Date:   Mon, 23 Nov 2020 14:13:41 -0800
From:   Daniel Latypov <dlatypov@...gle.com>
To:     Brendan Higgins <brendanhiggins@...gle.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        David Gow <davidgow@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH] Documentation: kunit: provide guidance for testing many inputs

On Mon, Nov 23, 2020 at 10:32 AM Brendan Higgins
<brendanhiggins@...gle.com> wrote:
>
> On Mon, Nov 2, 2020 at 1:37 PM Daniel Latypov <dlatypov@...gle.com> wrote:
> >
> > usage.rst goes into a detailed about faking out classes, but currently
> > lacks wording about how one might idiomatically test a range of inputs.
> >
> > 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@...gle.com>
>
> Aside from the minor comment I made below, I like the patch; it is a
> definite improvement, but I think the test you wrote that ultimately
> led to this documentation fix had more information in it than this
> documentation. I think it only contains the pattern that you outlined
> here, but I think it does include some other best practices. Maybe we
> should add some more documentation patches with more code examples in
> the future?
>
> Anyway, like I said, I think this patch in and of itself looks pretty good.
>
> Reviewed-by: Brendan Higgins <brendanhiggins@...gle.com>
>
> > ---
> >  Documentation/dev-tools/kunit/usage.rst | 66 +++++++++++++++++++++++++
> >  1 file changed, 66 insertions(+)
> >
> > diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
> > index 62142a47488c..317390df2b96 100644
> > --- a/Documentation/dev-tools/kunit/usage.rst
> > +++ b/Documentation/dev-tools/kunit/usage.rst
> > @@ -451,6 +451,72 @@ We can now use it to test ``struct eeprom_buffer``:
> >                 destroy_eeprom_buffer(ctx->eeprom_buffer);
> >         }
> >
> > +Testing various inputs
> > +----------------------
>
> Since this, by my count, the second test pattern that we are
> introducing here, could we maybe call that out with a subheading or a
> new section or something? It would be nice if we could sort of build
> up a cookbook of testing patterns.

Good point, I noticed now the "Organization of this document" section
would need to be updated.
Perhaps something like

-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.

I'll send out a V2 shortly, changing the example per David's
suggestion and with the above.

>
> > +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 ``md5sum(1)``
> > +
> > +.. code-block:: c
> > +
> > +       /* Note: the cast is to satisfy overly strict type-checking. */
> > +       #define TEST_MD5(in, want) \
> > +               md5sum(in, out); \
> > +               KUNIT_EXPECT_STREQ_MSG(test, (char *)out, want, "md5sum(%s)", in);
> > +
> > +       char out[16];
> > +       TEST_MD5("hello world",   "5eb63bbbe01eeed093cb22bb8f5acdc3");
> > +       TEST_MD5("hello world!",  "fc3ff98e8c6a0d3087d515c0473f8677");
> > +
> > +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[16];
> > +
> > +       struct md5_test_case {
> > +               const char *str;
> > +               const char *md5;
> > +       };
> > +
> > +       struct md5_test_case cases[] = {
> > +               {
> > +                       .str = "hello world",
> > +                       .md5 = "5eb63bbbe01eeed093cb22bb8f5acdc3",
> > +               },
> > +               {
> > +                       .str = "hello world!",
> > +                       .md5 = "fc3ff98e8c6a0d3087d515c0473f8677",
> > +               },
> > +       };
> > +       for (i = 0; i < ARRAY_SIZE(cases); ++i) {
> > +               md5sum(cases[i].str, out);
> > +               KUNIT_EXPECT_STREQ_MSG(test, (char *)out, cases[i].md5,
> > +                                     "md5sum(%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 had a magical ``undo_md5sum`` function, we could reuse ``cases``.
> > +
> >  .. _kunit-on-non-uml:
> >
> >  KUnit on non-UML architectures
> >
> > base-commit: 77c8473edf7f7664137f555cfcdc8c460bbd947d
> > --
> > 2.29.1.341.ge80a0c044ae-goog
> >

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