<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-writing-nixos-tests">
  <title>Writing Tests</title>
  <para>
    A NixOS test is a Nix expression that has the following structure:
  </para>
  <programlisting language="bash">
import ./make-test-python.nix {

  # Either the configuration of a single machine:
  machine =
    { config, pkgs, ... }:
    { configuration…
    };

  # Or a set of machines:
  nodes =
    { machine1 =
        { config, pkgs, ... }: { … };
      machine2 =
        { config, pkgs, ... }: { … };
      …
    };

  testScript =
    ''
      Python code…
    '';
}
</programlisting>
  <para>
    The attribute <literal>testScript</literal> 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 <literal>machine</literal> (if you need
    only one machine in your test) or by the attribute
    <literal>nodes</literal> (if you need multiple machines). For
    instance,
    <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/login.nix"><literal>login.nix</literal></link>
    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. On the other hand,
    <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/nfs/simple.nix"><literal>nfs/simple.nix</literal></link>,
    which tests NFS client and server functionality in the Linux kernel
    (including whether locks are maintained across server crashes),
    requires three machines: a server and two clients.
  </para>
  <para>
    There are a few special NixOS configuration options for test VMs:
  </para>
  <variablelist>
    <varlistentry>
      <term>
        <literal>virtualisation.memorySize</literal>
      </term>
      <listitem>
        <para>
          The memory of the VM in megabytes.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>virtualisation.vlans</literal>
      </term>
      <listitem>
        <para>
          The virtual networks to which the VM is connected. See
          <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/nat.nix"><literal>nat.nix</literal></link>
          for an example.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>virtualisation.writableStore</literal>
      </term>
      <listitem>
        <para>
          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.
        </para>
      </listitem>
    </varlistentry>
  </variablelist>
  <para>
    For more options, see the module
    <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/virtualisation/qemu-vm.nix"><literal>qemu-vm.nix</literal></link>.
  </para>
  <para>
    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 <literal>name</literal> if this is also the
    identifier of the machine in the declarative config. If you didn't
    specify multiple machines using the <literal>nodes</literal>
    attribute, it is just <literal>machine</literal>. 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:
  </para>
  <programlisting language="python">
machine.start()
machine.wait_for_unit(&quot;default.target&quot;)
if not &quot;Linux&quot; in machine.succeed(&quot;uname&quot;):
  raise Exception(&quot;Wrong OS&quot;)
</programlisting>
  <para>
    The first line is actually unnecessary; machines are implicitly
    started when you first execute an action on them (such as
    <literal>wait_for_unit</literal> or <literal>succeed</literal>). If
    you have multiple machines, you can speed up the test by starting
    them in parallel:
  </para>
  <programlisting language="python">
start_all()
</programlisting>
  <para>
    The following methods are available on machine objects:
  </para>
  <variablelist>
    <varlistentry>
      <term>
        <literal>start</literal>
      </term>
      <listitem>
        <para>
          Start the virtual machine. This method is asynchronous — it
          does not wait for the machine to finish booting.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>shutdown</literal>
      </term>
      <listitem>
        <para>
          Shut down the machine, waiting for the VM to exit.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>crash</literal>
      </term>
      <listitem>
        <para>
          Simulate a sudden power failure, by telling the VM to exit
          immediately.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>block</literal>
      </term>
      <listitem>
        <para>
          Simulate unplugging the Ethernet cable that connects the
          machine to the other machines.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>unblock</literal>
      </term>
      <listitem>
        <para>
          Undo the effect of <literal>block</literal>.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>screenshot</literal>
      </term>
      <listitem>
        <para>
          Take a picture of the display of the virtual machine, in PNG
          format. The screenshot is linked from the HTML log.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>get_screen_text_variants</literal>
      </term>
      <listitem>
        <para>
          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.
        </para>
        <note>
          <para>
            This requires passing <literal>enableOCR</literal> to the
            test attribute set.
          </para>
        </note>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>get_screen_text</literal>
      </term>
      <listitem>
        <para>
          Return a textual representation of what is currently visible
          on the machine's screen using optical character recognition.
        </para>
        <note>
          <para>
            This requires passing <literal>enableOCR</literal> to the
            test attribute set.
          </para>
        </note>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>send_monitor_command</literal>
      </term>
      <listitem>
        <para>
          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.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>send_key</literal>
      </term>
      <listitem>
        <para>
          Simulate pressing keys on the virtual keyboard, e.g.,
          <literal>send_key(&quot;ctrl-alt-delete&quot;)</literal>.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>send_chars</literal>
      </term>
      <listitem>
        <para>
          Simulate typing a sequence of characters on the virtual
          keyboard, e.g.,
          <literal>send_chars(&quot;foobar\n&quot;)</literal> will type
          the string <literal>foobar</literal> followed by the Enter
          key.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>execute</literal>
      </term>
      <listitem>
        <para>
          Execute a shell command, returning a list
          <literal>(status, stdout)</literal>. If the command detaches,
          it must close stdout, as <literal>execute</literal> will wait
          for this to consume all output reliably. This can be achieved
          by redirecting stdout to stderr <literal>&gt;&amp;2</literal>,
          to <literal>/dev/console</literal>,
