Running tests as components

Tests on Fuchsia can either be run as standalone executables or as components. Standalone executables are invoked in whatever environment the test runner happens to be in, whereas components executed in a test runner are run in a hermetic environment.

These hermetic environments are fully separated from the host services, and the test manifests can stipulate that new instances of services should be started in this environment, or services from the host should be plumbed in to the test environment.

This document aims to outline the idiomatic way for a developer to configure their test artifacts to be run as components. This document is targeted towards developers working inside of fuchsia.git, and the workflow described is unlikely to work for SDK consumers.

An example setup of a test component is available at //examples/hello_world/rust.

Building the test

The exact GN invocations that should be used to produce a test vary between different classes of tests and different languages. The rest of this document assumes that test logic is being built somewhere, and that the test output is something that can be run as a component. For C++ and Rust, this would be the executable file the build produces.

Further documentation for building tests is available for Rust.

Packaging and component-ifying the tests

Once the build rule for building a test executable exists, a component manifest referencing the executable and a package build rule containing the executable and manifest must be created.

Component manifests

The component manifest exists to inform the component framework how to run something. In this case, it's explaining how to run the test binary. This file typically lives in a meta directory next to the BUILD.gn file, and will be included in the package under a top level directory also called meta.

The simplest possible component manifest for running a test would look like this:

{
    "program": {
        "binary": "test/hello_world_rust_bin_test"
    }
}

This component, when run, would invoke the test/hello_world_rust_bin_test binary in the package.

This example manifest may be insufficient for many use cases as the program will have a rather limited set of capabilities, for example there will be no mutable storage available and no services it can access. The sandbox portion of the manifest can be used to expand on this. As an alternative to the prior example, this example will give the component access to storage at /cache and will allow it to talk to the service located at /svc/fuchsia.logger.LogSink.

{
    "program": {
        "binary": "test/hello_world_rust_bin_test"
    },
    "sandbox": {
        "features": [ "isolated-cache-storage" ],
        "services": [ "fuchsia.logger.LogSink" ]
    }
}

Test components can also have new instances of services created inside their test environment, thus isolating the impact of the test from the host. In the following example, the service available at /svc/fuchsia.example.Service will be handled by a brand new instance of the service referenced by the URL.

{
    "program": {
        "binary": "test/hello_world_rust_bin_test"
    },
    "facets": {
        "fuchsia.test": {
            "injected-services": {
                "fuchsia.example.Service": "fuchsia-pkg://fuchsia.com/example#meta/example_service.cmx"
            }
        }
    },
    "sandbox": {
        "services": [
            "fuchsia.example.Service"
        ]
    }
}

For a more thorough description of what is valid in a component manifest, please see the documentation on package metadata.

Component and package build rules

With a component manifest written the GN build rule can now be added to create a package that holds the test component.

import("//build/test/test_package.gni")

test_package("hello_world_rust_tests") {
  deps = [
    ":bin",
  ]
  tests = [
    {
      name = "hello_world_rust_bin_test"
    }
  ]
}

This example will produce a new package named hello_world_rust_tests that contains the artifacts necessary to run a test component. This example requires that the :bin target produce a test binary named hello_world_rust_bin_test.

The test_package template requires that meta/${TEST_NAME}.cmx exist and that the destination of the test binary match the target name. In this example, this means that meta/hello_world_rust_bin_test.cmx must exist. This template produces a package in the same way that the package template does, but it has extra checks in place to ensure that the test is set up properly. For more information, please see the documentation on test_package.

Running tests

Tests can be exercised with the fx run-test command by providing the name of the package containing the tests.

$ fx run-test ${TEST_PACKAGE_NAME}

This command will rebuild any modified files, push the named package to the device, and run it.

Tests can also be run directly from the shell on a Fuchsia device with the run_test_component command, which can take either a fuchsia-pkg URL or a prefix to search pkgfs for.

If using a fuchsia-pkg URL the test will be automatically updated on the device, but not rebuilt like if fx run-test was used. The test will be neither rebuilt nor updated if a prefix is provided.

In light of the above facts, the recommended way to run tests from a Fuchsia shell is:

$ run_test_component `locate ${TEST_PACKAGE_NAME}`

The locate tool will search for and return fuchsia-pkg URLs based on a given search query. If there are multiple matches for the query the above command will fail, so locate should be invoked directly to discover the URL that should be provided to run_test_component