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Test Execution

Test execution in Buck2 is a collaboration with a separate test runner process.

In its open-source build, Buck2 ships with a built-in simplistic test runner.

This test runner receives the commands defined by ExternalRunnerTestInfo and simply executes them. Exit code zero means the test passed, and one means it failed.

Users can of course develop their own test runners. Look at fbcode/buck2/app/buck2_test_runner as a sample. For comparison, here's how it's used at Meta:

At Meta, this test runner is Tpx


Tpx has a large number of responsibilities when used with Buck2, which can be grouped as follows:

  • Translation:
    • Understands the output formats of various supported test frameworks. This is used to identify test cases and collect test results.
    • Understands, to an extent, the input formats. For example, given a test case, Tpx can identify what command needs to run to execute just that test.
  • Orchestration:
    • Interacts with Test Infra to discover what tests should run, under a number of configurations.
    • Separates listing of tests (identifying what tests exists in a test target) and execution (running specific tests within that target).
    • Coordinates the execution of tests. For example, it may request retries, or choose to bundle multiple tests in a single execution (or not).
    • Reports test results to Test Infra as well.

In Buck2, rules interact with the test runner via a provider called ExternalRunnerTestInfo.

Anatomy of a test run

When a user runs buck2 test $targets:

  • Buck2 identifies all matching targets that have an ExternalRunnerTestInfo.
  • Buck2 builds all the artifacts referenced by those targets (this will likely change eventually to build them only if they are used).
  • Buck2 then notifies the test runner that those tests exist. Currently, the test runner receives a subset of ExternalRunnerTestInfo.
  • The test runner can request command execution from Buck2 to list and execute tests.
  • When it receives command results from Buck2, the test runner may fire off events that the end-user will see (such as test results), upload logs externally, request further executions, and so on.

If more than one target is being built, test building and execution will proceed concurrently.

Information available on ExternalRunnerTestInfo

As noted, rules communicate their testing capabilities via ExternalRunnerTestInfo. There are a number of fields available on ExternalRunnerTestInfo to control how a given target is tested, as detailed in the following sub-sections.

Fields exposed to the test runner

The following list shows what is available in ExternalRunnerTestInfo, with which the test runner can interact:

  • type - a string key that defines the type of test this is.
  • command and env - respectively, a list and a key-value mapping of arguments. They are not always visible to the test runner (for more details, see Verbatim arguments and handles, below).
  • labels - a set of string labels to pass to the test runner.
  • contacts - a list of contacts for the tests; usually oncalls.
  • executor_overrides - a key-value mapping of executor configurations that the test runner can use when requesting execution from Buck2.
  • local_resources - a key-value mapping from resource type to optional LocalResourceInfo provider. Provider is used for initialization of that resource type. If the value is None resource type is ignored even though test runner required it. For context see Local Resources For Tests Execution.

Fields pertinent for Remote Execution

For compatibility with Remote Execution (RE), there are two fields that rules should set in their ExternalRunnerTestInfo if they should be run on RE:

  • use_project_relative_paths - if true (the default is `true`), Buck2 will produce relative paths. If not, it'll produce absolute paths.
  • run_from_project_root - if true (the default is `true`), tests will run from the project root (their `cwd` will be the project root, which is the same as all build commands). If `false`, it'll be the cell root.

Note that passing --unstable-allow-all-tests-on-re to buck2 test will override those fields and set them to true, since they are a pre-requisite to run on RE. In contrast, passing --unstable-allow-compatible-tests-on-re will only allow tests that already set both those fields to true to execute on RE.

Also note that when executor_overrides are set, if an executor override is used and results in execution on RE, it'll happen on RE unconditionally. Therefore, it's a good idea to set those fields if RE-only executor overrides are provided.

Verbatim arguments and handles

As noted above, the test runner only interacts with a subset of arguments provided by rules in ExternalRunnerTestInfo. The reason for this is that the test runner doesn't get to access, for example, artifacts, that Buck2 knows about.

Consider the following example:

binary = ctx.attrs.dep[RunInfo]
test_info = ExternalRunnerTestInfo(command = [binary, "run-tests"], ...)

When Buck2 actually runs this command, binary is expanded to a path (and possibly to more args). Buck2 would also account for any hidden arguments and make those available where the command is executed. It is important for Buck2 to retain this capability when running with the test runner.

To that end, all non-trivial arguments present in command (and in the values of env), such as cmd_args or RunInfo, are exposed to the test runner as opaque handles, and simple string arguments are passed as-is to the test runner.

This means that the test runner would see the command described above as:

[ArgHandle(index = 0), Verbatim("foobar")]

When requesting execution from Buck2, the test runner can use the ArgHandle and Buck2 will swap it back for the underlying value that was set on the provider.

This allows the test runner to introspect and modify parts of the command lines it receives, as long as it doesn't need to access the actual text value of non-verbatim arguments. Usually, this works out to be sufficient (or can be made sufficient with a bit of refactoring in the test runner).

Execution Configurations

By default, tests execute using the execution configuration of the associated target. This is the execution configuration that would be used for run actions ( declared in the same target. This is a default that actually makes little sense but works out as long as cross-compiling is not the norm.

To support this, ExternalRunnerTestInfo allows specifying override platforms, which are given a name. The test runner can request execution on them by passing their name when it sends execution requests to Buck2, as shown in the following code:

executor_overrides = {
"ios-simulator": CommandExecutorConfig(
local_enabled = False,
remote_enabled = True,
remote_execution_properties = {
"platform": "ios-simulator-pure-re",
"subplatform": "iPhone 8.iOS 15.0",
"xcode-version": "xcodestable",
remote_execution_use_case = "tpx-default",
"static-listing": CommandExecutorConfig(local_enabled = True, remote_enabled = False),

The default execution platform can also be overridden:

default_executor = CommandExecutorConfig(
local_enabled = False,
remote_enabled = True,
remote_execution_properties = {
"platform": "ios-simulator-pure-re",
"subplatform": "iPhone 8.iOS 15.0",
"xcode-version": "xcodestable",
remote_execution_use_case = "tpx-default",

Working Directory

Tests can be run from the cell root by setting `run_from_project_root = False`.

To produce paths relative to the cell root for use by tests, use relative_to(ctx.label.cell_root) on cmd_args.