# Code Reviews

Code reviews are a central part of developing high-quality code for Chromium.
All changes must be reviewed.

The bigger patch-upload-and-land process is covered in more detail the
[contributing code](https://www.chromium.org/developers/contributing-code)
page.

# 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 (see below).

To indicate a positive review, the reviewer chooses "+1" in Code-Review field
on Gerrit, or types "LGTM" (case insensitive) into a comment on Rietveld. This
stands for "Looks Good To Me." "-1" in Code-Review field on Gerrit or the text
"not LGTM" on Rietveld will cancel out a previous positive review.

If you have multiple reviewers, make it clear in the message you send
requesting review what you expect from each reviewer. Otherwise people might
assume their input is not required or waste time with redundant reviews.

#### 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 have
    done a complete review, but you should be able to give some initial
    feedback, request more time, or suggest another reviewer.

  * It can be nice to indicate if you're away in your name in the code review
    tool. If you do this, indicate when you'll be back.

  * Don't generally discourage people from sending you code reviews. This
    includes writing a blanket ("slow") after your name in the review tool.

## 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 in the "More information"
section below.

*Tip:* The `git cl owners` command can help find owners.

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 must:

  * 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 6
    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 90 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.

Seldom-updated directories may have exceptions. Directories in `third_party`
should list those most familiar with the library.

## TBR ("To Be Reviewed")

"TBR" is our mechanism for post-commit review. It should be used rarely and
only in cases where a review is unnecessary or as described below. The most
common use of TBR is to revert patches that broke the build.

TBR does not mean "no review." A reviewer TBR-ed on a change should still
review the change. If there are comments after landing, the author is obligated
to address them in a followup patch.

Do not use TBR just because a change is urgent or the reviewer is being slow.
Contact the reviewer directly or find somebody.

To send a change TBR, annotate the description and send email like normal.
Otherwise the reviewer won't know to review the patch.

  * Add the reviewer's email address in the code review tool's reviewer field
    like normal.

  * Add a line "TBR=<reviewer's email>" to the bottom of the change list
    description. e.g. `TBR=reviewer1@chromium.org,reviewer2@chromium.org`

  * Type a message so that the owners in the TBR list can understand who is
    responsible for reviewing what, as part of their post-commit review
    responsibility. e.g.
    ```
    TBRing reviewers:
    reviewer1: Please review changes to foo/
    reviewer2: Please review changes to bar/
    ```

  * Push the "send mail" button.

### TBR-ing certain types of mechanical changes

Sometimes you might do something that affects many callers in different
directories. For example, adding a parameter to a common function in
`//base`, with callers in `//chrome/browser/foo`, `//net/bar`, and many other
directories. If the updates to the callers is mechanical, you can:

  * Get a normal owner of the lower-level code you're changing (in this
    example, the function in `//base`) to do a proper review of those changes.

  * Get _somebody_ to review the downstream changes made to the callers as a
    result of the `//base` change. This is often the same person from the
    previous step but could be somebody else.

  * Add the owners of the affected downstream directories as TBR. (In this
    example, reviewers from `//chrome/browser/foo/OWNERS`, `//net/bar/OWNERS`,
    etc.)

This process ensures that all code is reviewed prior to checkin and that the
concept of the change is reviewed by a qualified person, but you don't have to
wait for many individual owners to review trivial changes to their directories.

### TBR-ing documentation updates

You can TBR documentation updates. Documentation means markdown files, text
documents, and high-level comments in code. At finer levels of detail, comments
in source files become more like code and should be reviewed normally (not
using TBR). Non-TBR-able stuff includes things like function contracts and most
comments inside functions.

  * Use good judgement. If you're changing something very important, tricky,
    or something you may not be very familiar with, ask for the code review
    up-front.

  * Don't TBR changes to policy documents like the style guide or this document.

  * Don't mix unrelated documentation updates with code changes.

  * Be sure to actually send out the email for the code review. If you get one,
    please actually read the changes.

## More information

### OWNERS file details

Refer to the [source code](https://chromium.googlesource.com/chromium/tools/depot_tools/+/master/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 is used for specialized code. In this example, only the two listed people
are owners:
```
set noparent
a@chromium.org
b@chromium.org
```

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=*
```

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
```
