RFC-0141: CTF Process

RFC-0141: CTF Process
  • Testing

Describes a process for driving CTF tests to full coverage.

Gerrit change
Date submitted (year-month-day)2021-09-20
Date reviewed (year-month-day)2021-11-15


The Compatibility Tests for Fuchsia (CTF) are now running in our commit queue (CQ), ensuring that new platform code doesn't break backwards compatibility. However, a test suite is only as good as the tests it contains, and ours contains few tests. This document outlines a CTF Program for the platform to get to full API and ABI coverage.


The CTF exists to determine whether a build of the Fuchsia platform, running on a particular device, correctly implements (or is compatible with) the API and ABI exposed by a particular Fuchsia SDK. To put it another way, it demonstrates that the build correctly implements Fuchsia.

If a system running Fuchsia passes the CTF tests for a particular ABI revision, then its developers can have confidence that components built for that revision will work on the system, and that the system is backwards compatible with that revision. The tests associated with particular ABI and API revisions we care about are run in our CQ system.

Fuchsia Software Development Kits (SDKs) contain tools, libraries, and headers that allow developers to target Fuchsia's APIs and ABIs. We refer to the API and ABI exposed to out-of-tree developers via SDKs as Platform Surface Area (PlaSA). Each SDK is paired with a set of CTF tests that exercise the surface area it exposes. The tests are available in both source and binary form.

In order for this system to provide value, we must have a robust, complete set of tests. Robust tests are repeatable: they allow the developer to identify the platform behavior they are testing easily and do not exhibit flakiness. A set of tests is complete if it exercises all of the documented behavior for that interface.

As of September 2021, there are a handful of tests in the CTF suite. In the rest of this document, we identify how we will build out a robust, complete set of tests over time.

We note that there are a lot of process issues related to how each team will build out coverage that are not addressed in this RFC. For example, how we track progress is not specifically called out, and we do not provide implementation guidance. These issues are out of scope; if necessary, subsequent RFCs will address them.


Section to be filled out later.


The person appointed by FEC to shepherd this RFC through the RFC process.


shayba@google.com, test components and libraries ananthak@google.com, test infrastructure abarth@google.com, Fuchsia TL

List people whose vote (+1 or -1) will be taken into consideration by FEC when deciding whether this RFC is accepted or rejected.


List people who should review the RFC, but whose approval is not required.


This document was socialized with the CTF and test infrastructure teams.



We have two basic requirements that drive CTF policy.

First, CTF tests should, in the long term, provide complete coverage of the PlaSA, which consists of ABI (currently defined as anything in the Fuchsia System Interface) and API (currently defined as anything requiring API Review +1) exposed to end-developers via an SDK, and expected tooling behavior. A discussion of what constitutes coverage can be found in the next section.

Second, CTF tests should, to the extent possible, cover both intended and real-world use cases of the PlaSA. We need to cover real world cases because we cannot claim that our tests provide any degree of compatibility if they do not reflect what developers do with surface area elements. We need to cover intended cases for two reasons. First, when the developer writes the API for the first time, they will only have intended cases. Second, we believe that it is a useful exercise for developers to write intended client code when developing an API.

In this section, we present a path forward to providing broad platform coverage by the CTF.


Before we discuss what it means for our platform to reach complete coverage, we will discuss what coverage looks like for the CTF.

Note that, in this document, "complete coverage" does not mean it has to exercise every possible behavior - merely that it has to exercise every documented behavior - both success and error conditions - and all of the interfaces. Informally, each interface a test exercises is called an element.

The CTF team is tracking coverage of FIDL and C++ methods exported via SDKs. The team will be driving a process to ensure that we cover every FIDL and C/C++ method, and that test code covers code shipped with SDKs (e.g., the code in libfdio). In this document, when we refer to full coverage, we mean as tracked by the CTF program.

Tests that are not part of the CTF do not count towards coverage. Proprietary tests - that is, tests that are not open and part of the Fuchsia platform - also do not count towards coverage.

Note that full coverage does not ensure high quality, useful testing of all APIs. There is much the CTF program will not track; for example, we cannot have tests that exercise every possible set of parameter values (every API that takes a 32-bit integer can have 2^32 possible values; we are not likely to exercise it with 2^32 possible tests). API developers and reviewers are therefore responsible for ensuring that we have high quality coverage.

To phrase it another way, CTF is a mechanism that helps us enforce compatibility, but is not enough by itself. It is necessary, but not sufficient.

To ensure quality coverage, we provide two rules of thumb. They are intended to be used by developers to inform whether they have adequate coverage.

First, every behavior exposed via the PlaSA that should be documented per the API documentation rubric should also have a test that guarantees that behavior. For example, the documentation rubric states that behavior when a parameter is null should be documented. That behavior should be exercised with a CTF test.

Note that many APIs do not conform to the documentation rubric. Since we encourage writing tests as part of API development, the process of writing such tests is a good time to flesh out the documentation and ensure that it meets the rubric requirements.

Second, if a change in behavior causes a regression in application code running out of tree, then there is likely a CTF test missing. CTF tests are supposed to be the guarantee that new releases exhibit behavior compatible with older releases. If a Fuchsia build is compatible with an older release, that build must be able to run software that targets that release. Therefore, if a change causes a regression that breaks application code running out of tree, there is a compatibility behavior that was not tested - a missing CTF test.

In some cases, platform behavior may be changed to make platform behavior conform better to the documentation. In this case, the change should come with a new CTF test.

