Documentation: Add Function Redirection API docs

.. SPDX-License-Identifier: GPL-2.0

========================

Function Redirection API

========================

Overview

========

When writing unit tests, it's important to be able to isolate the code being

tested from other parts of the kernel. This ensures the reliability of the test

(it won't be affected by external factors), reduces dependencies on specific

hardware or config options (making the test easier to run), and protects the

stability of the rest of the system (making it less likely for test-specific

state to interfere with the rest of the system).

While for some code (typically generic data structures, helpers, and other

"pure functions") this is trivial, for others (like device drivers,

filesystems, core subsystems) the code is heavily coupled with other parts of

the kernel.

This coupling is often due to global state in some way: be it a global list of

devices, the filesystem, or some hardware state. Tests need to either carefully

manage, isolate, and restore state, or they can avoid it altogether by

replacing access to and mutation of this state with a "fake" or "mock" variant.

By refactoring access to such state, such as by introducing a layer of

indirection which can use or emulate a separate set of test state. However,

such refactoring comes with its own costs (and undertaking significant

refactoring before being able to write tests is suboptimal).

A simpler way to intercept and replace some of the function calls is to use

function redirection via static stubs.

Static Stubs

============

Static stubs are a way of redirecting calls to one function (the "real"

function) to another function (the "replacement" function).

It works by adding a macro to the "real" function which checks to see if a test

is running, and if a replacement function is available. If so, that function is

called in place of the original.

Using static stubs is pretty straightforward:

1. Add the KUNIT_STATIC_STUB_REDIRECT() macro to the start of the "real"

   function.

   This should be the first statement in the function, after any variable

   declarations. KUNIT_STATIC_STUB_REDIRECT() takes the name of the

   function, followed by all of the arguments passed to the real function.

   For example:

   .. code-block:: c

	void send_data_to_hardware(const char *str)

	{

		KUNIT_STATIC_STUB_REDIRECT(send_data_to_hardware, str);

		/* real implementation */

	}

2. Write one or more replacement functions.

   These functions should have the same function signature as the real function.

   In the event they need to access or modify test-specific state, they can use

   kunit_get_current_test() to get a struct kunit pointer. This can then

   be passed to the expectation/assertion macros, or used to look up KUnit

   resources.

   For example:

   .. code-block:: c

	void fake_send_data_to_hardware(const char *str)

	{

		struct kunit *test = kunit_get_current_test();

		KUNIT_EXPECT_STREQ(test, str, "Hello World!");

	}

3. Activate the static stub from your test.

   From within a test, the redirection can be enabled with

   kunit_activate_static_stub(), which accepts a struct kunit pointer,

   the real function, and the replacement function. You can call this several

   times with different replacement functions to swap out implementations of the

   function.

   In our example, this would be

   .. code-block:: c

	kunit_activate_static_stub(test,

				   send_data_to_hardware,

				   fake_send_data_to_hardware);

4. Call (perhaps indirectly) the real function.

   Once the redirection is activated, any call to the real function will call

   the replacement function instead. Such calls may be buried deep in the

   implementation of another function, but must occur from the test's kthread.

   For example:

   .. code-block:: c

	send_data_to_hardware("Hello World!"); /* Succeeds */

	send_data_to_hardware("Something else"); /* Fails the test. */

5. (Optionally) disable the stub.

   When you no longer need it, disable the redirection (and hence resume the

   original behaviour of the 'real' function) using

   kunit_deactivate_static_stub(). Otherwise, it will be automatically disabled

   when the test exits.

   For example:

   .. code-block:: c

	kunit_deactivate_static_stub(test, send_data_to_hardware);

It's also possible to use these replacement functions to test to see if a

function is called at all, for example:

.. code-block:: c

	void send_data_to_hardware(const char *str)

	{

		KUNIT_STATIC_STUB_REDIRECT(send_data_to_hardware, str);

		/* real implementation */

	}

	/* In test file */

	int times_called = 0;

	void fake_send_data_to_hardware(const char *str)

	{

		times_called++;

	}

	...

	/* In the test case, redirect calls for the duration of the test */

	kunit_activate_static_stub(test, send_data_to_hardware, fake_send_data_to_hardware);

	send_data_to_hardware("hello");

	KUNIT_EXPECT_EQ(test, times_called, 1);

	/* Can also deactivate the stub early, if wanted */

	kunit_deactivate_static_stub(test, send_data_to_hardware);

	send_data_to_hardware("hello again");

	KUNIT_EXPECT_EQ(test, times_called, 1);

API Reference

=============

.. kernel-doc:: include/kunit/static_stub.h

   :internal:

API Reference

=============

.. toctree::

	:hidden:

	test

	resource

	functionredirection

This section documents the KUnit kernel testing API. It is divided into the

This page documents the KUnit kernel testing API. It is divided into the

following sections:

Documentation/dev-tools/kunit/api/test.rst

 - documents all of the standard testing API

 - Documents all of the standard testing API

Documentation/dev-tools/kunit/api/resource.rst

 - documents the KUnit resource API

 - Documents the KUnit resource API

Documentation/dev-tools/kunit/api/functionredirection.rst

 - Documents the KUnit Function Redirection API