nixpkgs/nixos/doc/manual/development/writing-nixos-tests.section.md

Writing Tests

A NixOS test is a module that has the following structure:

{

  # One or more machines:
  nodes =
    { machine =
        { config, pkgs, ... }: { /* ... */ };
      machine2 =
        { config, pkgs, ... }: { /* ... */ };
      # …
    };

  testScript =
    ''
      Python code…
    '';
}

We refer to the whole test above as a test module, whereas the values in nodes.<name> are NixOS modules themselves.

The option testScript is a piece of Python code that executes the test (described below). During the test, it will start one or more virtual machines, the configuration of which is described by the option nodes.

An example of a single-node test is login.nix. It only needs a single machine to test whether users can log in on the virtual console, whether device ownership is correctly maintained when switching between consoles, and so on. An interesting multi-node test is nfs/simple.nix. It uses two client nodes to test correct locking across server crashes.

Calling a test

Tests are invoked differently depending on whether the test is part of NixOS or lives in a different project.

Testing within NixOS

Tests that are part of NixOS are added to nixos/tests/all-tests.nix.

{
  hostname = runTest ./hostname.nix;
}

Overrides can be added by defining an anonymous module in all-tests.nix.

{
  hostname = runTest {
    imports = [ ./hostname.nix ];
    defaults.networking.firewall.enable = false;
  };
}

You can run a test with attribute name hostname in nixos/tests/all-tests.nix by invoking:

cd /my/git/clone/of/nixpkgs
nix-build -A nixosTests.hostname

Testing outside the NixOS project

Outside the nixpkgs repository, you can instantiate the test by first importing the NixOS library,

let nixos-lib = import (nixpkgs + "/nixos/lib") { };
in

nixos-lib.runTest {
  imports = [ ./test.nix ];
  hostPkgs = pkgs;  # the Nixpkgs package set used outside the VMs
  defaults.services.foo.package = mypkg;
}

runTest returns a derivation that runs the test.

Configuring the nodes

There are a few special NixOS options for test VMs:

virtualisation.memorySize

The memory of the VM in megabytes.

virtualisation.vlans

The virtual networks to which the VM is connected. See nat.nix for an example.

virtualisation.writableStore

By default, the Nix store in the VM is not writable. If you enable this option, a writable union file system is mounted on top of the Nix store to make it appear writable. This is necessary for tests that run Nix operations that modify the store.

For more options, see the module qemu-vm.nix.

The test script is a sequence of Python statements that perform various actions, such as starting VMs, executing commands in the VMs, and so on. Each virtual machine is represented as an object stored in the variable name if this is also the identifier of the machine in the declarative config. If you specified a node nodes.machine, the following example starts the machine, waits until it has finished booting, then executes a command and checks that the output is more-or-less correct:

machine.start()
machine.wait_for_unit("default.target")
if not "Linux" in machine.succeed("uname"):
  raise Exception("Wrong OS")

The first line is technically unnecessary; machines are implicitly started when you first execute an action on them (such as wait_for_unit or succeed). If you have multiple machines, you can speed up the test by starting them in parallel:

start_all()

If the hostname of a node contains characters that can't be used in a Python variable name, those characters will be replaced with underscores in the variable name, so nodes.machine-a will be exposed to Python as machine_a.

Machine objects

The following methods are available on machine objects:

@PYTHON_MACHINE_METHODS@

To test user units declared by systemd.user.services the optional user argument can be used:

machine.start()
machine.wait_for_x()
machine.wait_for_unit("xautolock.service", "x-session-user")

This applies to systemctl, get_unit_info, wait_for_unit, start_job and stop_job.

For faster dev cycles it's also possible to disable the code-linters (this shouldn't be committed though):

{
  skipLint = true;
  nodes.machine =
    { config, pkgs, ... }:
    { # configuration…
    };

  testScript =
    ''
      Python code…
    '';
}

This will produce a Nix warning at evaluation time. To fully disable the linter, wrap the test script in comment directives to disable the Black linter directly (again, don't commit this within the Nixpkgs repository):

{
  testScript =
    ''
      # fmt: off
      Python code…
      # fmt: on
    '';
}

Similarly, the type checking of test scripts can be disabled in the following way:

{
  skipTypeCheck = true;
  nodes.machine =
    { config, pkgs, ... }:
    { # configuration…
    };
}

Failing tests early

To fail tests early when certain invariants are no longer met (instead of waiting for the build to time out), the decorator polling_condition is provided. For example, if we are testing a program foo that should not quit after being started, we might write the following:

@polling_condition
def foo_running():
    machine.succeed("pgrep -x foo")


machine.succeed("foo --start")
machine.wait_until_succeeds("pgrep -x foo")

with foo_running:
    ...  # Put `foo` through its paces

polling_condition takes the following (optional) arguments:

seconds_interval

specifies how often the condition should be polled:

@polling_condition(seconds_interval=10)
def foo_running():
    machine.succeed("pgrep -x foo")

description

is used in the log when the condition is checked. If this is not provided, the description is pulled from the docstring of the function. These two are therefore equivalent:

@polling_condition
def foo_running():
    "check that foo is running"
    machine.succeed("pgrep -x foo")

@polling_condition(description="check that foo is running")
def foo_running():
    machine.succeed("pgrep -x foo")

Adding Python packages to the test script

When additional Python libraries are required in the test script, they can be added using the parameter extraPythonPackages. For example, you could add numpy like this:

{
  extraPythonPackages = p: [ p.numpy ];

  nodes = { };

  # Type checking on extra packages doesn't work yet
  skipTypeCheck = true;

  testScript = ''
    import numpy as np
    assert str(np.zeros(4)) == "[0. 0. 0. 0.]"
  '';
}

In that case, numpy is chosen from the generic python3Packages.

Test Options Reference

The following options can be used when writing tests.

id-prefix: test-opt-
list-id: test-options-list
source: @NIXOS_TEST_OPTIONS_JSON@