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 third party code should be checked into
[//third_party](../third_party/), for the reasons given above.

There is one primary exception to this, which is that if a third_party
dependency has its own dependencies *and* it can be built on its own (without
Chromium), you can check its dependencies into its third_party. For example,
Dawn is a project that is developed independently of Chromium, and
it has a dependency on GLFW (which Chromium does not have). Dawn
can check that dependency into its `//third_party/glfw`, and in a Chromium
checkout, that will show up at `//third_party/dawn/third_party/glfw`.
That is okay, but it'd be better if we could add GLFW to a Chromium
checkout (in chromium/src's `third_party/glfw`) and configure Dawn
to use that location when it is being built as part of Chromium.

However, if that dependency is also needed by Chromium or another
of Chromium's dependencies, then it must be checked out into Chromium's
//third_party (i.e., now you have to use `//third_party/glfw`). This
prevents us from possibly needing to use two different versions of a
dependency.

Apart from that, 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
   * This refers to building `chrome` or test targets in the critical
     development path.
   * 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 increase of 16 KB or more 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)

### Binaries, obfuscated or minified code

The addition of third-party dependencies that contain binaries, obfuscated
code, or minified code is strongly discouraged. Code review is an important
part of reducing risk to Chromium and a reviewer asked to approve a change
that contains any of these has no way to determine the legitimacy of what
they are approving. Minification for performance optimization is
[usually not necessary](speed/binary_size/optimization_advice.md), and the
trade-off in terms of understandability and security is rarely worth
it.

Where your dependency will form part of a release binary where size is a concern,
there are existing tools which handle [compression for distribution](speed/binary_size/optimization_advice.md).

You should not check in any pre-built binaries where there is an alternate,
supported solution for getting them. If you need to compile from source,
consider using [CIPD](cipd_and_3pp.md) instead.

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

See [Chrome Code Policy](https://goto.google.com/chrome-code-policy)


## 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

This is 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 the email addresses of two Chromium
committers on 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)

**CPE Prefix**
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).
* Use CPE format 2.3 (preferred) or CPE format 2.2 (supported).
* If the CPE uses the 2.3 URI binding or 2.2 format (i.e. starts with "cpe:/"),
and no version is explicitly specified within the `CPEPrefix`, the `Version`
in the `README.chromium` file will be appended to the `CPEPrefix`, if available.
  * Note: if the `Version` field is set to a git hash value, version matching
  for vulnerabilities will fail.

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).


**Shipped**
Your README.chromium should also specify whether your third party dependency
will be shipped as part of a final binary. The "Shipped" field replaces the now
deprecated special value of "NOT_SHIPPED" which was previously allowed in the
"License File" field. This use is no longer supported and all third party
dependencies must include a valid license regardless of whether it is shipped
or not.


**Multiple packages**
Each package should have its own README.chromium. However, if this is not
possible and the information for multiple packages must be placed in a single
README.chromium, use the below line to separate the data for each package:
```
-------------------- DEPENDENCY DIVIDER --------------------
```


### Add a LICENSE file and run related checks

You need a LICENSE file. Example:
[//third_party/libjpeg/LICENSE](../third_party/libjpeg/LICENSE). Dependencies
should not be added without a license file and license type, even if they are
not shipped in a final product. Existing dependencies without a license file or
license type are currently being cleaned up as part of the metadata uplift
effort. If you are an OWNER of a dependency missing license fields, there will
soon be a bug filed to fix it.

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 the "Shipped" field to "no" so that the license
is not included in about:credits page ([more on this below](#credits)).

When a dependency allows a choice of license, OWNERS should choose the least
restrictive license that meets Chromium's needs and document only the chosen
license(s) in the README.chromium file.

Multiple licenses apply when there are dependencies bundled together, or
different parts have different restrictions, these are inherently 'and'. This is
very different to a project allowing multiple license options.

The license field in README.chromium must use a _comma-separated list_ of licenses
that are actively in use. Complex license expressions are not allowed or
supported.

## 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 path to the
LICENSE file and set the Shipped value, if it passes the checks, it’ll be
included automatically.