<section xmlns="http://docbook.org/ns/docbook"
        xmlns:xlink="http://www.w3.org/1999/xlink"
        xmlns:xi="http://www.w3.org/2001/XInclude"
        version="5.0"
        xml:id="sec-writing-nixos-tests">
 <title>Writing Tests</title>

 <para>
  A NixOS test is a Nix expression that has the following structure:
<programlisting>
import ./make-test-python.nix {

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

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

  testScript =
    ''
      <replaceable>Python code…</replaceable>
    '';
}
</programlisting>
  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,
  <filename
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/login.nix">login.nix</filename>
  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,
  <filename
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/nfs/simple.nix">nfs/simple.nix</filename>,
  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:
<!-- FIXME: would be nice to generate this automatically. -->
  <variablelist>
   <varlistentry>
    <term>
     <option>virtualisation.memorySize</option>
    </term>
    <listitem>
     <para>
      The memory of the VM in megabytes.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <option>virtualisation.vlans</option>
    </term>
    <listitem>
     <para>
      The virtual networks to which the VM is connected. See
      <filename
    xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/nat.nix">nat.nix</filename>
      for an example.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <option>virtualisation.writableStore</option>
    </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>
  For more options, see the module
  <filename
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/virtualisation/qemu-vm.nix">qemu-vm.nix</filename>.
 </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><replaceable>name</replaceable></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:
<programlisting>
machine.start()
machine.wait_for_unit("default.target")
if not "Linux" in machine.succeed("uname"):
  raise Exception("Wrong OS")
</programlisting>
  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:
<programlisting>
start_all()
</programlisting>
 </para>

 <para>
  The following methods are available on machine objects:
  <variablelist>
   <varlistentry>
    <term>
     <methodname>start</methodname>
    </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>
     <methodname>shutdown</methodname>
    </term>
    <listitem>
     <para>
      Shut down the machine, waiting for the VM to exit.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>crash</methodname>
    </term>
    <listitem>
     <para>
      Simulate a sudden power failure, by telling the VM to exit immediately.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>block</methodname>
    </term>
    <listitem>
     <para>
      Simulate unplugging the Ethernet cable that connects the machine to the
      other machines.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>unblock</methodname>
    </term>
    <listitem>
     <para>
      Undo the effect of <methodname>block</methodname>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>screenshot</methodname>
    </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>
     <methodname>get_screen_text</methodname>
    </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 <option>enableOCR</option> to the test attribute
       set.
      </para>
     </note>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>send_monitor_command</methodname>
    </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>
     <methodname>send_key</methodname>
    </term>
    <listitem>
     <para>
      Simulate pressing keys on the virtual keyboard, e.g.,
      <literal>send_key("ctrl-alt-delete")</literal>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>send_chars</methodname>
    </term>
    <listitem>
     <para>
      Simulate typing a sequence of characters on the virtual keyboard, e.g.,
      <literal>send_chars("foobar\n")</literal> will type the string
      <literal>foobar</literal> followed by the Enter key.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>execute</methodname>
    </term>
    <listitem>
     <para>
      Execute a shell command, returning a list
      <literal>(<replaceable>status</replaceable>,
      <replaceable>stdout</replaceable>)</literal>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>succeed</methodname>
    </term>
    <listitem>
     <para>
      Execute a shell command, raising an exception if the exit status is not
      zero, otherwise returning the standard output.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>fail</methodname>
    </term>
    <listitem>
     <para>
      Like <methodname>succeed</methodname>, but raising an exception if the
      command returns a zero status.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>wait_until_succeeds</methodname>
    </term>
    <listitem>
     <para>
      Repeat a shell command with 1-second intervals until it succeeds.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>wait_until_fails</methodname>
    </term>
    <listitem>
     <para>
      Repeat a shell command with 1-second intervals until it fails.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>wait_for_unit</methodname>
    </term>
    <listitem>
     <para>
      Wait until the specified systemd unit has reached the “active” state.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>wait_for_file</methodname>
    </term>
    <listitem>
     <para>
      Wait until the specified file exists.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>wait_for_open_port</methodname>
    </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>
     <methodname>wait_for_closed_port</methodname>
    </term>
    <listitem>
     <para>
      Wait until nobody is listening on the given TCP port.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>wait_for_x</methodname>
    </term>
    <listitem>
     <para>
      Wait until the X11 server is accepting connections.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>wait_for_text</methodname>
    </term>
    <listitem>
     <para>
      Wait until the supplied regular expressions matches the textual contents
      of the screen by using optical character recognition (see
      <methodname>get_screen_text</methodname>).
     </para>
     <note>
      <para>
       This requires passing <option>enableOCR</option> to the test attribute
       set.
      </para>
     </note>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>wait_for_console_text</methodname>
    </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>
     <methodname>wait_for_window</methodname>
    </term>
    <listitem>
     <para>
      Wait until an X11 window has appeared whose name matches the given
      regular expression, e.g., <literal>wait_for_window("Terminal")</literal>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <methodname>copy_from_host</methodname>
    </term>
    <listitem>
     <para>
      Copies a file from host to machine, e.g.,
      <literal>copy_from_host("myfile", "/etc/my/important/file")</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>
     <methodname>systemctl</methodname>
    </term>
    <listitem>
     <para>
      Runs <literal>systemctl</literal> commands with optional support for
      <literal>systemctl --user</literal>
     </para>
     <para>
<programlisting>
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`
</programlisting>
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </para>

 <para>
  To test user units declared by <literal>systemd.user.services</literal> the
  optional <literal>user</literal> argument can be used:
<programlisting>
machine.start()
machine.wait_for_x()
machine.wait_for_unit("xautolock.service", "x-session-user")
</programlisting>
  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):
<programlisting>
import ./make-test-python.nix {
  skipLint = true;
  machine =
    { config, pkgs, ... }:
    { <replaceable>configuration…</replaceable>
    };

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