docs/concepts/source_code/third-party-management.md

# Adding third-party code to Fuchsia

Third-party code is part of the Fuchsia checkout but is neither copyrighted by
the Fuchsia authors nor subject to Fuchsia's [license]. In other words, any code
that is not 100% owned by the Fuchsia authors is managed as third-party code.

The Fuchsia project maintains copies of third-party code dependencies under the
`//third_party/` directory in the checkout. This is also known as vendoring.
Vendoring ensures that third-party code is served from Fuchsia-owned source
repositories and is served at revisions that are known to work with other code
in the Fuchsia checkout.

When adding third-party code, follow the steps below to ensure the code complies
with the Fuchsia project policies.

## Before you start

All external code must go through the [Open Source Review Board (OSRB)
process][osrb-process] to be added to the Fuchsia Platform Source Tree. Once the
OSRB request is approved, continue with the steps below.

### Language-specific guides

If you are adding Rust, Dart or Go dependencies, follow the guides below:

- **Rust**: Follow the [external Rust crates][rust-third-party] guide.

- **Dart**: Follow the [third-party Dart packages][dart-third-party] guide.

- **Go**: See [`//third_party/golibs/`][golibs].

For all other languages, continue with the steps below.

## Get the code

All external code must follow the third_party source layout below (using
`googletest` as example):

```none {:.devsite-disable-click-to-copy}
root [fuchsia.googlesource.com/fuchsia]
  third_party/
    googletest/
      src/ [fuchsia.googlesource.com/third_party/github.com/google/googletest]
      BUILD.gn
      OWNERS
      README.fuchsia
```

`//third_party/googletest/src/` is the root of the [Fuchsia-owned mirror
repository][third-party-googletest], that contains a copy of the [upstream
repository][googletest] for `googletest`. (_Note:_ For Python repositories,
replace `/src` with `/<module_name>` to follow Python's convention. This
convention is expected by common Python tools like [pyright][pyrightconfig].)

The `//third_party/googletest/` directory is part of the [`fuchsia.git`][fuchsia-git]
repository.

`//third_party/googletest/BUILD.gn` defines build targets for the `googletest`
library. Since this file belongs to [`fuchsia.git`][fuchsia-git] (not the
[`googletest` repository][third-party-googletest]), it can be updated in
lockstep with other Fuchsia `BUILD.gn` files that depend on `googletest`. This
makes build refactors and other large-scale changes easier.

Additional files that are required to adapt the third-party code to the Fuchsia
project may be present under (in this case) `//third_party/googletest`.

### Add OWNERS

Each dependency must have an associated [`OWNERS`][owners] file.  Because it's
defined in `fuchsia.git`, it is possible to include owners from other files
elsewhere in the Fuchsia project.

The OWNERS file must either list two Fuchsia developer accounts as the first
two lines or include a `file:` directive to another OWNERS file. This will ensure
accountability for maintenance of the code over time.

The OWNERS are typically the owners of the code that use the dependency, unless
specified otherwise.

The dependency's OWNERS help keep Fuchsia and its users safe by:
* Removing the dependency when/if it is no longer needed
* Updating the dependency when a security or stability bug is fixed upstream
* Helping ensure the Fuchsia feature that uses the dependency continues to use the
dependency in the best way, as the feature and the dependency change over time.

### Add README.fuchsia

You need a README.fuchsia file with information about the project from which
you're reusing code. Check out [`README.fuchsia`][readme-fuchsia] for the list
of required fields to include.

### Exceptional cases

Most third-party dependencies can follow the layout described above. However, a
small fraction of dependencies that are subject to uncommon circumstances are
managed differently.

Having exotic dependencies can increase complexity and maintenance costs, which
are incurred by direct dependencies of the third-party code. Additionally, they
add complexity to common global maintenance tasks such as:

- Performing git administration tasks.
- Updating and maintaining toolchains.
- Responding to disclosed security vulnerabilities by updating vulnerable
  third-party code from upstream sources.
- Refactoring build rules, such as to enforce new compile-time checks.

Please exercise careful deliberation when stepping off the beaten path.


# Migrating legacy third-party code to current layout

Bringing all the existing //third_party code to the layout documented above
is WIP, and contributions are welcome.

