories that depends on that line of code or else the line of code should be removed.

    When you are adding support for a new OS, a new architecture, a new port or a new top-level directory, please send an email to [email protected] and get approval. For long-term maintenance reasons, we will accept only things that are used by the Chromium project (including Chromium-supported projects like V8 and Skia) and things whose benefit to Chromium outweighs any cost increase in maintaining Chromium's supported architectures / platforms (e.g. adding one ifdef branch for an unsupported architecture / platform has negligible cost and is likely fine, but introducing new abstractions or changes to higher level directories has a high cost and would need to provide Chromium with corresponding benefit). Note that an unsupported architecture / platform will not have bots on Google-managed waterfalls (even FYI bots) or maintained by Chromium developers. Please use existing ifdef branches as much as possible.

  • Code should only be moved to a central location (e.g., //base) when multiple consumers would benefit. We should resist the temptation to build overly generic common libraries as that can lead to code bloat and unnecessary complexity in common code.

  • The code likely wasn't designed for everything we are trying to do with it now. Take time to refactor existing code to make sure the new feature or subcomponent you are developing fits properly within the system. Technical debt is easy to accumulate and is everyone's responsibility to avoid.

  • Common code is everyone's responsibility. Large files that are at the cross-roads of many subsystems, where integration happens, can be some of the most fragile in the system. As a companion to the previous point, be cognizant of how you may be adding more complexity to the commons as you venture to complete your task.

  • Changes should include corresponding tests. Automated testing is at the heart of how we move forward as a project. All changes should include corresponding tests so we can ensure that there is good coverage for code and that future changes will be less likely to regress functionality. Protect your code with tests!

  • Stick to the current set of supported languages as described in the styleguide. While there is likely always a slightly better tool for any particular job, maintainability of the codebase is paramount. Reducing the number of languages eases toolchain and infrastructure requirements, and minimizes the learning hurdles for developers to be successful contributing across the codebase. Additions of new languages must be approved by //ATL_OWNERS.

  • When your team is making API changes or migrating between services, the team mandating the change needs to do at least 80% of the work. The rationale is to reduce externalities by having the team that requires a change spend the vast majority of the time required to make it happen. This naturally encourages designing to minimize the cost of change, be it through automation, tooling, or pooled centralized expertise. You can find more detailed rationale in this doc (Google internal). If you need an exception or help, please contact [email protected].

Tips

Review etiquette

During the lifetime of a review, you may want to rebase your change onto a newer source revision to minimize merge conflicts. The reviewer-friendly way to do this is to first address any unresolved comments and upload those changes as a patchset. Then, rebase to the newer revision and upload that as its own patchset (with no other changes). This makes it easy for reviewers to see the changes made in response to their comments, and then quickly verify the diffs from the rebase.

Code authors and reviewers should keep in mind that Chromium is a global project: contributors and reviewers are often in time zones far apart. Please read these guidelines on minimizing review lag and take them in consideration both when writing reviews and responding to review feedback.

Watchlists

If you would like to be notified about changes to a set of files covering a topic or an area of Chromium, you may use the watchlists feature in order to receive email notifications.

Appendix: CL footer reference {#cl-footer-reference}

Chromium stores a lot of information in footers at the bottom of commit messages. With the exception of R=, these footers are only valid in the last paragraph of a commit message; any footers separated from the last line of the message by whitespace or non-footer lines will be ignored. This includes everything from the unique Change-Id which identifies a Gerrit change, to more useful metadata like bugs the change helps fix, trybots which should be run to test the change, and more. This section includes a listing of well-known footers, their meanings, and their formats.

  • Bug:
    • A comma-separated list of bug references.
    • A bug reference
      • can be a bare number, e.g. Bug: 123456, or
      • can specify a project and a number, e.g. Bug: skia:1234.
    • On chromium-review, the default project is assumed to be chromium, so all bugs in non-chromium projects on bugs.chromium.org should be qualified by their project name.
    • The Google-internal issue tracker is accessible by using the b: project prefix.
  • Fixed: The same as Bug:, but will automatically close the bug(s) as fixed when the CL lands.
  • R=
  • Cq-Include-Trybots:
    • A comma-separated list of trybots which should be triggered and checked by the CQ in addition to the normal set.
    • Trybots are indicated in bucket:builder format (e.g. luci.chromium.try:android-asan).
    • The "Choose Tryjobs" UI in the "Checks" tab in Gerrit shows (and has a button to copy) the Cq-Include-Trybots syntax for the currently selected tryjobs.
  • No-Presubmit:
    • If present, the value should always be the string true.
    • Indicates to the CQ that it should not run presubmit checks on the CL.
    • Used primarily on automated reverts.
  • No-Try:
    • If present, the value should always be the string true.
    • Indicates to the CQ that it should not start or check the results of any tryjobs.
    • Used primarily on automated reverts.
  • No-Tree-Checks:
    • If present, the value should always be the string true.
    • Indicates to the CQ that it should ignore the tree status and submit the change even to a closed tree.
    • Used primarily on automated reverts.
  • Test:
    • A freeform description of manual testing performed on the change.
    • Not necessary if all testing is covered by trybots.
  • Reviewed-by:
    • Automatically added by Gerrit when a change is submitted.
    • Lists the names and email addresses of the people who approved (set the Code-Review label on) the change prior to submission.
  • Reviewed-on:
    • Automatically added by Gerrit when a change is submitted.
    • Links back to the code review page for easy access to comment and patch set history.
  • Change-Id:
    • Automatically added by git cl upload.
    • A unique ID that helps Gerrit keep track of commits that are part of the same code review.
  • Cr-Commit-Position:
    • Automatically added by the git-numberer Gerrit plugin when a change is submitted.
    • This is of the format fully/qualified/ref@{#123456} and gives both the branch name and "sequence number" along that branch.
    • This approximates an SVN-style monotonically increasing revision number.
  • Cr-Branched-From:
    • Automatically added by the git-numberer Gerrit plugin on changes which are submitted to non-main branches.
    • Aids those reading a non-main branch history in finding when a given commit diverged from main.