# Writing Tests

A NixOS test is a Nix expression that has the following structure:

    import ./make-test-python.nix {

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

      testScript =
        ''
          Python code…
        '';
    }

The attribute `testScript` is a bit 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 attribute `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.

There are a few special NixOS configuration 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()

## Machine objects

The following methods are available on machine objects:

`start`

  Start the virtual machine. This method is asynchronous — it does not wait for the machine to finish booting.

`shutdown`

  Shut down the machine, waiting for the VM to exit.

`crash`

  Simulate a sudden power failure, by telling the VM to exit immediately.

`block`

  Simulate unplugging the Ethernet cable that connects the machine to the other machines.

`unblock`

  Undo the effect of `block`.

`screenshot`

  Take a picture of the display of the virtual machine, in PNG format. The screenshot is linked from the HTML log.

`get_screen_text_variants`

  Return a list of different interpretations of what is currently visible on the machine's screen using optical character recognition. The number and order of the interpretations is not specified and is subject to change, but if no exception is raised at least one will be returned.

  This requires passing `enableOCR` to the test attribute set.

`get_screen_text`

  Return a textual representation of what is currently visible on the machine's screen using optical character recognition.

  This requires passing `enableOCR` to the test attribute set.

`send_monitor_command`

  Send a command to the QEMU monitor. This is rarely used, but allows doing stuff such as attaching virtual USB disks to a running machine.

`send_key`

  Simulate pressing keys on the virtual keyboard, e.g., `send_key("ctrl-alt-delete")`.

`send_chars`

  Simulate typing a sequence of characters on the virtual keyboard, e.g., `send_chars("foobar\n")` will type the string `foobar` followed by the Enter key.

`send_console`

  Send keys to the kernel console. This allows interaction with the systemd emergency mode, for example. Takes a string that is sent, e.g., `send_console("\n\nsystemctl default\n")`.

`execute`

  Execute a shell command, returning a list `(status, stdout)`.

  Commands are run with `set -euo pipefail` set:

  - If several commands are separated by `;` and one fails, the command as a whole will fail.

  - For pipelines, the last non-zero exit status will be returned (if there is one; otherwise zero will be returned).

  - Dereferencing unset variables fails the command.

  - It will wait for stdout to be closed.

  If the command detaches, it must close stdout, as `execute` will wait for this to consume all output reliably. This can be achieved by redirecting stdout to stderr `>&2`, to `/dev/console`, `/dev/null` or a file. Examples of detaching commands are `sleep 365d &`, where the shell forks a new process that can write to stdout and `xclip -i`, where the `xclip` command itself forks without closing stdout.

  Takes an optional parameter `check_return` that defaults to `True`. Setting this parameter to `False` will not check for the return code and return -1 instead. This can be used for commands that shut down the VM and would therefore break the pipe that would be used for retrieving the return code.

  A timeout for the command can be specified (in seconds) using the optional `timeout` parameter, e.g., `execute(cmd, timeout=10)` or `execute(cmd, timeout=None)`. The default is 900 seconds.

`succeed`

  Execute a shell command, raising an exception if the exit status is not zero, otherwise returning the standard output. Similar to `execute`, except that the timeout is `None` by default. See `execute` for details on command execution.

`fail`

  Like `succeed`, but raising an exception if the command returns a zero status.

`wait_until_succeeds`

  Repeat a shell command with 1-second intervals until it succeeds. Has a default timeout of 900 seconds which can be modified, e.g. `wait_until_succeeds(cmd, timeout=10)`. See `execute` for details on command execution.

`wait_until_fails`

  Like `wait_until_succeeds`, but repeating the command until it fails.

`wait_for_unit`

  Wait until the specified systemd unit has reached the active state.

`wait_for_file`

  Wait until the specified file exists.

`wait_for_open_port`

  Wait until a process is listening on the given TCP port (on `localhost`, at least).

`wait_for_closed_port`

  Wait until nobody is listening on the given TCP port.

`wait_for_x`

  Wait until the X11 server is accepting connections.

`wait_for_text`

  Wait until the supplied regular expressions matches the textual contents of the screen by using optical character recognition (see `get_screen_text` and `get_screen_text_variants`).

  This requires passing `enableOCR` to the test attribute set.

`wait_for_console_text`

  Wait until the supplied regular expressions match a line of the serial console output. This method is useful when OCR is not possibile or accurate enough.

`wait_for_window`

  Wait until an X11 window has appeared whose name matches the given regular expression, e.g., `wait_for_window("Terminal")`.

`copy_from_host`

  Copies a file from host to machine, e.g., `copy_from_host("myfile", "/etc/my/important/file")`.

  The first argument is the file on the host. The file needs to be accessible while building the nix derivation. The second argument is the location of the file on the machine.

`systemctl`

  Runs `systemctl` commands with optional support for `systemctl --user`

      machine.systemctl("list-jobs --no-pager") # runs `systemctl list-jobs --no-pager`
      machine.systemctl("list-jobs --no-pager", "any-user") # spawns a shell for `any-user` and runs `systemctl --user list-jobs --no-pager`

`shell_interact`

  Allows you to directly interact with the guest shell. This should only be used during test development, not in production tests. Killing the interactive session with `Ctrl-d` or `Ctrl-c` also ends the guest session.

`console_interact`

  Allows you to directly interact with QEMU’s stdin. This should only be used during test development, not in production tests. Output from QEMU is only read line-wise. `Ctrl-c` kills QEMU and `Ctrl-d` closes console and returns to the test runner.

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 commited though):

    import ./make-test-python.nix {
      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
        '';

## Failing tests early

To fail tests early when certain invariables 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:

    ```py
    @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:

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

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