docs/adding_to_third_party.md

# Adding third_party Libraries

[TOC]

Using third party code can save time and is consistent with our values - no need
to reinvent the wheel! We put all code that isn't written by Chromium developers
into `//third_party` (even if you end up modifying just a few functions). We do
this to make it easy to track license compliance, security patches, and supply
the right credit and attributions. It also makes it a lot easier for other
projects that embed our code to track what is Chromium licensed and what is
covered by other licenses.

## Put the code in //third_party

By default, all code should be checked into [//third_party](../third_party/),
for the reasons given above. Other locations are only appropriate in a few
situations and need explicit approval; don't assume that because there's some
other directory with third_party in the name it's okay to put new things
there.

## Before you start

To make sure the inclusion of a new third_party project makes sense for the
Chromium project, you should first obtain
[Chrome ATL](../ATL_OWNERS) approval. Please include the following information in an
email to chrome-atls-discuss@google.com:
* Motivation of your project
* Design docs
* Additional checkout size
   * If the increase is significant (e.g., 20+ MB), can we consider limiting the
   files to be checked in?
* Build time increase
   * If the increase is significant (e.g., 30+ seconds), can we consider making
   this an optional build target?
* Binary size increase on Android ([official](https://www.chromium.org/developers/gn-build-configuration) builds)
   * Any 16 KB increase on Android is flagged on the build bots and
   justification is needed.
* Binary size increase on Windows
* Is this library maintained on all platforms that we will use it on?
   * If not, will the Chrome org be expected to maintain this for some or all
   platforms?
* Does it have any performance / memory implications (esp. on Android)? Was the
library designed with intended use on Android?
* Do we really need the library? Is there any alternative such as an existing
library already in Chromium? If introducing a library with similar functionality
as existing, will it be easy for another developer to understand which should be
used where? Will you commit to consolidating uses in Chromium and remove the
alternative libraries?
* For desktop (Win/Mac/Linux/ChromeOS), does the dependency introduce closed
source components (e.g., binaries, WASM binaries, obfuscated code)? If yes,
please reach out to Chrome ATLs.


Googlers can access [go/chrome-atls](https://goto.google.com/chrome-atls) and review
existing topics in g/chrome-atls, and can also come to office hours to ask
questions.

### Rust

Rust is allowed for third-party libraries as long as there is a business need,
which includes the following:

* The Rust implementation is the best (e.g., speed, memory, lack of bugs) or
only existing implementation available for the third-party library.
* The Rust implementation allows the operation to move to a higher privileged
process, and this benefits the product by improving on guardrail metrics (e.g.
through avoiding process startup, IPC overheads, or C++ memory-unsafety
mitigations).
* The Rust implementation can meaningfully reduce our expected risk of
(memory/crashes/undefined behavior) bugs, when compared to the existing
third-party library and related C++ code required to use the library. We realize
assessing risk is quite complex and very nuanced. If this is the criteria by
which the third-party library is being added, chrome-atls-discuss@google.com and
chrome-rust@google.com may ask for more data.

Support for third-party libraries written in Rust is in active development. If
the library you wish to add is in Rust, reach out to chrome-rust@google.com
first.

### A note on size constraints

The size of Chromium derived executables can impact overall performance of those binaries as they
need to run on a wide range of devices including those with extremely limited RAM. Additionally, we
have experience from Windows of the binary size impacting successful patch rate of updates as well
as constraints from the Android Ecosystem where APKs included in the system image have hard
limits on their size due to allocation size of the system partition. For more details and
guidelines on size increases see
[//docs/speed/binary_size/binary_size_explainer.md](speed/binary_size/binary_size_explainer.md) and Googlers can
additionally check [go/chrome-binary-size](https://goto.google.com/chrome-binary-size)

## Get the code

There are two common ways to depend on third-party code: you can reference a
Git repo directly (via entries in the DEPS file) or you can check in a
snapshot. The former is preferable in most cases:

1. If you are actively developing in the upstream repo, then having the DEPS
   file include the upstream (that's been mirrored to GoB, see below) can be a
   way to include those changes into Chromium at a particular revision. The
   DEPS file will be updated to a new revision when you are ready to "roll" the
   new version into Chromium. This also avoids duplicate copies of the code
   showing up in multiple repos leading to contributor confusion.
1. This interacts favorably with our upstream tracking automation. We
   automatically consume the upstream Git hashes and match them against a
   database of known upstreams to tracking drift between Chromium and upstream
   sources.
1. This makes adding deps that don't need local changes easier. E.g. some of
   our automation automatically converts non-GN build rules into GN build rules
   without any additional CLs.

