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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()
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.
::: {.note} This requires
enableOCR
to be set totrue
. ::: get_screen_text
-
Return a textual representation of what is currently visible on the machine's screen using optical character recognition.
::: {.note} This requires
enableOCR
to be set totrue
. ::: 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 stringfoobar
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 aresleep 365d &
, where the shell forks a new process that can write to stdout andxclip -i
, where thexclip
command itself forks without closing stdout.Takes an optional parameter
check_return
that defaults toTrue
. Setting this parameter toFalse
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)
orexecute(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 isNone
by default. Seeexecute
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)
. Seeexecute
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
andget_screen_text_variants
).::: {.note} This requires
enableOCR
to be set totrue
. ::: 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 possible 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 forsystemctl --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
orCtrl-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 andCtrl-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 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) == "array([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.
<xi:include href="../../generated/test-options-db.xml" xpointer="test-options-list"/>