The Compatibility Tests for Fuchsia (CTF) are a set of tests designed to provide coverage for Application Programming Interface (API) and Application Binary Interface (ABI) elements (e.g., headers, FIDL files) made available to developers via a Fuchsia SDK. It was originally proposed as RFC 0015, and the project code is located at //sdk/ctf.
Background, Motivation, and Goals
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.
Each release of the Fuchsia platform is bundled with a set of Software Development Kits (SDKs), which contain tools, libraries, and headers that allow developers to target Fuchsia's APIs and ABIs. We refer to the API and ABI as Platform Surface Area (or PlaSA). Each SDK will be paired with a set of CTF tests that exercise the surface area it exposes. The tests will be available in both source and binary form.
CTF tests are not comprehensive. They cannot guarantee that any component that is built against one SDK will run against any particular build of Fuchsia. CTF must, therefore, be complemented with product-specific testing.
With that in mind, the CTF can then be used to enforce compatibility in the following ways:
For platform developers, including those working in fuchsia.git, system integrators, product developers, and device developers
The CTF binary tests can be run against a device running Fuchsia to ensure that the build on that device is ABI compatible with the CTF's associated SDK. This can provide enforcement of backwards compatibility guarantees: if the CTF from a given SDK passes, that is a strong indicator (although not a conclusive one) that programs built against that SDK will continue to work. It can also provide confidence that software running on the device that is not exercised by in-tree tests, such as out-of-tree device drivers, does not change the behavior of the platform.
As a table:
|Run → Against ↓||CTF N||CTF N + 1|
|SDK / Product Build at version N||A||B|
|SDK / Product Build at version N + 1||C||A|
A = Someone who wants to make sure a product build correctly implements the ABI revision they claim it does.
B = Someone who wants to make sure that a product build is forward compatible with the a newer ABI revision. Fuchsia org doesn't provide this kind of guarantee.
C = Someone who wants to make sure that a product build is backwards compatible with an older ABI revision. Fuchsia org provides this kind of guarantee to customers whose code targets older SDKs.
For SDK vendors
The CTF source tests can be built against an SDK to ensure that the SDK is API compatible with the CTF's associated SDK. Additionally, CTF contains a suite of tests for SDK host tools. These tests can provide confidence that changes to the SDK do not break developer code and workflows. For example, we can build the CTF for API version N against an SDK that contains support for API version N+1 to see if the SDK has broken API compatibility. Currently, the only SDK vendor supported by the CTF project is the Fuchsia organization itself.
As a table:
|Build → Against ↓||CTF N||CTF N + 1|
|SDK at version N||A||B|
|SDK at version N + 1||C||A|
A = Someone who wants to make sure an SDK correctly implements the API level they claim it does. This includes Fuchsia org (testing at tip of tree).
B = Someone who wants to make sure that an SDK is forward compatible with the a newer API level. Fuchsia org doesn't provide this kind of guarantee.
C = Someone who wants to make sure that an SDK is backwards compatible with an older API level. Fuchsia org provides this kind of guarantee to customers whose code targets older SDKs.