Testing: Questions and Answers

You are encouraged to add your own questions (and answers) here!

Q: How do I define a new unit test?

A: Use language-appropriate constructs, like GTest for C++. You can define a new file if need be, such as:

(in a BUILD.gn file)

executable("unittests") {
  output_name = "scenic_unittests"
  testonly = true
  sources = ["some_test.cc"],
  deps = [":some_dep"],

Q: What ensures it is run?

A: An unbroken chain of dependencies that roll up to your fx set command's universe of available packages (expandable using the --with flag), typically going through the all target of //<layer>/packages/tests/BUILD.gn, such as //garnet/packages/tests:all.

For example:


is an executable, listed under the “tests” stanza of


which is a package, which is listed in a package group


which is itself included in the catch-all group,


Your product definition (typically one found in products/ may or may not transitively include this test group. If it doesn't, add it to your fx set command, like so:

fx set ... --with //garnet/packages/tests:scenic_tests

Typically, one just adds a new test to an existing binary, or a new test binary to an existing package.

Q: How do I run this unit test on a QEMU instance?

There‘s the easy way if your QEMU has networking, and the hard way if it doesn’t.

A (with networking): In one terminal, start your QEMU instance with fx run -N. Next, on another terminal, type in fx run-test scenic_tests.

This invocation runs all the test executables in the scenic_tests package.

A (no networking): Start a QEMU instance (fx run), and then manually invoke the runtests command.

In the QEMU shell, type in runtests /pkgfs/packages/scenic_tests/0/test. The argument is a specific directory containing the test executables.

Note Well! Without networking, the files are loaded into the QEMU instance at startup. So after rebuilding a test, you'll need to shutdown and re-start the QEMU instance to see the rebuilt test.

To exit QEMU, dm shutdown.

Q: How do I run this unit test on my development device?

A: Either manual invocation, like in QEMU, or fx run-test to a running device.

Note that the booted device may not contain your binary at startup, but fx run-test will build the test binary, ship it over to the device, and run it, while piping the output back to your workstation terminal. Slick!

Make sure your device is running (hit Ctrl-D to boot an existing image) and connected to your workstation.

From your workstation, fx run-test scenic_tests will serially run through all test executables contained in the scenic_tests package.

To run just one test executable, fx run-test scenic_test -t scenic_unittests, where the argument to -t is the executable name.

You can automatically rebuild, install, and run your tests on every source file change with fx -i. For instance: fx -i run-test scenic_tests.

Q: Where are the test results captured?

A: The output is directed to your terminal.

There does exist a way to write test output into files (including a summary JSON file), which is how CQ bots collect the test output for automated runs.

Q: How to disable a test? How to find and run disabled tests?

A: To temporary prevent a particular test to being run as part of a test suite, you can mark it as disabled. Disabled tests are defined by having their name prefixed with DISABLED_. One way to find them is therefore simply git grep DISABLED_.

If running the test outputs YOU HAVE 1 DISABLED TEST, you can also pass the following flags to find out which test is disabled: fx run-test scenic_tests -- --gtest_list_tests --gtest_filter=*DISABLED_*.

To force-run disabled tests: fx run-test scenic_tests -- --gtest_also_run_disabled_tests.

Alternatively, you may also want to disable an entire test executable within a package containing several test executables. To do this, edit the BUILD.gn as follows: tests = [ { name = "scenic_unittests", disabled = true } ]. As a result, scenic_unittests will be put in a disabled sub-directory of /pkgfs/packages/<package_name>/0/test, and will not be run by the CQ system.

Q: How do I run a bunch of tests automatically? How do I ensure all dependencies are tested?

A: Upload your patch to Gerrit and do a CQ dry run.

Q: How do I run this unit test in a CQ dry run?

A: Clicking on CQ dry run (aka +1) will take your CL's properly defined unit test and run it on multiple bots, one for each build target (x86-64 versus arm64, release versus debug). Each job will have an output page showing all the tests that ran.