nix.dev/source/tutorials/nixos/integration-testing-using-virtual-machines.md

(integration-testing-vms)=

Integration testing with NixOS virtual machines

This tutorial aims to be compatible with NixOS release 22.11.

What will you learn?

This tutorial introduces the functionality of Nixpkgs to write automated tests to debug NixOS configurations independent of a working NixOS installation.

What do you need?

• A working Nix installation or NixOS.
• Basic knowledge of the Nix language.
• Basic knowledge of NixOS configuration.

Introduction

Nixpkgs provides a test environment to automate integration testing for distributed systems. It allows defining tests based on a set of declarative NixOS configurations and using a Python shell to interact with them through QEMU as the backend. Those tests are widely used to ensure that NixOS works as intended, so in general they are called NixOS Tests. They can be written and launched outside of NixOS, on any Linux machine¹.

Integration tests are reproducible due to the design properties of Nix, making them a valuable part of a Continuous Integration (CI) pipeline.

The nixosTest function

NixOS VM tests are defined using the nixosTest function. The pattern for NixOS VM tests looks like this:

let
  nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/nixos-22.11";
  pkgs = import nixpkgs { config = {}; overlays = []; };
in

pkgs.nixosTest {
  name = "test-name";
  nodes = {
    machine1 = { config, pkgs, ... }: {
      # ...
    };
    machine2 = { config, pkgs, ... }: {
      # ...
    };
  }
  testScript = { nodes, ... }: ''
    # ...
  '';
}

The function nixosTest takes a module to specify the test options. Because this module only sets configuration values, one can use the abbreviated form of the module convention. The following configuration values must be set:

• name defines the name of the test.

• nodes contains a set of named configurations, because a test script can involve more than one virtual machine. Each virtual machine is created from a NixOS configuration.

• testScript defines the Python test script, either as literal string or as a function that takes a nodes attribute. This Python test script can access the virtual machines via the names used for the nodes. It has super user rights in the virtual machines. In the Python script each virtual machine is accessible via the machine object. NixOS provides various methods to run tests on these configurations.

The test framework automatically starts the virtual machines and runs the Python script.

Minimal example

As a minimal test on the default configuration, we will check if the user root and alice can run Firefox. We will build the example up from scratch.

1. As recommended we use an explicitly pinned version of Nixpkgs, and explicitly set configuration options and overlays to avoid them being inadvertently overridden by global configuration:

   let
     nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/nixos-22.11";
     pkgs = import nixpkgs { config = {}; overlays = []; };
   in
   
   pkgs.nixosTest {
     # ...
   }
   

2. Label the test with a descriptive name such as "minimal-test":

   name = "minimal-test";
   

3. Because this example only uses one virtual machine, the node we specify is simply called machine. This name is arbitrary and can be chosen freely. As configuration you use the relevant parts of the default configuration, that we used in a previous tutorial:

   nodes.machine = { config, pkgs, ... }: {
     users.users.alice = {
       isNormalUser = true;
       extraGroups = [ "wheel" ];
       packages = with pkgs; [
         firefox
         tree
       ];
     };
   
     system.stateVersion = "22.11";
   };
   

4. This is the test script:

   machine.wait_for_unit("default.target")
   machine.succeed("su -- alice -c 'which firefox'")
   machine.fail("su -- root -c 'which firefox'")
   

   This Python script refers to machine which is the name chosen for the virtual machine configuration used in the nodes attribute set.

   The script waits until systemd reaches default.target. It uses the su command to switch between users and the which command to check if the user has access to firefox. It expects that the command which firefox to succeed for user alice and to fail for root.

   This script will be the value of the testScript attribute.