          <literal>/dev/null</literal> or a file. Examples of detaching
          commands are <literal>sleep 365d &amp;</literal>, where the
          shell forks a new process that can write to stdout and
          <literal>xclip -i</literal>, where the
          <literal>xclip</literal> command itself forks without closing
          stdout. Takes an optional parameter
          <literal>check_return</literal> that defaults to
          <literal>True</literal>. Setting this parameter to
          <literal>False</literal> 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.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>succeed</literal>
      </term>
      <listitem>
        <para>
          Execute a shell command, raising an exception if the exit
          status is not zero, otherwise returning the standard output.
          Commands are run with <literal>set -euo pipefail</literal>
          set:
        </para>
        <itemizedlist>
          <listitem>
            <para>
              If several commands are separated by <literal>;</literal>
              and one fails, the command as a whole will fail.
            </para>
          </listitem>
          <listitem>
            <para>
              For pipelines, the last non-zero exit status will be
              returned (if there is one, zero will be returned
              otherwise).
            </para>
          </listitem>
          <listitem>
            <para>
              Dereferencing unset variables fail the command.
            </para>
          </listitem>
          <listitem>
            <para>
              It will wait for stdout to be closed. See
              <literal>execute</literal> for the implications.
            </para>
          </listitem>
        </itemizedlist>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>fail</literal>
      </term>
      <listitem>
        <para>
          Like <literal>succeed</literal>, but raising an exception if
          the command returns a zero status.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>wait_until_succeeds</literal>
      </term>
      <listitem>
        <para>
          Repeat a shell command with 1-second intervals until it
          succeeds.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>wait_until_fails</literal>
      </term>
      <listitem>
        <para>
          Repeat a shell command with 1-second intervals until it fails.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>wait_for_unit</literal>
      </term>
      <listitem>
        <para>
          Wait until the specified systemd unit has reached the
          <quote>active</quote> state.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>wait_for_file</literal>
      </term>
      <listitem>
        <para>
          Wait until the specified file exists.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>wait_for_open_port</literal>
      </term>
      <listitem>
        <para>
          Wait until a process is listening on the given TCP port (on
          <literal>localhost</literal>, at least).
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>wait_for_closed_port</literal>
      </term>
      <listitem>
        <para>
          Wait until nobody is listening on the given TCP port.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>wait_for_x</literal>
      </term>
      <listitem>
        <para>
          Wait until the X11 server is accepting connections.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>wait_for_text</literal>
      </term>
      <listitem>
        <para>
          Wait until the supplied regular expressions matches the
          textual contents of the screen by using optical character
          recognition (see <literal>get_screen_text</literal> and
          <literal>get_screen_text_variants</literal>).
        </para>
        <note>
          <para>
            This requires passing <literal>enableOCR</literal> to the
            test attribute set.
          </para>
        </note>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>wait_for_console_text</literal>
      </term>
      <listitem>
        <para>
          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.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>wait_for_window</literal>
      </term>
      <listitem>
        <para>
          Wait until an X11 window has appeared whose name matches the
          given regular expression, e.g.,
          <literal>wait_for_window(&quot;Terminal&quot;)</literal>.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>copy_from_host</literal>
      </term>
      <listitem>
        <para>
          Copies a file from host to machine, e.g.,
          <literal>copy_from_host(&quot;myfile&quot;, &quot;/etc/my/important/file&quot;)</literal>.
        </para>
        <para>
          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.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>systemctl</literal>
      </term>
      <listitem>
        <para>
          Runs <literal>systemctl</literal> commands with optional
          support for <literal>systemctl --user</literal>
        </para>
        <programlisting language="python">
machine.systemctl(&quot;list-jobs --no-pager&quot;) # runs `systemctl list-jobs --no-pager`
machine.systemctl(&quot;list-jobs --no-pager&quot;, &quot;any-user&quot;) # spawns a shell for `any-user` and runs `systemctl --user list-jobs --no-pager`
</programlisting>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>shell_interact</literal>
      </term>
      <listitem>
        <para>
          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
          <literal>Ctrl-d</literal> or <literal>Ctrl-c</literal> also
          ends the guest session.
        </para>
      </listitem>
    </varlistentry>
  </variablelist>
  <para>
    To test user units declared by
    <literal>systemd.user.services</literal> the optional
    <literal>user</literal> argument can be used:
  </para>
  <programlisting language="python">
machine.start()
machine.wait_for_x()
machine.wait_for_unit(&quot;xautolock.service&quot;, &quot;x-session-user&quot;)
</programlisting>
  <para>
    This applies to <literal>systemctl</literal>,
    <literal>get_unit_info</literal>, <literal>wait_for_unit</literal>,
    <literal>start_job</literal> and <literal>stop_job</literal>.
  </para>
  <para>
    For faster dev cycles it's also possible to disable the code-linters
    (this shouldn't be commited though):
  </para>
  <programlisting language="bash">
import ./make-test-python.nix {
  skipLint = true;
  machine =
    { config, pkgs, ... }:
    { configuration…
    };

  testScript =
    ''
      Python code…
    '';
}
</programlisting>
  <para>
    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):
  </para>
  <programlisting language="bash">
  testScript =
    ''
      # fmt: off
      Python code…
      # fmt: on
    '';
</programlisting>
</section>
