Fuchsia Testability Rubrics


Goals of this document

Fuchsia Testability is a form of Readability that focuses on ensuring that changes to Fuchsia introduce testable and tested code.

As with any Readability process, the guidelines and standards are best made publicly available, for reviewers and reviewees. This document is the Fuchsia Testability equivalent of a style guide for a programming language readability process.

It’s valuable to apply the Testability standards consistently, hence it is important that anyone involved in Testability should study the doc (whether you’re reviewing changes or authoring them).

The exact contents of the doc may change over time.

Your goals as a Testability reviewer

  • Determine if the change is tested. Apply Code-Review+2 if you agree that it’s tested, and reply with a note for what’s missing if it’s not.
  • Apply the standard (this doc) consistently.
  • If the change needs to be amended to meet the standards, provide actionable feedback.
  • Promote Fuchsia testing & testability.
  • Identify cases not handled by this doc and propose changes.
  • Uphold the standard but also apply your good judgement. The goal is to improve quality and promote a culture of testing. You’re not expected to execute a precise decision algorithm.

Your goals as a change author

  • Inform yourself of the standards by reading this doc (you’re doing it right now!).
  • Respect that your Testability reviewer is volunteering for the role, and treat them appropriately.
  • Consider feedback given for ways that you could improve confidence in your change using testing.

What to test? How to test?

  • Changes to functionality should have a test that would have failed without said change.
  • Tests must be local to the code being changed: dependencies with test coverage do not count as test coverage. For example, if “A” is used by a “B”, and the “B” contains tests, this does not provide coverage for “A”. If bugs are caught with “B”'s tests, they will manifest indirectly, making them harder to pinpoint to “A”. Similarly, if “B” is deprecated (or just changes its dependencies) all coverage for “A” would be lost.
  • Tests must be automated (CI/CQ when supported). A manual test is not sufficient, because there is no guarantee that a future change to the code (especially when authored by another engineer) will exercise the same manual tests. Exceptions may apply to some parts of the codebase to recognize ongoing automation challenges.
  • Tests must minimize their external dependencies. Our test infrastructure explicitly provisions each test with certain resources, but tests are able to access more than those that are provisioned. Examples of resources include hardware, CPU, memory, persistent storage, network, other IO devices, reserved network ports, and system services. The stability and availability of resources that are not provisioned explicitly for a test cannot be guaranteed, so tests that access such resources are inherently flaky and / or difficult to reproduce. Tests must not access external resources beyond the control of the test infrastructure. For example, tests must not access services on the Internet. Tests should only use resources beyond those provisioned explicitly for that test when necessary. For example, tests might have to access system services that do not have test doubles available. A small number of exceptions to this rule are made for end-to-end tests.
  • Changes to legacy code (old code that predates Testability requirements and is poorly tested) must be tested. Proximity to poorly-tested code is not a reason to not test new code. Untested legacy code isn’t necessarily old and crufty, it may be proven and battle-hardened, whereas new code that isn’t tested is more likely to fail!
  • Changes you are making to someone else’s code are subject to the same Testability requirements. If the author is changing code they’re not familiar with or responsible for, that’s more reason to test it well. The author can be expected to work with the responsible individual or team to find effective ways to test the change. Individuals responsible for the code under change are expected to help the author with testability with the same priority as the author’s change.

What does not require testing

Missing testing coverage for the below should not prevent a change from receiving Code-Review+2.

  • Logging. In most cases, it’s probably not worth testing the log output of components. The log output is usually treated as opaque data by the rest of the system, which means changes to log output are unlikely to break other system. However, if the log output is load bearing in some way (e.g., perhaps some other system depends on observing certain log messages), then that contract is worth testing. This can also apply to other forms of instrumentation, such as Tracing. This does not apply to instrumentation when it is used as a contract, for instance Inspect usage can be tested, and should be if you rely on it working as intended (for instance, in ffx inspect or feedback reports).
  • Code that we don’t own (the source of truth is not in Fuchsia tree). Changes that pick up an update to source code that’s copied from elsewhere don’t bear testability requirements.
  • Pure refactors (changes that could have entirely been made by an automated refactor tool), such as moving files, renaming symbols, or deleting them, don’t bear testability requirements. Some languages can have behavior that’s exposed to such changes (e.g. runtime reflection), so exceptions may apply.
  • Generated code. Changes that are generated by tools (such as formatting, or generated code checked in as a golden file) don’t bear testability requirements. As an aside, it’s generally discouraged to check in generated code (rather harness the tools and have the code be generated at build time), but in the exceptional case don’t require tests for code written by machines.
  • Testability bootstrapping. In cases where the change is in preparation for introducing testability to the code, and this is explicitly documented by the author, then Testability reviewers may exercise discretion and take an IOU.
  • Manual tests. Manual tests are often themselves used to test or demonstrate functionality that is hard to test in an automated fashion. Additions or modifications to manual tests therefore do not require automated tests. However, it is strongly recommended that manual tests be paired with a README.md or TESTING.md document describing how to run them.
  • Hardcoded values. Additions or changes to hardcoded values do not necessarily require tests. Oftentimes, these values control behaviors that are not easily observable, such as unexposed implementation details, heuristics, or “cosmetic” changes (e.g. background color of a UI). Tests of the style assert_eq!(CONFIG_PARAM, 5); are not considered useful and are not required by testability. However, if the contribution results in an easily observable behavioral change, that contirbution should include a test for the new behavior.

What does require testing

If fixing a flake, test for flakiness in CQ

If fixing a flake, verify the fix by testing for flakiness in CQ.

Tests should not sleep

Sleeps can lead to flaky tests because timing is difficult to control across different test environments. Some factors that can contribute to this difficulty this are the test target's CPU speed, number of cores, and system load along with environmental factors like temperature.

  • Avoid something like:

    // Check if the callback was called.
    EXPECT_EQ(true, callback_happened);
  • Instead, explicitly wait for the condition:

    // In callback
    callback() {
    // In test
    sync_completion_wait(&event_, ZX_TIME_INFINITE);

    This code sample was adapted from task_test.cc.

Regression tests for race conditions

It is difficult to write regression tests for race conditions that don't have a high false-pass rate. If you can write a test that deterministically reproduces the issue, you should do that. See Write reproducible, deterministic tests for tips. Otherwise, if the data race was fixed by improving the locking scheme used, you can add thread annotations as a regression test. For other races, you should attempt to design APIs that prevent the existence of the race condition.

Recently removed exemptions

  • Engprod scripts (e.g. fx commands) and associated configuration files no longer have an exemption from testability. fx must have integration tests before further changes land. Exceptions may be granted by the fx team after consulting with a testability reviewer.

Temporary testability exemptions

The following are currently exempt from Testability, while ongoing work aims to change that.

  • Engprod scripts in the tools/devshell/contrib and associated configuration are exempt.
  • GN templates are not easily testable. We are working on a test framework for GN templates. Until then, it's permitted for build template changes to be manually tested only.
  • Resource leaks are not easily preventable in C-style code. In the longer term, such code should be refactored to use Rust or modern C++ idioms to reduce the chances of leaks, and automation should exist that is capable of automatically detecting leaks.
  • Gigaboot is a UEFI bootloader in //src/firmware/gigaboot that predates testability policy. At present there is not an infrastructure available to write integration tests for the UEFI code. Introducing that infrastructure is tracked in fxbug.dev/34478. A testability exception is granted until fxbug.dev/34478 is addressed.