[PATCH 7/7] Documentation: kunit: rewrite "Writing tests"

Rewrite the documentation for clarity. Major points:

* Switch to third person point of view * Briefly describe code examples before giving them out * Use "base" and "derived" terminology on class inheritance

Signed-off-by: Bagas Sanjaya bagasdotme@gmail.com --- Documentation/dev-tools/kunit/usage.rst | 322 +++++++++++++----------- 1 file changed, 173 insertions(+), 149 deletions(-)

diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst index 2737863ef36532..e529da3fd1d32b 100644 --- a/Documentation/dev-tools/kunit/usage.rst +++ b/Documentation/dev-tools/kunit/usage.rst @@ -6,9 +6,11 @@ Writing Tests Test Cases ----------

-The fundamental unit in KUnit is the test case. A test case is a function with -the signature ``void (*)(struct kunit *test)``. It calls the function under test -and then sets *expectations* for what should happen. For example: +The fundamental unit in KUnit is the test case. A test case in KUnit is a +function with the signature ``void (*)(struct kunit *test)``. It calls the +function under test and then sets *expectations* for what should happen. + +Below are the simplest examples:

.. code-block:: c

@@ -22,18 +24,17 @@ and then sets *expectations* for what should happen. For example: }

In the above example, ``example_test_success`` always passes because it does -nothing; no expectations are set, and therefore all expectations pass. On the -other hand ``example_test_failure`` always fails because it calls ``KUNIT_FAIL``, -which is a special expectation that logs a message and causes the test case to -fail. +nothing (there are no expectations set). On the +other hand ``example_test_failure`` always fails because it calls +``KUNIT_FAIL``, which is a special function that logs the message string and +signal the test as failed.

Expectations ~~~~~~~~~~~~ -An *expectation* specifies that we expect a piece of code to do something in a -test. An expectation is called like a function. A test is made by setting -expectations about the behavior of a piece of code under test. When one or more -expectations fail, the test case fails and information about the failure is -logged. For example: +An *expectation* specifies the expected behavior of tested function. It is +written as regular function call. A test is made by asserting one or more +expectations in the test case. When any of these is not satisfied, the test +case fails and information about the failure is logged. For example:

.. code-block:: c

@@ -43,18 +44,16 @@ logged. For example: KUNIT_EXPECT_EQ(test, 2, add(1, 1)); }

-In the above example, ``add_test_basic`` makes a number of assertions about the -behavior of a function called ``add``. The first parameter is always of type +In the above example, ``add_test_basic`` tests a function called ``add()``. +The first parameter to ``KUNIT_EXPECT_EQ`` is always of type ``struct kunit *``, which contains information about the current test context. -The second parameter, in this case, is what the value is expected to be. The -last value is what the value actually is. If ``add`` passes all of these -expectations, the test case, ``add_test_basic`` will pass; if any one of these -expectations fails, the test case will fail. +The second parameter is what the value is expected to be returned by the +function. The last is value returned by calling the tested function.

-A test case *fails* when any expectation is violated; however, the test will -continue to run, and try other expectations until the test case ends or is -otherwise terminated. This is as opposed to *assertions* which are discussed -later. +Even though a single expectation is not satisfied, the test will +continue to run and try asserting the following expectations until either +the test case ends or is terminated. This is as opposed to *assertions* which +are discussed later.

To learn about more KUnit expectations, see Documentation/dev-tools/kunit/api/test.rst.

@@ -62,9 +61,8 @@ To learn about more KUnit expectations, see Documentation/dev-tools/kunit/api/te A single test case should be short, easy to understand, and focused on a single behavior.

-For example, if we want to rigorously test the ``add`` function above, create -additional tests cases which would test each property that an ``add`` function -should have as shown below: +The example below extends ``add()`` tests with cases for negative values +and edge cases involving ``INT_MIN`` and ``INT_MAX`` constants:

.. code-block:: c

@@ -93,8 +91,10 @@ should have as shown below: Assertions ~~~~~~~~~~

-An assertion is like an expectation, except that the assertion immediately -terminates the test case if the condition is not satisfied. For example: +An assertion is like an expectation, except that it immediately +terminates the test case if the condition is false. + +The following example demonstrate testing ``sort()`` function:

.. code-block:: c

@@ -112,21 +112,22 @@ terminates the test case if the condition is not satisfied. For example: KUNIT_EXPECT_LE(test, a[i], a[i + 1]); }

-In this example, the method under test should return pointer to a value. If the -pointer returns null or an errno, we want to stop the test since the following -expectation could crash the test case. `ASSERT_NOT_ERR_OR_NULL(...)` allows us -to bail out of the test case if the appropriate conditions are not satisfied to -complete the test. +In this example, the method under test should return pointer to a value. If +it instead returns null pointer or errno, the test case should be bailed out +with ``KUNIT_ASSERT_NOT_ERR_OR_NULL()`` since the following expectation that +asserts the array order could crash it.

