kernel/Documentation/dev-tools/kunit/tips.rst

.. SPDX-License-Identifier: GPL-2.0

============================
Tips For Writing KUnit Tests
============================

Exiting early on failed expectations
------------------------------------

``KUNIT_EXPECT_EQ`` and friends will mark the test as failed and continue
execution.  In some cases, it's unsafe to continue and you can use the
``KUNIT_ASSERT`` variant to exit on failure.

.. code-block:: c

	void example_test_user_alloc_function(struct kunit *test)
	{
		void *object = alloc_some_object_for_me();

		/* Make sure we got a valid pointer back. */
		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, object);
		do_something_with_object(object);
	}

Allocating memory
-----------------

Where you would use ``kzalloc``, you should prefer ``kunit_kzalloc`` instead.
KUnit will ensure the memory is freed once the test completes.

This is particularly useful since it lets you use the ``KUNIT_ASSERT_EQ``
macros to exit early from a test without having to worry about remembering to
call ``kfree``.

Example:

.. code-block:: c

	void example_test_allocation(struct kunit *test)
	{
		char *buffer = kunit_kzalloc(test, 16, GFP_KERNEL);
		/* Ensure allocation succeeded. */
		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, buffer);

		KUNIT_ASSERT_STREQ(test, buffer, "");
	}


Testing static functions
------------------------

If you don't want to expose functions or variables just for testing, one option
is to conditionally ``#include`` the test file at the end of your .c file, e.g.

.. code-block:: c

	/* In my_file.c */

	static int do_interesting_thing();

	#ifdef CONFIG_MY_KUNIT_TEST
	#include "my_kunit_test.c"
	#endif

Injecting test-only code
------------------------

Similarly to the above, it can be useful to add test-specific logic.

.. code-block:: c

	/* In my_file.h */

	#ifdef CONFIG_MY_KUNIT_TEST
	/* Defined in my_kunit_test.c */
	void test_only_hook(void);
	#else
	void test_only_hook(void) { }
	#endif

This test-only code can be made more useful by accessing the current kunit
test, see below.

Accessing the current test
--------------------------

In some cases, you need to call test-only code from outside the test file, e.g.
like in the example above or if you're providing a fake implementation of an
ops struct.
There is a ``kunit_test`` field in ``task_struct``, so you can access it via
``current->kunit_test``.

Here's a slightly in-depth example of how one could implement "mocking":

.. code-block:: c

	#include <linux/sched.h> /* for current */

	struct test_data {
		int foo_result;
		int want_foo_called_with;
	};

	static int fake_foo(int arg)
	{
		struct kunit *test = current->kunit_test;
		struct test_data *test_data = test->priv;

		KUNIT_EXPECT_EQ(test, test_data->want_foo_called_with, arg);
		return test_data->foo_result;
	}

	static void example_simple_test(struct kunit *test)
	{
		/* Assume priv is allocated in the suite's .init */
		struct test_data *test_data = test->priv;

		test_data->foo_result = 42;
		test_data->want_foo_called_with = 1;

		/* In a real test, we'd probably pass a pointer to fake_foo somewhere
		 * like an ops struct, etc. instead of calling it directly. */
		KUNIT_EXPECT_EQ(test, fake_foo(1), 42);
	}


Note: here we're able to get away with using ``test->priv``, but if you wanted
something more flexible you could use a named ``kunit_resource``, see
Documentation/dev-tools/kunit/api/test.rst.

Failing the current test
------------------------

But sometimes, you might just want to fail the current test. In that case, we
have ``kunit_fail_current_test(fmt, args...)`` which is defined in ``<kunit/test-bug.h>`` and
doesn't require pulling in ``<kunit/test.h>``.

E.g. say we had an option to enable some extra debug checks on some data structure:

.. code-block:: c

	#include <kunit/test-bug.h>

	#ifdef CONFIG_EXTRA_DEBUG_CHECKS
	static void validate_my_data(struct data *data)
	{
		if (is_valid(data))
			return;

		kunit_fail_current_test("data %p is invalid", data);

		/* Normal, non-KUnit, error reporting code here. */
	}
	#else
	static void my_debug_function(void) { }
	#endif


Customizing error messages
--------------------------

Each of the ``KUNIT_EXPECT`` and ``KUNIT_ASSERT`` macros have a ``_MSG`` variant.
These take a format string and arguments to provide additional context to the automatically generated error messages.

.. code-block:: c

	char some_str[41];
	generate_sha1_hex_string(some_str);

	/* Before. Not easy to tell why the test failed. */
	KUNIT_EXPECT_EQ(test, strlen(some_str), 40);

	/* After. Now we see the offending string. */
	KUNIT_EXPECT_EQ_MSG(test, strlen(some_str), 40, "some_str='%s'", some_str);

Alternatively, one can take full control over the error message by using ``KUNIT_FAIL()``, e.g.

.. code-block:: c

	/* Before */
	KUNIT_EXPECT_EQ(test, some_setup_function(), 0);

	/* After: full control over the failure message. */
	if (some_setup_function())
		KUNIT_FAIL(test, "Failed to setup thing for testing");

Next Steps
==========
*   Optional: see the Documentation/dev-tools/kunit/usage.rst page for a more
    in-depth explanation of KUnit.