Checking in a snapshot is useful if this is effectively taking on maintenance
of an unmaintained project (e.g. an ancient library that we're going to GN-ify
that hasn't been updated in years). And, of course, if the code you need isn't
in a Git repo, then you have to snapshot.

### Node packages

To include a Node package, add the dependency to the
[Node package.json](../third_party/node/package.json). Make sure to update
the corresponding [`npm_exclude.txt`](../third_party/node/npm_exclude.txt)
and [`npm_include.txt`](../third_party/node/npm_include.txt) to make the code
available during checkout.

### Pulling the code via DEPS

If the code is in a Git repo that you want to mirror, please file an [infra git
ticket](https://bugs.chromium.org/p/chromium/issues/entry?template=Infra-Git)
to get the repo mirrored onto chromium.googlesource.com; we don't allow direct
dependencies on non-Google-hosted repositories, so that we can still build
if an external repository goes down.

Once the mirror is set up, add an entry to [//DEPS](../DEPS) so that gclient
will pull it in. If the code is only needed on some platforms, add a condition
to the deps entry so that developers on other platforms don't pull in things
they don't need.

As for specifying the path where the library is fetched, a path like
`//third_party/<project_name>/src` is highly recommended so that you can put
the file like OWNERS or README.chromium at `//third_party/<project_name>`. If
you have a wrong path in DEPS and want to change the path of the existing
library in DEPS, please ask the infrastructure team before committing the
change.

Lastly, add the new directory to Chromium's `//third_party/.gitignore`, so that
it won't show up as untracked files when you run `git status` on the main
repository.

### Checking in the code directly

If you are checking in a snapshot, please describe the source in the
README.chromium file, described below.  For security reasons, please retrieve
the code as securely as you can, using HTTPS and GPG signatures if available.
If retrieving a tarball, please do not check the tarball itself into the tree,
but do list the source and the SHA-512 hash (for verification) in the
README.chromium and Change List. The SHA-512 hash can be computed via
`sha512sum` or `openssl dgst -sha512`.  If retrieving from a git
repository, please list the revision that the code was pulled from.

If you are checking the files in directly, you do not need an entry in DEPS
and do not need to modify `//third_party/.gitignore`.

### Checking in large files

_Accessible to Googlers only. Non-Googlers can email one of the people in
third_party/OWNERS for help.

See [Moving large files to Google Storage](https://goto.google.com/checking-in-large-files)

## Document the code's context

### Add OWNERS

Your OWNERS file must either list two Chromium developer accounts as the first
two lines or include a `file:` directive to an OWNERS file within the
`third_party` directory that itself conforms to this criterion. This will ensure
accountability for maintenance of the code over time. While there isn't always
an ideal or obvious set of people that should go in OWNERS, this is critical for
first-line triage of any issues that crop up in the code.

As an OWNER, you're expected to:

* Remove the dependency when/if it is no longer needed
* Update the dependency when a security or stability bug is fixed upstream
* Help ensure the Chrome feature that uses the dependency continues to use the
  dependency in the best way, as the feature and the dependency change over
  time.

### Add a README.chromium

You need a README.chromium file with information about the project from which
you're re-using code. See
[//third_party/README.chromium.template](../third_party/README.chromium.template)
for a list of fields to include. A presubmit check will check this has the right
format.