Test Suites ~~~~~~~~~~~

-We need many test cases covering all the unit's behaviors. It is common to have -many similar tests. In order to reduce duplication in these closely related -tests, most unit testing frameworks (including KUnit) provide the concept of a -*test suite*. A test suite is a collection of test cases for a unit of code -with optional setup and teardown functions that run before/after the whole -suite and/or every test case. For example: +It is common to have many similar tests cases in order to test the code +behavior. In order to reduce duplication in these closely related +tests, most unit testing frameworks (including KUnit) provide the *test suite* +concept. A test suite is a collection of test cases for a particularcode +with optional setup and cleanup functions that run before/after the whole +suite and/or every test case. + +Below is an example of writing a test suite:

.. code-block:: c

@@ -147,21 +148,21 @@ suite and/or every test case. For example: }; kunit_test_suite(example_test_suite);

-In the above example, the test suite ``example_test_suite`` would first run -``example_suite_init``, then run the test cases ``example_test_foo``, -``example_test_bar``, and ``example_test_baz``. Each would have -``example_test_init`` called immediately before it and ``example_test_exit`` -called immediately after it. Finally, ``example_suite_exit`` would be called -after everything else. ``kunit_test_suite(example_test_suite)`` registers the -test suite with the KUnit test framework. +In the above example, running ``example_test_suite`` will initialize the +suite with ``example_suite_init``, then run three cases ``example_test_foo``, +``example_test_bar``, and ``example_test_baz``. Each case will be initialized +with ``example_test_init`` and ``example_test_exit`` will take care of +case-specific cleanup after the case have been finished. Finally, +``example_suite_exit`` will do suite-wide cleanup, then finish the suite. +``kunit_test_suite(example_test_suite)`` registers the test suite with KUnit.

.. note:: A test case will only run if it is associated with a test suite.

-``kunit_test_suite(...)`` is a macro which tells the linker to put the -specified test suite in a special linker section so that it can be run by KUnit -either after ``late_init``, or when the test module is loaded (if the test was -built as a module). +``kunit_test_suite()`` is a macro which tells the linker to put the specified +test suite in a special linker section so that it can be run either after +``late_init``, or when the test module is loaded (if the test was built as +module).

For more information, see Documentation/dev-tools/kunit/api/test.rst.

@@ -170,25 +171,23 @@ For more information, see Documentation/dev-tools/kunit/api/test.rst. Writing Tests For Other Architectures -------------------------------------

-It is better to write tests that run on UML to tests that only run under a -particular architecture. It is better to write tests that run under QEMU or -another easy to obtain (and monetarily free) software environment to a specific -piece of hardware. +In many cases, it is better to write tests that can be run on UML or QEMU +without requiring any specific architecture or hardware, since these can +be practically run by any machine that is capable of cross-compiling.

-Nevertheless, there are still valid reasons to write a test that is architecture -or hardware specific. For example, we might want to test code that really -belongs in ``arch/some-arch/*``. Even so, try to write the test so that it does -not depend on physical hardware. Some of our test cases may not need hardware, -only few tests actually require the hardware to test it. When hardware is not -available, instead of disabling tests, we can skip them. +Nevertheless, there are still valid reasons to write architecture-specific +or hardware-specific tests. For example, you might want to test codes that +are in ``arch/some-arch/*``. Even so, try to write tests so that it does +not require specific hardware to run them. A test suite may contain +hardware-specific cases. These can be skipped if the hardware is not +available.

-Now that we have narrowed down exactly what bits are hardware specific, the -actual procedure for writing and running the tests is same as writing normal -KUnit tests. +Writing architecture-specific and hardware-specific tests is the same as +writing any other tests.

.. important:: - We may have to reset hardware state. If this is not possible, we may only - be able to run one test case per invocation. + In some cases you need to reset hardware state after each test case, + otherwise only one case can be run per test suite.

