Documentation: kunit: provide guidance for testing many inputs

usage.rst goes into a detailed section about faking out classes, but
currently lacks wording about how one might idiomatically test a range
of inputs.

Add a new chapter for "Common Patterns" and group "Isolating behvaior"
and this new section under there.

Give an example of how one might test a hash function via macros/helper
funcs and a table-driven test and very briefly discuss pros and cons.

Also highlight the KUNIT_EXPECT_*_MSG() variants (that aren't mentioned
elsewhere [1]) which are particularly useful in these situations.

It is also criminally underused at the moment, only appearing in 2
tests (both written by people involved in KUnit).

[1] not even on
https://www.kernel.org/doc/html/latest/dev-tools/kunit/api/test.html

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: dlatypov

Documentation/dev-tools/kunit/usage.rst

@@ -15,10 +15,10 @@ project, see :doc:`start`.
 Organization of this document
 =============================
 
-This document is organized into two main sections: Testing and Isolating
-Behavior. The first covers what unit tests are and how to use KUnit to write
-them. The second covers how to use KUnit to isolate code and make it possible
-to unit test code that was otherwise un-unit-testable.
+This document is organized into two main sections: Testing and Common Patterns.
+The first covers what unit tests are and how to use KUnit to write them. The
+second covers common testing patterns, e.g. how to isolate code and make it
+possible to unit test code that was otherwise un-unit-testable.
 
 Testing
 =======
@@ -218,8 +218,11 @@ test was built in or not).
 
 For more information on these types of things see the :doc:`api/test`.
 
+Common Patterns
+===============
+
 Isolating Behavior
-==================
+------------------
 
 The most important aspect of unit testing that other forms of testing do not
 provide is the ability to limit the amount of code under test to a single unit.
@@ -233,7 +236,7 @@ implementer, and architecture-specific functions which have definitions selected
 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, pretty much every project
@@ -451,6 +454,74 @@ We can now use it to test ``struct eeprom_buffer``:
 		destroy_eeprom_buffer(ctx->eeprom_buffer);
 	}
 
+Testing against multiple inputs
+-------------------------------
+
+Testing just a few inputs might not be enough to have confidence that the code
+works correctly, e.g. for a hash function.
+
+In such cases, it can be helpful to have a helper macro or function, e.g. this
+fictitious example for ``sha1sum(1)``
+
+.. code-block:: c
+
+	/* Note: the cast is to satisfy overly strict type-checking. */
+	#define TEST_SHA1(in, want) \
+		sha1sum(in, out); \
+		KUNIT_EXPECT_STREQ_MSG(test, (char *)out, want, "sha1sum(%s)", in);
+
+	char out[40];
+	TEST_SHA1("hello world",  "2aae6c35c94fcfb415dbe95f408b9ce91ee846ed");
+	TEST_SHA1("hello world!", "430ce34d020724ed75a196dfc2ad67c77772d169");
+
+
+Note the use of ``KUNIT_EXPECT_STREQ_MSG`` to give more context when it fails
+and make it easier to track down. (Yes, in this example, ``want`` is likely
+going to be unique enough on its own).
+
+The ``_MSG`` variants are even more useful when the same expectation is called
+multiple times (in a loop or helper function) and thus the line number isn't
+enough to identify what failed, like below.
+
+In some cases, it can be helpful to write a *table-driven test* instead, e.g.
+
+.. code-block:: c
+
+	int i;
+	char out[40];
+
+	struct sha1_test_case {
+		const char *str;
+		const char *sha1;
+	};
+
+	struct sha1_test_case cases[] = {
+		{
+			.str = "hello world",
+			.sha1 = "2aae6c35c94fcfb415dbe95f408b9ce91ee846ed",
+		},
+		{
+			.str = "hello world!",
+			.sha1 = "430ce34d020724ed75a196dfc2ad67c77772d169",
+		},
+	};
+	for (i = 0; i < ARRAY_SIZE(cases); ++i) {
+		sha1sum(cases[i].str, out);
+		KUNIT_EXPECT_STREQ_MSG(test, (char *)out, cases[i].sha1,
+		                      "sha1sum(%s)", cases[i].str);
+	}
+
+
+There's more boilerplate involved, but it can:
+
+* be more readable when there are multiple inputs/outputs thanks to field names,
+
+  * E.g. see ``fs/ext4/inode-test.c`` for an example of both.
+* reduce duplication if test cases can be shared across multiple tests.
+
+  * E.g. if we wanted to also test ``sha256sum``, we could add a ``sha256``
+    field and reuse ``cases``.
+
 .. _kunit-on-non-uml:
 
 KUnit on non-UML architectures