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Contributing Code

If you want to contribute code to Chromium, there are a few things you can do to make the process flow smoothly.  For Chromium OS, please go here.

Quick start guide: Life of a Chromium Developer

Note: These instructions apply only if your changes are in Chromium code. If your changes are in a directory that we pull from another repository (e.g. Blink, v8, skia, gtest, gmock), you need to use that project's way of getting code submitted. For example, here is a cheatsheet for contributing to Blink. Once your change has landed upstream, you need to update Chromium's DEPS file to pull in the new revision (we update Blink very often, so you don't have to take care of this if you're contributing to Blink).

Contents

  1. 1 Communicate
  2. 2 Get your code ready
  3. 3 Review Tools
  4. 4 Life of a Change list
    1. 4.1 Create a change list (CL)
      1. 4.1.1 GCL
      2. 4.1.2 GIT CL
    2. 4.2 Upload your change to Rietveld
      1. 4.2.1 GCL
      2. 4.2.2 GIT CL
  5. 5 Request review
    1. 5.1 Find a reviewer
    2. 5.2 The review process
      1. 5.2.1 GCL
      2. 5.2.2 GIT CL
    3. 5.3 Approval
    4. 5.4 Committing changes
      1. 5.4.1 Committing a patch for a non-committer
      2. 5.4.2 See also

Communicate

  • Whether you're writing a new feature or fixing an existing bug, it pays to get a second opinion before you get too far. If it's a new feature idea, post to the appropriate discussion group (Chromium | Chromium OS) and propose it.  If it is in the existing code base, it is a good idea to talk to some of the folks in the OWNERS file for the code you want to change.
    Start by looking for an OWNERS file in the directory of the file you wish to change, then keep moving up the tree until you find one.
  • Not all bugs in our bug system (Chromium | Chromium OS) are assigned, but if the one you're interested in fixing is, send a note to the person it's assigned to and ask if they would like a patch.
  • Behavior changes and anything nontrivial (i.e. anything other than simple cleanups and style fixes) should generally be tracked in the bug system. Please file a bug (Chromium | Chromium OS) and describe what you're doing if there isn't one already.

Get your code ready

  1. Code
    1. must conform to the Chromium style guidelines.
    2. must be tested, preferably with unit tests.
    3. should be a reasonable size to review. Giant patches are unlikely to get reviewed quickly.
  2. Run the unit tests and the UI tests to make sure you haven't broken anything. You can ask someone to pass it to the try server.
  3. Legal
    1. You must complete the Individual Contributor License Agreement. You can do this online, and it only takes a minute. If you are contributing on behalf of a corporation, you must fill out the Corporate Contributor License Agreement and send it to us as described on that page.
    2. If you've never submitted code before, you must add your (or your organization's) name and contact info to the AUTHORS file (Chromium | Chromium OS).
    3. NOTE TO REVIEWERS: Follow the External Contributor Checklist.

Review Tools

You either use gcl or git-cl but not both. Both gcl and git-cl comes with depot tools.
  • Inside a subversion checkout, you need to use gcl. This is the case by default for a chromium checkout. See UsingGit if you want to use git.
  • Inside a git checkout, you need to use "git cl". This is the case by default for a chromiumos checkout. For help, run git cl help

Life of a Change list

Create a change list (CL)

GCL

Run gcl help for some doc.

Make a changelist with the gcl change command . The changelist name is only to help you refer to it on your local computer, so call it whatever you want:

C:\code\src\chromium\src> gcl change mychange

This will open your text editor. Write the change description at the top of the file. The description should describe what your patch changes and why. This is important for people who are looking at commit logs in the future to track down an issue: they should be able to see why you changed it without going anywhere else. If there's an associated bug file, you should also add a BUG=bug_number line so they can find the associated bug.   If you have manual test instructions that you want to pass to the test team so that they can validate the fix, add TEST=how_test_test at the end of the description.  If there is no bug or test, leave out the lines.  Multiple bugs can be listed, comma separated. Example:
  
Increase the goat teleporter timeout threshold to 100 because the old
value of 10 caused problems for extremely overweight goats. Tests show
that the largest goat in existence should be teleported in 50ms, so...