.. TODO(brendanhiggins@google.com): Add an actual example of an architecture- dependent KUnit test. @@ -200,38 +199,41 @@ Isolating Behavior ------------------

Unit testing limits the amount of code under test to a single unit. It controls -what code gets run when the unit under test calls a function. Where a function -is exposed as part of an API such that the definition of that function can be +what code gets run when the unit under test calls a function. When a function +is exposed as part of an API, the function definition can be changed without affecting the rest of the code base. In the kernel, this comes from two constructs: classes, which are structs that contain function pointers -provided by the implementer, and architecture-specific functions, which have -definitions selected at compile time. +provided by the implementer; and architecture-specific functions, which have +definitions determined at compile time.

Classes ~~~~~~~

-Classes are not a construct that is built into the C programming language; -however, it is an easily derived concept. Accordingly, in most cases, every -project that does not use a standardized object oriented library (like GNOME's -GObject) has their own slightly different way of doing object oriented -programming; the Linux kernel is no exception. +The C programming language does not have the formal notion of class-based +object-oriented programming, hovewer the paradigm can be applied within +procedural framework. Accordingly, every project has their own slightly +different way of doing object oriented programming (such as GNOME's GObject); +the Linux kernel is no exception.

The central concept in kernel object oriented programming is the class. In the kernel, a *class* is a struct that contains function pointers. This creates a contract between *implementers* and *users* since it forces them to use the -same function signature without having to call the function directly. To be a -class, the function pointers must specify that a pointer to the class, known as -a *class handle*, be one of the parameters. Thus the member functions (also -known as *methods*) have access to member variables (also known as *fields*) -allowing the same implementation to have multiple *instances*. +same function signature without having to call the function directly. In order +to be a class, the function pointers must specify that a pointer to the class +(known as a *class handle*) be one of the parameters; thus the member functions +(also known as *methods*) have access to member variables (also known as +*fields*), allowing the same implementation to have multiple *instances*.

-A class can be *overridden* by *child classes* by embedding the *parent class* -in the child class. Then when the child class *method* is called, the child -implementation knows that the pointer passed to it is of a parent contained -within the child. Thus, the child can compute the pointer to itself because the -pointer to the parent is always a fixed offset from the pointer to the child. -This offset is the offset of the parent contained in the child struct. For -example: +A class can be *overridden* by *derived classes* by embedding the *base class* +in the derived class. Then when the dervied class *method* is called, the +derived class implementation knows that the pointer passed to it is of a base +class contained within the derived class. Thus, the child can compute the +pointer to itself because the pointer to the base class is always a fixed +offset from the pointer to the derived class. This offset is the offset of the +base class contained in the struct of derived class. + +The example below defines a base class ``shape`` and derived class +``rectangle`` along with class implementations:

.. code-block:: c

@@ -259,8 +261,8 @@ example: self->width = width; }

-In this example, computing the pointer to the child from the pointer to the -parent is done by ``container_of``. +In this example, computing the pointer to ``rectangle`` from the pointer to +the ``shape`` is taken care of by ``container_of`` method.

Faking Classes ~~~~~~~~~~~~~~ @@ -269,11 +271,13 @@ In order to unit test a piece of code that calls a method in a class, the behavior of the method must be controllable, otherwise the test ceases to be a unit test and becomes an integration test.

-A fake class implements a piece of code that is different than what runs in a -production instance, but behaves identical from the standpoint of the callers. -This is done to replace a dependency that is hard to deal with, or is slow. For -example, implementing a fake EEPROM that stores the "contents" in an -internal buffer. Assume we have a class that represents an EEPROM: +A fake class implements a piece of code that interfaces to actual code +used in production. This is done to replace code dependencies that is hard to +deal with (expensive or impossible to duplicate), or is slow. + +The examples below shows how to test fake EEPROM implementation that stores +its contents in an internal buffer. Assume that there is ``eeprom`` class, +which is defined as:

.. code-block:: c

@@ -282,7 +286,8 @@ internal buffer. Assume we have a class that represents an EEPROM: ssize_t (*write)(struct eeprom *this, size_t offset, const char *buffer, size_t count); };

-And we want to test code that buffers writes to the EEPROM: +Supposes that you want to test ``eeprom_buffer`` class, which writes the +contents to actual EEPROM:

.. code-block:: c

@@ -295,7 +300,8 @@ And we want to test code that buffers writes to the EEPROM: struct eeprom_buffer *new_eeprom_buffer(struct eeprom *eeprom); void destroy_eeprom_buffer(struct eeprom *eeprom);

-We can test this code by *faking out* the underlying EEPROM: +In order to test ``eeprom_buffer``, you need *faking out* the underlying +EEPROM with ``fake_eeprom``, which is derived from ``eeprom``:

.. code-block:: c

@@ -331,7 +337,7 @@ We can test this code by *faking out* the underlying EEPROM: memset(this->contents, 0, FAKE_EEPROM_CONTENTS_SIZE); }

-We can now use it to test ``struct eeprom_buffer``: +You can now use it to test ``eeprom_buffer``:

.. code-block:: c

@@ -425,11 +431,12 @@ We can now use it to test ``struct eeprom_buffer``: Testing Against Multiple Inputs -------------------------------

-Testing just a few inputs is not enough to ensure that the code works correctly, -for example: testing a hash function. +Sometimes in order to correctly test the code, many inputs are required. In +such cases, you can write a helper macro or function. The helper can be +called for each test input.

