docs/code_reviews.md

# Code Reviews

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](contributing.md) page. To learn about the code review changes
and OWNERS policy changes launched on March 24, 2021, see
[Mandatory Code Review and Native OWNERS](code_review_owners.md).

# Code review policies

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](#owners-files)).

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](cl_respect.md) and
[Respectful Code Reviews](cr_respect.md).

#### Expectations for all reviewers

  * 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.

## OWNERS files

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](#owners-file-details).

*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.

#### Expectations of owners

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.

### OWNERS file details

Refer to the [source code](https://chromium.googlesource.com/chromium/tools/depot_tools/+/main/owners.py)
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
```

### Owners-Override

Setting the `Owners-Override +1` label will bypass OWNERS enforcement. Active
[sheriffs](sheriffs.md), Release Program Managers, 
[Large Scale Changes](#large-scale-changes), 
[Global Approvers](#global-approvals) reviewers, 
[Chrome Eng Review members](https://chromium.googlesource.com/chromium/src/+/HEAD/ENG_REVIEW_OWNERS) 
have this capability. The power to use Owners-Override should be restricted 
as follows:

  * Active sheriffs can set Owners-Override only on changes needed for 
    sheriffing (e.g., revert, reland, test fix).
  * Release Program Managers can set Owners-Override only on changes needed 
    for releasing (e.g., revert, reland, test fix, cherry-pick).
  * Large Scale Change reviewers can set Owners-Override only on changes 
    about the approved Large Scale Change.
  * Global approvers can set Owners-Override only on mechanical changes 
    associated with their API changes. For example, //base/OWNERS can set 
    Owners-Override only on mechanical changes 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. However, please 
    use one of the above groups before asking Chrome Eng Review members.
  
Note that Owners-Override by itself is not enough on your own CLs. Where this 
matters is when you're 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.

## Mechanical changes

### Global Approvals
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.

### Large Scale Changes
You can use the [Large Scale Changes](process/lsc/large_scale_changes.md)
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

Documentation updates require code review. We may revisit this decision in the
future.

## Automated code-review

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`](#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.