R=someone@chromium.org
BUG=31337,2754
TEST=Try loading an overweight goat and confirm the teleporter works.

NOTE: "BUG=xxx" will update the bug in the default issue tracker for the repository you're committing to.  That means if you're checking into the chromium-os repository, the bug is assumed to be a chromium-os bug number, and if the chrome repository, a chrome bug number.  If this isn't right, you can preface each bug number with chromium: or chromium-os: as appropriate to cause the correct bug to be updated.  (Failing to do this will lead to very confused engineers who get the wrong bug mailed to them.)

Broken frame code in Chrome led to black holes spontaneously appearing in ChromeOS devices.

R=someone@chromium.org
BUG=chromium-os:1234
TEST=The world is not converted to a small supermassive point on startup.

 Cut and paste the filenames above or below the divider to add files to or remove files from the changelist. You can use the gcl opened command to see your changed files and changelists:

C:\code\src\chromium\src> gcl opened
M       chrome\browser\browser.vcproj       ← this is a modified file not in any changelist

--- Changelist mychange:
M       chrome\browser\browser.cc

GIT CL

Creating a change in git is really just creating a branch.

git checkout -b mychange origin/master
echo "This describes the goat teleporter." > GOATS
git add GOATS
git commit

Upload your change to Rietveld

Rietveld is the open source code review tool we use. We use an AppEngine instance at http://codereview.chromium.org.

GCL

Use the gcl upload command:

C:\code\src\chromium\src> gcl upload mychange
Issue created. URL: http://codereview.chromium.org/123456
Uploading chrome\browser\browser.cc (1/1)

You may have to enter a Google Account username and password, for example your Gmail credentials but it's not required to be a Gmail account. The URL it gives you is the page where the review can be accessed. If you want to upload a new copy of your patch, optionally add or remove files using gcl change and upload it again using the same gcl upload command.

Once you've uploaded your change and know its patch URL, add that to the bug comments.  That way someone coming along later can make comments or see how the bug was fixed.

GIT CL

Use the git cl upload command:

git cl upload --send-mail

If you specified R=foo@chromium.org in the CL description, they will be emailed automatically so use --send-mail so you don't need to visit the web UI.

Request review

Go to the supplied URL or just go to the respective code review page (Chromium | Chromium OS) and click Issues created by me.  Select the change you want to submit for review and click Edit Issue.  Enter at least one reviewer's email address and click Update Issue.  Now click on Publish+Mail Comments, add any optional notes, and send your change off for review. All Chromium code reviews are automatically cc'd to their respective review groups, so that everyone has an opportunity to see and comment on the changes being made.

Note: If you don't see editing commands on the review page, click Log In in the upper right.
Hint: You can add -r reviewer@example.com --send-mail to send the email directly when uploading a change in both gcl and git-cl.

Find a reviewer

Ideally, the reviewer is someone who is familiar with the area of code you are touching. If you have doubts, look at the svn blame or git blame for the file to see who else has been editing it. Failing that, ask on #chromium or #chromium-os on irc.freenode.net. You can also just request review from the respective lists (Chromium | Chromium OS), but more specific review requests will be attended to sooner.  Your review must include one person from the OWNERS file for every directory you change code in.  

The review process

Chromium and Chromium OS reviewers try to review code that conforms to these guidelines as quickly as possible. You should hear back within 24-48 hours. Remember though, if you submit a giant patch, or do a bunch of work without discussing it with the relevant people, you may have a hard time convincing anyone to review it! Speak with others early and often. Sometimes, the tree will be locked down and certain types of changes won't be allowed. This is true when we're preparing a release. During times like these, reviewers may be focused on fixing the bugs required to get the release out the door.