-We can write a helper macro or function. The function is called for each input. -For example, to test ``sha1sum(1)``, we can write: +The following example defines ``TEST_SHA1`` helper macro for testing +:manpage:`sha1sum(1)`. The macro is called on 2 test cases:

.. code-block:: c

@@ -444,12 +451,15 @@ For example, to test ``sha1sum(1)``, we can write: Note the use of the ``_MSG`` version of ``KUNIT_EXPECT_STREQ`` to print a more detailed error and make the assertions clearer within the helper macros.

-The ``_MSG`` variants are useful when the same expectation is called multiple -times (in a loop or helper function) and thus the line number is not enough to -identify what failed, as shown below. +The ``_MSG`` variants are useful when the same expectation is asserted +multiple times (in a loop or helper function), since the line number alone is +not enough to identify the failure, as shown below.

-In complicated cases, we recommend using a *table-driven test* compared to the -helper macro variation, for example: +In complicated cases, it is recommended to use *table-driven test* pattern +instead. + +The following example does the same test as above, but the test cases are +defined in an array of struct:

.. code-block:: c

@@ -478,18 +488,14 @@ helper macro variation, for example: }

-There is more boilerplate code involved, but it can: +There are advantages of *table-driven tests*:

-* be more readable when there are multiple inputs/outputs (due to field names). - - * For example, see ``fs/ext4/inode-test.c``. - -* reduce duplication if test cases are shared across multiple tests. - - * For example: if we want to test ``sha256sum``, we could add a ``sha256`` - field and reuse ``cases``. - -* be converted to a "parameterized test". +* The test is more readable when there are many inputs and expected outputs. + See ``fs/ext4/inode-test.c`` for example. +* It can reduce duplication if test cases are shared across multiple tests. + For example, if you want to also test :manpage:`sha256sum(1)`, you can + simply add ``sha256`` field to ``cases``. +* The test can be turned into "parameterized test", see below subsection.

Parameterized Testing ~~~~~~~~~~~~~~~~~~~~~ @@ -497,8 +503,8 @@ Parameterized Testing The table-driven testing pattern is common enough that KUnit has special support for it.

-By reusing the same ``cases`` array from above, we can write the test as a -"parameterized test" with the following. +The following example does the same :manpage:`sha1sum(1)` test as above, +but is written using parameterized testing facilities:

.. code-block:: c

@@ -526,7 +532,7 @@ By reusing the same ``cases`` array from above, we can write the test as a // Creates `sha1_gen_params()` to iterate over `cases`. KUNIT_ARRAY_PARAM(sha1, cases, case_to_desc);

- // Looks no different from a normal test. + // Looks no different from other tests. static void sha1_test(struct kunit *test) { // This function can just contain the body of the for-loop. @@ -539,7 +545,7 @@ By reusing the same ``cases`` array from above, we can write the test as a "sha1sum(%s)", test_param->str); }

- // Instead of KUNIT_CASE, we use KUNIT_CASE_PARAM and pass in the + // Instead of KUNIT_CASE, use KUNIT_CASE_PARAM and pass in the // function declared by KUNIT_ARRAY_PARAM. static struct kunit_case sha1_test_cases[] = { KUNIT_CASE_PARAM(sha1_test, sha1_gen_params), @@ -549,9 +555,13 @@ By reusing the same ``cases`` array from above, we can write the test as a Exiting Early on Failed Expectations ------------------------------------

-We can use ``KUNIT_EXPECT_EQ`` to mark the test as failed and continue -execution. In some cases, it is unsafe to continue. We can use the -``KUNIT_ASSERT`` variant to exit on failure. +All the tests until now uses ``KUNIT_EXPECT`` macros to assert expectations. +In case of any of these are failed, the test continues. However, in some +cases, continuing tests is not possible or is unsafe. For these cases, +you can use ``KUNIT_ASSERT`` variant to exit early on failure. + +The example below tests allocating objects then play with them. Only if +the allocation succeed, these objects can be played with:

.. code-block:: c

@@ -567,12 +577,15 @@ execution. In some cases, it is unsafe to continue. We can use the Allocating Memory -----------------

-Where you might use ``kzalloc``, you can instead use ``kunit_kzalloc`` as KUnit -will then ensure that the memory is freed once the test completes. +When ``kzalloc`` may be used to allocate memory, you can instead use +``kunit_kzalloc`` as KUnit will then ensure that the memory is freed once +the test completes.

