Fuchsia Networking Contributor Guide

Fuchsia Networking welcomes contributions from all. This document defines contribution guidelines where they differ from or refine on the guidelines that apply to Fuchsia as a whole.

Getting Started

Consult the getting started document to set up your development environment.

Contributor Workflow

Consult the contribute changes document for general contribution guidance and project-wide best practices. The remainder of this document describes best practices specific to Fuchsia Networking.

Coding Guidelines

General Advice

Avoid duplicating code whenever possible. In cases where existing code is not exposed in a manner suitable to your needs, prefer to extract the necessary parts into a common dependency.

Avoid unhandled errors and APIs which inherently disallow proper error handling; for a common example, consider fuchsia_async::executor::spawn. spawn inherently precludes error passing (since the flow of execution is severed). In most cases spawn can be replaced with a future that is later included in a select expression (example commit) or simply awaited on directly (example commit).

Prefer type safety over runtime invariant checking. In other words, arrange your abstractions such that they cannot express invalid conditions rather than relying on assertions at runtime.

Write testable code; testable code is modular and its dependencies are easily injected.

Avoid magic numbers.


When writing comments, take a moment to consider the future reader of your comment. Ensure that your comments are complete sentences with proper grammar and punctuation. Note that adding more comments or more verbose comments is not always better; for example, avoid comments that repeat the code they're anchored on.

Documentation comments should be self-contained; in other words, do not assume that the reader is aware of documentation in adjacent files or on adjacent structures. Avoid documentation comments on types which describe instances of the type; for example, AddressSet is a set of client addresses. is a comment that describes a field of type AddressSet, but the type may be used to hold any kind of Address, not just a client's.

Phrase your comments to avoid references that might become stale; for example: do not mention a variable or type by name when possible (certain doc comments are necessary exceptions). Also avoid references to past or future versions of or past or future work surrounding the item being documented; explain things from first principles rather than making external references (including past revisions).

When writing TODOs:

  1. Include an issue reference using the format TODO(1245):
  2. Phrase the text as an action that is to be taken; it should be possible for another contributor to pick up the TODO without consulting any external sources, including the referenced issue.

Error Messages

As with code comments, consider the future reader of the error messages emitted by your code. Ensure that your error messages are actionable. For example, avoid test failure messages such as “unexpected value” - always include the unexpected value; another example is “expected to be empty, was non-empty” - this message would be much more useful if it included the unexpected elements.

Always consider: what will the reader do with this message?


Consult the testability rubrics for general guidelines on test-writing and testability reviews on Fuchsia. In Fuchsia Networking, we define the following test classes:

  • Unit tests are fully local to a piece of code and all their external dependencies are faked or mocked.
  • Integration tests validate behavior between two or more different components.
  • End-to-end tests are driven by an external host machine and use the public APIs and bytes written to the network to perform behavior validation. Can be performed over a physical network or by virtualization of the DUT (qemu).

Consider the following guidelines considering test-writing:

  1. Always add tests for new features or bug fixes.
  2. Consider the guidelines in Error Messages when writing test assertions.
  3. Tests must be deterministic. Threaded or time-dependent code, Random Number Generators (RNGs), and cross-component communication are common sources of nondeterminism.
    • Don't use sleep in tests as a means of weak synchronization. Only sleep when strictly necessary (e.g. when polling is required).
    • Time-dependent tests can use fake or mocked clocks to provide determinism. See fuchsia_async::Executor::new_with_fake_time and fake-clock.
    • Threaded code must always use the proper synchronization primitives to avoid flakes. Whenever possible, prefer single-threaded tests.
    • Always provide a mechanism to inject seeds for RNGs and use them in tests.
    • Test for flakes locally whenever possible; use repeat flags in test binaries (-count in Go, --gtest_repeat for googletest) and aim for at least 100-1000 runs locally if your test is prone to flakes before merging.

      Rust test binaries currently don't have an equivalent flag, you may need to resort to a bash loop or equivalent to get repeated runs. See #65218.

  4. Avoid tests with hard-coded timeouts. Prefer relying on the framework/fixture to time out tests.
  5. Prefer hermetic tests; test set-up routines should be explicit and deterministic. Be mindful of test fixtures that run cases in parallel (such as Rust's) when using “ambient” services. Prefer explicitly injecting component dependencies that are vital to the test.
  6. Tests should always be components.
  7. Prefer virtual devices and networks for non-end-to-end tests. See netemul for guidance on virtual network environments.
  8. Avoid change detector tests; tests that are unnecessarily sensitive to changes, especially ones external to the code under test, can hamper feature development and refactoring.
  9. Do not encode implementation details in tests, prefer testing through a module's public API.

Source Control Best Practices

Commits should be arranged for ease of reading; that is, incidental changes such as code movement or formatting changes should be committed separately from actual code changes.

Commits should always be focused. For example, a commit could add a feature, fix a bug, or refactor code, but not a mixture.

Commits should be thoughtfully sized; avoid overly large or complex commits which can be logically separated, but also avoid overly separated commits that require code reviews to load multiple commits into their mental working memory in order to properly understand how the various pieces fit together. If your changes require multiple commits, consider whether those changes warrant a design doc or RFC.

Commit Messages

Commit messages should be concise but self-contained (avoid relying on bug references as explanations for changes) and written such that they are helpful to people reading in the future (include rationale and any necessary context).

Avoid superfluous details or narrative.

Commit messages should consist of a brief subject line and a separate explanatory paragraph in accordance with the following:

  1. Separate subject from body with a blank line
  2. Limit the subject line to 50 characters
  3. Capitalize the subject line
  4. Do not end the subject line with a period
  5. Use the imperative mood in the subject line
  6. Wrap the body at 72 characters
  7. Use the body to explain what and why vs. how

The body may be omitted if the subject is self-explanatory; e.g. when fixing a typo. The git book contains a Commit Guidelines section with much of the same advice, and the list above is part of a blog post by Chris Beams.

Commit messages should make use of issue tracker integration. See Commit-log message integration in the monorail documentation.

Commit messages should never contain references to any of:

  1. Relative moments in time
  2. Non-public URLs (such as go/, fxb/, fxr/, and the like)
  3. Individuals
  4. Hosted code reviews (such as on fuchsia-review.googlesource.com)
  5. Other entities which may not make sense to arbitrary future readers

Adding a Test: line to the commit message is encouraged. A Test: line should:

  1. Justify that any behavior changes or additions are thoroughly tested.
  2. Describe how to run new/affected test cases.

For example: Test: Added new unit tests. `fx test netstack_gotests`.


This section is inspired by Flutter's style guide, which contains many general principles that you should apply to all your programming work. Read it. The below calls out specific aspects that we feel are particularly important.

Be Lazy

Do not implement features you don‘t need. It is hard to correctly design unused code. This is closely related to the commit sizing advice given above; adding a new data structure to be used in some future commit is akin to adding a feature you don’t need - it is exceedingly hard for your code reviewer to determine if you‘ve designed the structure correctly because they (and you!) can’t see how it is to be used.

Go Down the Rabbit Hole

You will occasionally encounter behaviour that surprises you or seems wrong. It probably is! Invest the time to find the root cause - you will either learn something, or fix something, and both are worth your time. Do not work around behaviour you don't understand.

Tips & Tricks

fx set

Run the following command to build all tests and their dependencies:

fx set core.x64 --with //src/connectivity/network:tests

If you're working on changes that affect fdio and third_party/go, add:

--with-base //garnet/packages/tests:zircon --with //third_party/go:go_stdlib_tests