The complete minimal-test.nix file content looks like the following:

let
  nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/nixos-22.11";
  pkgs = import nixpkgs { config = {}; overlays = []; };
in

pkgs.nixosTest {
  name = "minimal-test";

  nodes.machine = { config, pkgs, ... }: {

    users.users.alice = {
      isNormalUser = true;
      extraGroups = [ "wheel" ];
      packages = with pkgs; [
        firefox
        tree
      ];
    };

    system.stateVersion = "22.11";
  };

  testScript = ''
    machine.wait_for_unit("default.target")
    machine.succeed("su -- alice -c 'which firefox'")
    machine.fail("su -- root -c 'which firefox'")
  '';
}

Running tests

To set up all machines and run the test script:

$ nix-build minimal-test.nix

...
test script finished in 10.96s
cleaning up
killing machine (pid 10)
(0.00 seconds)
/nix/store/bx7z3imvxxpwkkza10vb23czhw7873w2-vm-test-run-minimal-test

Interactive Python shell in the virtual machine

When developing tests or when something breaks, it’s useful to interactively tinker with the test or access a terminal for a machine.

To start an interactive Python session with the testing framework:

$ $(nix-build -A driverInteractive minimal-test.nix)/bin/nixos-test-driver

Here you can run any of the testing operations. Execute the testScript attribute from minimal-test.nix with the test_script() function.

If a virtual machine is not yet started, the test environment takes care of it on the first call of a method on a machine object.

But you can also manually trigger the start of the virtual machine with:

>>> machine.start()

for a specific node,

or

>>> start_all()

for all nodes.

You can enter a interactive shell on the virtual machine using:

>>> machine.shell_interact()

and run shell commands like:

uname -a

Linux server 5.10.37 #1-NixOS SMP Fri May 14 07:50:46 UTC 2021 x86_64 GNU/Linux

Re-running successful tests

Because test results are kept in the Nix store, a successful test is cached. This means that Nix will not run the test a second time as long as the test setup (node configuration and test script) stays semantically the same. Therefore, to run a test again, one needs to remove the result.

If you would try to delete the result using the symbolic link, you will get the following error:

nix-store --delete ./result

finding garbage collector roots...
0 store paths deleted, 0.00 MiB freed
error: Cannot delete path '/nix/store/4klj06bsilkqkn6h2sia8dcsi72wbcfl-vm-test-run-unnamed' since it is still alive. To find out why, use: nix-store --query --roots

Instead, remove the symbolic link and only then remove the cached result:

rm ./result
nix-store --delete /nix/store/4klj06bsilkqkn6h2sia8dcsi72wbcfl-vm-test-run-unnamed

This can be also done with one command:

result=$(readlink -f ./result) rm ./result && nix-store --delete $result

Tests with multiple virtual machines

Tests can involve multiple virtual machines, for example to test client-server-communication.

The following example setup includes:

• A virtual machine named server running nginx with default configuration.
• A virtual machine named client that has curl available to make an HTTP request.
• A testScript orchestrating testing logic between client and server.

The complete client-server-test.nix file content looks like the following:

let
  nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/nixos-22.11";
  pkgs = import nixpkgs { config = {}; overlays = []; };
in

pkgs.nixosTest {
  name = "client-server-test";

  nodes.server = { pkgs, ... }: {
    networking = {
      firewall = {
        allowedTCPPorts = [ 80 ];
      };
    };
    services.nginx = {
      enable = true;
      virtualHosts."server" = {};
    };
  };

  nodes.client = { pkgs, ... }: {
    environment.systemPackages = with pkgs; [
      curl
    ];
  };

  testScript = ''
    server.wait_for_unit("default.target")
    client.wait_for_unit("default.target")
    client.succeed("curl http://server/ | grep -o \"Welcome to nginx!\"")
  '';
}

The test script performs the following steps:

1. Start the server and wait for it to be ready.
2. Start the client and wait for it to be ready.
3. Run curl on the client and use grep to check the expected return string. The test passes or fails based on the return value.

Run the test:

$ nix-build server-client-test.nix

Additional information regarding NixOS tests

• Running integration tests on CI requires hardware acceleration, which many CIs do not support.

  To run integration tests in GitHub Actions see how to disable hardware acceleration.

• NixOS comes with a large set of tests that can serve as educational examples.

  A good inspiration is Matrix bridging with an IRC.

---

1. Support for running NixOS VM tests on macOS is also implemented but currently undocumented. ↩︎