README.chromium files contain a field indicating whether the package is
security-critical or not. A package is security-critical if it is compiled
into the product and does any of the following:

* Accepts untrustworthy inputs from the internet
* Parses or interprets complex input formats
* Sends data to internet servers
* Collects new data
* Influences or sets security-related policy (including the user experience)

One of the fields is CPEPrefix. This is used by Chromium and Google systems to
spot known upstream security vulnerabilities, and ensure we merge the fixes
into our third-party copy. These systems are not foolproof, so as the OWNER,
it's up to you to keep an eye out rather than solely relying on these
automated systems. But, adding CPEs decreases the chances of us missing
vulnerabilities, so they should always be added if possible.

The CPE is a common format shared across the industry; you can look up the CPE
for your package [here](https://nvd.nist.gov/products/cpe/search). Please use
CPE format 2.2. When searching for a CPE, you may find that there is not yet
a CPE for the specific upstream version you're using. This is normal, as CPEs
are typically allocated only when a vulnerability is found. You should follow
the version number convention such that, when that does occur in future, we'll
be notified. If no CPE is available, please specify "unknown".

If you're using a patched or modified version which is halfway between two
public versions, please "round downwards" to the lower of the public versions
(it's better for us to be notified of false-positive vulnerabilities than
false-negatives).

### Add a LICENSE file and run related checks

You need a LICENSE file. Example:
[//third_party/libjpeg/LICENSE](../third_party/libjpeg/LICENSE).

Run `//tools/licenses/licenses.py scan`; this will complain about incomplete or missing
data for third_party checkins. We use `licenses.py credits` to generate the
about:credits page in Google Chrome builds.

If the library will never be shipped as a part of Chrome (e.g. build-time tools,
testing tools), make sure to set "License File" as "NOT_SHIPPED" so that the
license is not included in about:credits page ([more on this below](#credits)).

## Get a review

All third party additions and substantive changes like re-licensing need the
following sign-offs. Some of these are accessible to Googlers only.
Non-Googlers can email one of the people in
[//third_party/OWNERS](../third_party/OWNERS) for help.

* Make sure you have the approval from Chrome ATLs as mentioned
  [above](#before-you-start).
* Get security@chromium.org (or chrome-security@google.com, Google-only)
  approval. Email the list with relevant details and a link to the CL.
  Third party code is a hot spot for security vulnerabilities.
  When adding a new package that could potentially carry security risk, make
  sure to highlight risk to security@chromium.org. You may be asked to add
  a README.security or, in dangerous cases, README.SECURITY.URGENTLY file.
* Add chromium-third-party@google.com as a reviewer on your change. This
  will trigger an automatic round-robin assignment to a reviewer who will check
  licensing matters. These reviewers may not be able to +1 a change so look for
  verbal approval in the comments. (This list does not receive or deliver
  email, so only use it as a reviewer, not for other communication. Internally,
  see [cl/221704656](http://cl/221704656) for details about how
  this is configured.). If you have questions about the third-party process,
  ask one of the [//third_party/OWNERS](../third_party/OWNERS) instead.
* Lastly, if all other steps are complete, get a positive code review from a
  member of [//third_party/OWNERS](../third_party/OWNERS) to land the change.

Please send separate emails to the ATLs and security@chromium.org.
You can skip the ATL review and security@chromium.org when you are only moving
existing directories in Chromium to //third_party/.

Subsequent changes don't normally require third-party-owners or security
approval; you can modify the code as much as you want. When you update code, be
mindful of security-related mailing lists for the project and relevant CVE to
update your package.

## How we ensure that the right credits are displayed {#credits}

As we said at the beginning, it is important that Chrome displays the
right credit and attributions for all of the third_party code we use.

To view this in chrome, you can open chrome://credits.

That page displays a resource embedded in the browser as part of the
[//components/resources/components_resources.grd](../components/resources/components_resource.grd)
GRIT file; the actual HTML text is generated in the
[//components/resources:about_credits](../components/resources/BUILD.gn)
build target using a template from the output of the
[//tools/licenses/licenses.py](../tools/licenses/licenses.py) script. Assuming you've followed
the rules above to ensure that you have the proper LICENSE file and it passes
the checks, it'll be included automatically.