To migrate legacy third-party repositories to this layout, follow these
steps:


1. Move Fuchsia-specific `BUILD.gn` files to
   [`//build/secondary`][build-secondary].

   1. Copy `BUILD.gn` files from `//third_party/<name>` to
      `//build/secondary/third_party/<name>`. If there is more than one
      `BUILD.gn` file, maintain the same subtree under `//build/secondary`.
   1. In the copied `BUILD.gn` files, update references to paths to third-party
      files in the form of `//third_party/<name>/` to the form of
      `//third_party/<name>/src/`.
   1. Copy `OWNERS` from `//third_party/<name>` to `//build/secondary/<name>`,
      or create it if it does not exist. Review the `OWNERS` file to ensure that
      it follows the [best practices][owners-best-practices].
   1. Copy `README.fuchsia` from `//third_party/<name>` to
      `//build/secondary/<name>`. Review the contents of this file and ensure
      that the metadata is correct. In uncommon cases there are modifications
      made to third-party code in third-party repositories, and such changes are
      listed in `README.fuchsia`. Local modifications will often require you to
      make special accommodations that are not covered in this guide.
   1. Review `//third_party/<name>` for any other first party `.gni` files and
      move those to `//build/secondary/<name>` as well.
   1. Update `//build/secondary/third_party/<name>/BUILD.gn` (and other files
      containing source paths such as `.gni` files) to use the new source
      location `//third_party/<name>/src`. This requires updating all sources,
      including directory paths and more.

   Example: [https://fxrev.dev/622785](https://fxrev.dev/622785)

   Note: CQ won't catch errors in this change (for example, typos in paths)
   because the secondary build files are not yet active. You can validate that
   your `BUILD.gn` files are correct by staging the next commit (update the
   integration manifest), running `jiri update -local-manifest`, then building
   (such as with `fx build`).

1. Update the integration manifest.

   Note: This step can only be performed in Google's internal repositories at the
   moment.

   Replace `path` (not `name`) of the existing third-party project at
   `//third_party/<name>` with `//third_party/<name>/src`, while keeping the
   revision unchanged. With this change merged, the Fuchsia build will switch to
   using the `BUILD.gn` files from the previous step.

   Example: [http://tqr/457911](http://tqr/457911)

1. Move Fuchsia-specific files added in step 1 to `//third_party/<name>`.

   Now that third-party code is nested under `//third_party/<name>/src` and
   `//third_party/<name>` is part of [`fuchsia.git`][fuchsia-git], you can undo
   the transitional step 1.

   1. Wait for the integration manifest change to merge and roll, then run
      `jiri update`. Or stage the integration manifest change from the previous
      step in your local checkout, then run `jiri update -local-manifest`.

   1. Move `BUILD.gn` and other Fuchsia-specific files from
      `//build/secondary/<name>` to `//third_party/<name>`.

   1. Update [`//.gitignore`][gitignore] so that `//third_party/<name>` is
      tracked but `//third_party/<name>/src` is not tracked.

   Example: [https://fxrev.dev/622789](https://fxrev.dev/622789)

1. Turn `//third_party/<name>/src` into a mirror.

   Note: This step can only be performed in Google's internal repositories at the
   moment.

   Change `//third_party/<name>/src` to track upstream such that it only has
   upstream changes in its `git log`. You can do this by updating the
   integration manifest to reference an upstream commit hash.

   Example: [http://tqr/427570](http://tqr/427570)

## Additional reading

- [Fuchsia open source licensing policies][oss-licensing]
- [Source code layout][source-layout]

[build-secondary]: /build/secondary/
[dart-third-party]: /docs/development/languages/dart/third_party.md
[fuchsia-git]: https://fuchsia.googlesource.com/fuchsia/+/refs/heads/main
[gitignore]: https://fuchsia.googlesource.com/fuchsia/+/refs/heads/main/.gitignore
[golibs]: /third_party/golibs/
[googletest]: https://github.com/google/googletest
[license]: /LICENSE
[osrb-process]: /docs/contribute/governance/policy/osrb-process.md
[oss-licensing]: /docs/contribute/governance/policy/open-source-licensing-policies.md
[owners]: /docs/development/source_code/owners.md
[owners-best-practices]: /docs/development/source_code/owners.md#best_practices
[readme-fuchsia]: /docs/development/source_code/third-party-metadata.md
[rust-third-party]: /docs/development/languages/rust/external_crates.md
[source-layout]: /docs/development/source_code/layout.md
[third-party-googletest]: https://fuchsia.googlesource.com/third_party/github.com/google/googletest/
[pyrightconfig]: https://fuchsia.googlesource.com/fuchsia/+/refs/heads/main/pyrightconfig.json