-This is useful because it lets us use the ``KUNIT_ASSERT_EQ`` macros to exit -early from a test without having to worry about remembering to call ``kfree``. -For example: +This is useful because it lets you to use ``KUNIT_ASSERT_EQ`` macros to exit +early from a test without having to worry about remembering to freeing +memory with ``kfree``. + +The following example tests allocating buffer memory:

.. code-block:: c

@@ -589,9 +602,11 @@ For example: Testing Static Functions ------------------------

-If we do not want to expose functions or variables for testing, one option is to -conditionally ``#include`` the test file at the end of your .c file. For -example: +If you do not want to expose testing functions or variables unconditionally, +you can ``#include`` the test file inside ``#ifdef`` guard. + +In the example below, the test code in ``my_kunit_test.c`` is included and +compiled only if ``CONFIG_MY_KUNIT_TEST`` is enabled:

.. code-block:: c

@@ -606,7 +621,11 @@ example: Injecting Test-Only Code ------------------------

-Similar to as shown above, we can add test-specific logic. For example: +You can also add test-only logic inside the guard. + +In the following example, prototype of ``test_only_hook()`` is defined with +``CONFIG_MY_KUNIT_TEST`` both enabled and disabled. In case the configuration +is disabled, the hook is defined as empty function:

.. code-block:: c

@@ -619,19 +638,21 @@ Similar to as shown above, we can add test-specific logic. For example: void test_only_hook(void) { } #endif

-This test-only code can be made more useful by accessing the current ``kunit_test`` -as shown in next section: *Accessing The Current Test*. +This can be made more useful by accessing the current ``kunit_test`` +as shown in next section below.

Accessing The Current Test --------------------------

-In some cases, we need to call test-only code from outside the test file. -For example, see example in section *Injecting Test-Only Code* or if -we are providing a fake implementation of an ops struct. Using -``kunit_test`` field in ``task_struct``, we can access it via -``current->kunit_test``. +In some cases, it is desired to call test-only code from outside the test +file. See example from previous section for how this can be done by +including the test file.

-The example below includes how to implement "mocking": +Another way is to provide the fake implementation of an ops struct. For +example, given ``kunit_test`` field in ``task_struct``, the field can be +accessed via ``current->kunit_test``. + +The example below shows how to implement "mocking" pattern:

.. code-block:: c

@@ -665,25 +686,28 @@ The example below includes how to implement "mocking": KUNIT_EXPECT_EQ(test, fake_foo(1), 42); }

-In this example, we are using the ``priv`` member of ``struct kunit`` as a way +In this example, ``kunit->priv`` is used as a way of passing data to the test from the init function. In general ``priv`` is pointer that can be used for any user data. This is preferred over static variables, as it avoids concurrency issues.

-Had we wanted something more flexible, we could have used a named ``kunit_resource``. -Each test can have multiple resources which have string names providing the same -flexibility as a ``priv`` member, but also, for example, allowing helper +In cases where you want something more flexible, ``kunit_resource`` can be +used instead. Each test can have multiple resources which have names providing +the same flexibility as ``priv``, but also, for example, allowing helper functions to create resources without conflicting with each other. It is also -possible to define a clean up function for each resource, making it easy to -avoid resource leaks. For more information, see Documentation/dev-tools/kunit/api/test.rst. +possible to define clean up routines for each resource, making it easy to +avoid leaking. For more information, see +Documentation/dev-tools/kunit/api/test.rst.

Failing The Current Test ------------------------

-If we want to fail the current test, we can use ``kunit_fail_current_test(fmt, args...)`` -which is defined in ``<kunit/test-bug.h>`` and does not require pulling in ``<kunit/test.h>``. -For example, we have an option to enable some extra debug checks on some data -structures as shown below: +If you want to fail the current test with a meaningful reason, you can use +``kunit_fail_current_test()``, which is defined in ``<kunit/test-bug.h>``. +It does not require pulling in ``<kunit/test.h>``. + +The following example have the extra validation over ``data`` struct, which +is only done when ``CONFIG_EXTRA_DEBUG_CHECKS`` is enabled:

.. code-block:: c