path: root/chromium/docs/contributing.md

# Contributing to Chromium

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

[TOC]

## Related resources

- [Life of a Chromium Developer][life-of-a-chromium-developer], which is mostly
  up-to-date.
- [Tutorial][noms-tutorial] by committer emeritus noms@chromium.org.
- [Commit Checklist][commit-checklist], a useful checklist to go through before
  submitting each CL on Gerrit.

## 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][discussion-groups]. If it's in the existing code base, talk
to some of the folks in the "OWNERS" file (see [code review
policies][code-reviews] for more) for the code being changed.

- If a change needs further context outside the CL, it should be tracked in the
  [bug system][crbug]. 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][crbug-new].
- 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][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
[chromium-design-docs@chromium.org][chromium-design-docs]. Google internal Chrome
design docs should follow the process at
[go/chrome-dd-review-process][chrome-dd-review-process].

## Legal stuff

All contributors must complete the contributor license agreement. For
individual contributors, please complete the [Individual Contributor License
Agreement][individual-cla] online. Corporate contributors must fill out the
[Corporate Contributor License Agreement][corporate-cla] 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][cr-authors] or [Chromium OS][cros-authors]. 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][cr-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.
  - 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 "myemail@chromium.org"
   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/master` as
the upstream branch.

```
git checkout -b mychange -t origin/master
```

Write and test your change.

- Conform to the [style guide][cr-styleguide].
- 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][github-tutorial] 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][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][depot-tools-setup] 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][crbug] (see more on [CL footer syntax][cl-footer-syntax]). Also
see [How to Write a Git Commit Message][good-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 forwards to [Gitiles][cr-gitiles], rather than linking to the review at
  <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 foo@example.com,bar@example.com -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][code-reviews] 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. Otherwise, people might assume their input is not required or
  waste time with redundant reviews.
- `git cl owners` automatically suggests reviewers based on the OWNERS files.

### Requesting review

Open the change on [the web][crrev]. 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](#running-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][code-reviews]).

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][code-reviews-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][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][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][direct-commit] a change, bypassing the commit queue. This should only
be used in emergencies because it will bypass all the safety nets.

## Code guidelines

In addition to the adhering to the [styleguide][cr-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 other words, 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.

- **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!

## 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][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][watchlist-doc]
feature in order to receive email notifications.


[//]: # (the reference link section should be alphabetically sorted)
[checkout-and-build]: https://chromium.googlesource.com/chromium/src/+/master/docs/#checking-out-and-building
[chrome-dd-review-process]: http://go/chrome-dd-review-process
[chromium-design-docs]: https://groups.google.com/a/chromium.org/forum/#!forum/chromium-design-docs
[cl-footer-syntax]: https://dev.chromium.org/developers/contributing-code/-bug-syntax
[code-reviews-owners]: code_reviews.md#OWNERS-files
[code-reviews]: code_reviews.md
[commit-checklist]: commit_checklist.md
[commit-queue]: infra/cq.md
[core-principles]: https://www.chromium.org/developers/core-principles
[corporate-cla]: https://cla.developers.google.com/about/google-corporate?csw=1
[cr-authors]: https://chromium.googlesource.com/chromium/src/+/HEAD/AUTHORS
[cr-gitiles]: https://chromium.googlesource.com/chromium/src/+/master/
[cr-styleguide]: https://chromium.googlesource.com/chromium/src/+/master/styleguide/styleguide.md
[crbug-new]: https://bugs.chromium.org/p/chromium/issues/entry
[crbug]: https://bugs.chromium.org/p/chromium/issues/list
[cros-authors]: https://chromium.googlesource.com/chromium/src/+/master/AUTHORS
[cros-dev-guide]: https://chromium.googlesource.com/chromiumos/docs/+/master/developer_guide.md
[crrev]: https://chromium-review.googlesource.com
[depot-tools-setup]: https://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html#_setting_up
[design-doc-template]: https://docs.google.com/document/d/14YBYKgk-uSfjfwpKFlp_omgUq5hwMVazy_M965s_1KA
[direct-commit]: https://dev.chromium.org/developers/contributing-code/direct-commit
[discussion-groups]: https://www.chromium.org/developers/discussion-groups
[github-tutorial]: https://try.github.io
[good-git-commit-message]: https://chris.beams.io/posts/git-commit/
[individual-cla]: https://cla.developers.google.com/about/google-individual?csw=1
[life-of-a-chromium-developer]: https://docs.google.com/a/google.com/present/view?id=0AetfwCoL2lQAZGQ5bXJ0NDVfMGRtdGQ0OWM2
[noms-tutorial]: https://meowni.ca/posts/chromium-101
[review-lag]: https://dev.chromium.org/developers/contributing-code/minimizing-review-lag-across-time-zones
[skia-dev-guide]: https://skia.org/dev/contrib
[try-job-access]: https://www.chromium.org/getting-involved/become-a-committer#TOC-Try-job-access
[v8-dev-guide]: https://v8.dev/docs
[watchlist-doc]: infra/watchlists.md