Code reviews are a central part of developing high-quality code for Chromium. All change lists (CLs) must be reviewed.
The general patch, upload, and land process is covered in more detail in the contributing code page. To learn about the code review changes and OWNERS policy changes launched on March 24, 2021, see Mandatory Code Review and Native OWNERS.
Ideally the reviewer is someone who is familiar with the area of code you are
touching. Any committer can review code, but an owner must provide a review
for each directory you are touching. If you have doubts, look at the git blame
for the file and the OWNERS files (more info).
To indicate a positive review, the reviewer provides a Code-Review +1 in
Gerrit, also known as an LGTM ("Looks Good To Me"). A score of "-1" indicates
the change should not be submitted as-is.
If you have multiple reviewers, provide a message indicating what you expect from each reviewer. Otherwise people might assume their input is not required or waste time with redundant reviews.
Please also read Respectful Changes and Respectful Code Reviews.
-
Aim to provide some kind of actionable response within 24 hours of receipt (not counting weekends and holidays). This doesn't mean you have to do a complete review, but you should be able to give some initial feedback, request more time, or suggest another reviewer.
-
Use the status field in Gerrit settings to indicate if you're away and when you'll be back.
-
Don't generally discourage people from sending you code reviews. This includes using a blanket "slow" in your status field.
In various directories there are files named OWNERS that list the email
addresses of people qualified to review changes in that directory. You must
get a positive review from an owner of each directory your change touches.
Owners files are recursive, so each file also applies to its subdirectories.
It's generally best to pick more specific owners. People listed in higher-level
directories may have less experience with the code in question. For example,
the reviewers in the //chrome/browser/component_name/OWNERS file will likely
be more familiar with code in //chrome/browser/component_name/sub_component
than reviewers in the higher-level //chrome/OWNERS file.
More detail on the owners file format is provided here.
Tip: The git cl owners command can help find owners. Gerrit also provides
this functionality in the Reviewers field of CLs.
While owners must approve all patches, any committer can contribute to the review. In some directories the owners can be overloaded or there might be people not listed as owners who are more familiar with the low-level code in question. In these cases it's common to request a low-level review from an appropriate person, and then request a high-level owner review once that's complete. As always, be clear what you expect of each reviewer to avoid duplicated work.
Owners do not have to pick other owners for reviews. Since they should already be familiar with the code in question, a thorough review from any appropriate committer is sufficient.
The existing owners of a directory approve additions to the list. It is preferable to have many directories, each with a smaller number of specific owners rather than large directories with many owners. Owners should:
-
Demonstrate excellent judgment, teamwork and ability to uphold Chrome development principles.
-
Be already acting as an owner, providing high-quality reviews and design feedback.
-
Be a Chromium project member with full commit access of at least three months tenure.
-
Have submitted a substantial number of non-trivial changes to the affected directory.
-
Have committed or reviewed substantial work to the affected directory within the last ninety days.
-
Have the bandwidth to contribute to reviews in a timely manner. If the load is unsustainable, work to expand the number of owners. Don't try to discourage people from sending reviews, including writing "slow" or "emeritus" after your name.
The above are guidelines more than they are hard rules, and exceptions are
okay as long as there is a consensus by the existing owners for them.
For example, seldom-updated directories may have exceptions to the
"substantiality" and "recency" requirements. Directories in third_party
should list those most familiar with the library, regardless of how often
the code is updated.
Refer to the source code for all details on the file format.
This example indicates that two people are owners, in addition to any owners
from the parent directory. git cl owners will list the comment after an
owner address, so this is a good place to include restrictions or special
instructions.
# You can include comments like this.
a@chromium.org
b@chromium.org # Only for the frobinator.
A * indicates that all committers are owners:
*
The text set noparent will stop owner propagation from parent directories.
This should be rarely used. If you want to use set noparent except for IPC
related files, please first reach out to chrome-eng-review@google.com.
You have to use set noparent together with a reference to a file that lists
the owners for the given use case. Approved use cases are listed in
//build/OWNERS.setnoparent. Owners listed in those files are expected to
execute special governance functions such as eng review or ipc security review.