Remember that code reviews are an important part of the engineering process. The reviewer will almost always have suggestions or style fixes for you, and it's important not to take such suggestions personally or as a commentary on your abilities or ideas. This is a process where we work together to make sure that the highest quality code gets submitted!

You will likely get email back from the reviewer with comments. Fix these and update the patch set in the issue by uploading again.  The upload will explain that it is updating the current CL and ask you for a message explaining the change.
Important: during the lifetime of a review you may want to rebase your change onto a newer source revision to minimize eventual merge pain.  The reviewer-friendly way to do this is to upload the rebase as its own patchset (with no changes other than the rebase), and to only do this when there are no outstanding comments.  In other words, address all the comments from the latest patchset (A), upload a new patchset (B) containing those updates, and only then rebase and upload a patchset (C).  This way your reviewer can view the A/B diff to see all the changes that resulted from the last round of review, and can view the B/C diff to ensure only rebase-related changes were introduced.  Doing things differently (addressing comments and rebasing in a single patchset, or rebasing first and then addressing comments) makes things harder than necessary for your reviewer.

GCL

Add any new files that need to be added to your checkout

gcl change changelist_name

gcl upload changelist_name

GIT CL

If you need to update the code on an already uploaded CL, simply edit the code, commit it again locally, and then run git cl upload again e.g.

echo '' >> GOATS
git add GOATS
git commit -m 'add newline fix to GOATS'
git cl upload

Once you're ready for another review, use Publish+Mail Comments again to send another notification (you can say something like "new snapshot uploaded" or PTAL "Please Take A/Another Look").

Note: As you work through the review process, both you and your reviewers should converse using the code review interface, and send notes using Publish+Mail Comments.  Avoid the temptation to respond directly to review mails (or, for reviewers, to send review comments) directly from your email client; doing that won't add your comments to the online record for the issue under review, and can make it hard for others to understand later what the history of a patch was.

Approval

When the reviewer is happy with your patch, they will say "LGTM" ("Looks Good To Me"). The reviewer types this exact text (case-insensitive, so "lgtm" also works) in their comment, which is detected by Rietveld and marks approval. If this is accidentally typed, writing "not LGTM" withdraws approval.
You need approval from OWNERS of all affected files, either before committing (ex ante) or after committing (ex post).
LGTM gives ex ante approval, which is generally best, and if you have a LGTM from OWNERS of all affected files, you can commit the CL.
Alternatively (instead of getting an LGTM), you can list the OWNERS in the TBR= "To Be Reviewed" field of the CL description, and they will review it after it has been committed. TBR is generally used either for urgent changes (when dcommitting), so the CL is reviewed but not blocked, or for routine changes, where it primarily functions as an FYI.
It is also possible to get an LGTM from some OWNERS and list others in TBR.

Committing changes

Only committers can commit changes. If you are not a committer, you should ask your reviewer to commit it for you, and add "Contributed by me@example.com" in your change description for clarity.

The preferred way of committing changes is via the commit queue. This is done by checking the "commit" check box in Rietveld, or from the command line via git cl set_commit. The commit queue will then send your patch to the try bots, which will eventually appear as bubbles on the Rietveld issue. If all tests pass, your change will be auto committed. If it fails, click on the red (failure) bubbles for a direct link to the failures. Alternatively, search the waterfall for your changelist number and clicking into the links where I see the red failure blocks. If the failure appears unrelated to your change, it is OK to check the commit check box again and retry; sometimes the tests or bots are flaky.

Alternatively, it is possible to directly commit your change, bypassing the commit queue. This is discouraged, due to not running the tests, and requires more supervision on the part of the committer, but is acceptable or necessary in certain circumstances.

Committing a patch for a non-committer

  • If this is the first patch from a contributor, make sure the contributor has signed the CLA or CCLA and the patch adds the right name to the AUTHORS file.
  • Make sure the description includes the committer's name. For example: "Add cool feature x.\n\nPatch from First Last <email@example.com>\nBUG=123"

See also