A change in platform behavior may also result from a change to undocumented behavior that we want to remain undocumented. What we do in that case is outside the scope of this document. CTF tests should not exercise deliberately undocumented behavior.

CTF tiers

In order to encourage as much CTF coverage as possible, we will gradually introduce more requirements for CTF coverage. As areas build their coverage, they move from less covered tiers to more covered tiers. Prior to adding tests, areas are in no tier at all.

Specific policy for the given tier is set off by blockquotes:

This is non-normative text.

This is normative text.

Tier 1 (starting)

Covering breakages

Because one of the goals of the CTF is to ensure compatibility, and a critical sign of incompatibility is when a platform or SDK release breaks a customer due to a change in behavior, we propose introducing the following policy as soon as possible:

If a platform or SDK release causes an SDK roll failure or a failed canary release due to backwards incompatible changes to Fuchsia PlaSA API or behavior, the author of that breaking change is responsible for ensuring there is documentation for the new behavior and a CTF test that covers the new behavior. If it is not currently possible to write CTF tests for that surface area element, then the team that owns that PlaSA element must prioritize a plan for developing them.

If the behavior is intended to be undocumented, then an exception to the above may be granted. The exception may involve documenting the fact that the behavior is intentionally undocumented, or providing tools to users to help them identify the fact that they are relying on undocumented behavior.

New SDK-facing features

We expect developers to start to develop platform testing capabilities alongside new SDK-facing features.

All additions to the PlaSA (e.g., new SDK tools, FIDL protocols or methods, and C++ headers) shipped with partner or public SDKs require CTF tests. Anyone taking a new API element through API Review and including it in a public or partner SDK must have a plan to include CTF tests with their new API.

It is the responsibility of the platform team owning the platform change to provide the CTF test, and work with the CTF program to deliver that test.

As API developers work through their test plans, we strongly recommend that they surface what kinds of tests they intend to write with their API reviewers so that they can develop a shared understanding of the test plan, including what tests are highest priority.

If a platform team responds to all backwards-compatibility-breaking changes to API and ABI you own with a test, and write tests for new platform contract functionality and behaviors, that team is in Tier 1.

Note that the requirement for CTF tests for API review only applies to PlaSA API elements. As long as an API is not exposed to end developers via an SDK, it does not need associated CTF tests.

Tier 2 (building)

API/ABI evolution

Because breakages in changing code are more likely than breakages in long-term stable code, and as a way of providing incremental coverage, we propose introducing the following policy:

All modifications to the API or behavior of a PlaSA element, regardless of whether it is a new PlaSA element or an existing one, must be accompanied by a CTF test that covers the change.

If it is not currently possible to write CTF tests for an existing public or partner surface area element, then the responsible team must provide a plan for developing those tests prior to making the change, and execute on that plan as soon as is feasible, as agreed upon with their API reviewer.

In this tier, we strongly recommend CTF tests be written prior to major rewrites or replacement of any services, tools, or libraries that affect PlaSA behavior, regardless of whether the new code is intended to change that behavior. For example, if we were to rewrite the kernel from scratch, but keep the same VDSO behavior, tests of that behavior should be written prior to the rewrite.

If you do everything you need for Tier 1, and you make sure that all changes you make to ABI or API are covered by CTF tests, you have achieved Tier 2.

Tier 3 (complete)

The long tail

We recognize that there are many long term stable PlaSA elements, many of which are not under active development. Ultimately, we want to provide coverage for the entire PlaSA.

Each area must develop a plan to have complete coverage of the PlaSA elements it owns and deliver the results of that plan.

If you do everything you need for Tier 2, and have complete coverage of the PlaSA elements you own, you are complete.


A CTF program will be initiated by the Fuchsia team to encourage full platform coverage. CTF coverage will be tracked by several dashboards. The details of these dashboards will evolve, and are outside the scope of this RFC, but they will track basic test coverage for each PlaSA element.

The CTF team is responsible for providing developers with the infrastructure needed to run their tests. If test developers do not see the functionality they need, they should partner with the CTF team to ensure that they can provide tests that can be run in CTF infrastructure. Developers can reach out to the CTF team by filing a bug.

API review will also evolve to accommodate the requirement that CTF tests accompany changes. For areas at Tier 2 or higher, modifications or additions to PlaSA elements must have CTF tests. Eventually, this requirement will be extended to all areas; by that point, we expect to formalize testing as part of API Review.


The existence of tests has no impact on platform performance. We have to track machine usage of the CTF tests carefully to make sure that they do not exceed Fuchsia machine capacity.


The CTF team is tasked with ensuring that it is easy for areas to write basic CTF tests and that infrastructure exists to run those tests. They are not responsible for domain-specific infrastructure and frameworks.

Backwards compatibility

This proposal does not break backwards compatibility. The goal is to create a mechanism to enforce backwards compatibility for the Fuchsia platform.

Security considerations

We do not believe the CTF program will negatively impact Fuchsia security.

Privacy considerations

We do not believe the CTF program will negatively impact Fuchsia privacy properties.


As this RFC largely details a process, it does not require a detailed test plan.


The team will work with DevRel and Tech Writers to provide detailed and useful guidance on how to write effective CTF tests.

Drawbacks, alternatives, and unknowns

This change creates substantial upfront work for API developers, who often do not include tests with their API changes. We believe that requiring developers to write tests that use their APIs prior to releasing them will improve the ergonomics of those APIs prior to release.

Prior art and references

The Android system has similar processes about CTF inclusion when developing new APIs.

Many programming languages have similar testing requirements for new APIs, as well.