Every set of owners should implement their own means of auditing membership. The
minimum expectation is that membership in those files is reevaluated on
project, or affiliation changes.
In this example, only the eng reviewers are owners:
set noparent
file://ENG_REVIEW_OWNERS
The per-file directive allows owners to be added that apply only to files
matching a pattern. In this example, owners from the parent directory
apply, plus one person for some classes of files, and all committers are
owners for the readme:
per-file foo_bar.cc=a@chromium.org
per-file foo.*=a@chromium.org
per-file readme.txt=*
Note that per-file directives cannot directly specify subdirectories, e.g:
per-file foo/bar.cc=a@chromium.org
is not OK; instead, place a per-file directive in foo/OWNERS.
Other OWNERS files can be included by reference by listing the path to the
file with file://.... This example indicates that only the people listed in
//ipc/SECURITY_OWNERS can review the messages files:
per-file *_messages*.h=set noparent
per-file *_messages*.h=file://ipc/SECURITY_OWNERS
Setting the Owners-Override +1 label will bypass OWNERS enforcement. Active
sheriffs, Release Program Managers,
Large Scale Changes,
Global Approvers reviewers,
Chrome Eng Review members
have this capability. The power to use Owners-Override should be restricted
as follows:
- Active sheriffs can set Owners-Override only on sheriffing CLs (e.g., revert, reland, test fix).
- Release Program Managers can set Owners-Override only on sheriffing CLs and CLs needed for releasing (e.g., cherry-pick).
- Large Scale Change reviewers can set Owners-Override only on sheriffing CLs and CLs about the approved Large Scale Change.
- Global approvers can set Owners-Override only on sheriffing CLs and mechanical CLs associated with their API changes. For example, //base/OWNERS can set Owners-Override on mechanical CLs associated with //base/ API changes.
- Chrome Eng Review members can set Owners-Override on any changes to help with cases that cannot be handled by the above groups and expedite CLs when LSC is too heavyweight.. However, please use one of the above groups before asking Chrome Eng Review members.
When you need Owners-Override on sheriffing CLs, please reach out to the Active Sheriffs and Release Program Managers first. If none of them is available, please send an email to lsc-owners-override@chromium.org for help.
Note that Owners-Override by itself is not enough on your own CLs. Where this matters is when you are sheriffing. For example, if you want to revert or disable a test, your Owners-Override on the CL is not enough. You need another committer to LGTM the CL.
For one-off CLs, API owners of base, build, content, third_party/blink
and url can Owners-Override +1 a change to their APIs to avoid waiting for
rubberstamp +1s from affected directories' owners. This should only be used for
mechanical updates to the affected directories.
If you are making one-off CLs that touch many directories and cannot be handled by the global approvers, you can ask one of Chrome Eng Review members.
You can use the Large Scale Changes process to get approval to bypass OWNERS enforcement for large changes like refactoring, architectural changes, or other repetitive code changes across the whole codebase. This is used for work that span many dozen CLs.
Documentation updates require code review. We may revisit this decision in the future.
For verifiably safe changes like translation files, clean reverts, and clean
cherry-picks, we have automation that will vote +1 on the Bot-Commit label
allowing the CL to be submitted without human code-review. Add Rubber Stamper
(rubber-stamper@appspot.gserviceaccount.com) to your CL as a reviewer to
activate this automation. It will scan the CL after about 1 minute and reply
with its verdict. Bot-Commit votes are not sticky between patchsets and so
only add the bot once the CL is finalized.
When combined with the Owners-Override power, sheriffs can
effectively revert and reland on their own.
Rubber Stamper never provides OWNERS approval, by design. It's intended to be used by those who have owners in the directory modified or who are sheriffs. If it provided both code review and OWNERS approval, that would be an abuse vector: that would allow anyone who can create a revert or cherry-pick to land it without any other person being involved (e.g. the silent revert of security patches).
Changes not supported by Rubber Stamper always need a +1 from another
committer.