Date:   Fri, 29 Jan 2021 12:51:03 +0800
From:   David Gow <davidgow@...gle.com>
To:     Daniel Latypov <dlatypov@...gle.com>
Cc:     Brendan Higgins <brendanhiggins@...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] kunit: don't show `1 == 1` in failed assertion messages

On Fri, Jan 29, 2021 at 10:26 AM Daniel Latypov <dlatypov@...gle.com> wrote:
>
> Currently, given something (fairly dystopian) like
> > KUNIT_EXPECT_EQ(test, 2 + 2, 5)
>
> KUnit will prints a failure message like this.
> >  Expected 2 + 2 == 5, but
> >      2 + 2 == 4
> >      5 == 5
>
> With this patch, the output just becomes
> >  Expected 2 + 2 == 5, but
> >      2 + 2 == 4
>
> This patch is slightly hacky, but it's quite common* to compare an
> expression to a literal integer value, so this can make KUnit less
> chatty in many cases. (This patch also fixes variants like
> KUNIT_EXPECT_GT, LE, et al.).
>
> It also allocates an additional string briefly, but given this only
> happens on test failures, it doesn't seem too bad a tradeoff.
> Also, in most cases it'll realize the lengths are unequal and bail out
> before the allocation.
>
> We could save the result of the formatted string to avoid wasting this
> extra work, but it felt cleaner to leave it as-is.
>
> Edge case: for something silly and unrealistic like
> > KUNIT_EXPECT_EQ(test, 4, 5);
>
> It'll generate this message with a trailing "but"
> >  Expected 2 + 2 == 5, but
> >  <next line of normal output>

I assume this is supposed to say "Expected 4 == 5" here.
(I tested it to make sure, and that's what it did here.)

Personally, I'd ideally like to get rid of the ", but", or even add a
"but 4 != 5" style second line. Particularly in case the next line in
the output might be confused for the rest of a sentence.

That being said, this is a pretty silly edge case: I'd be worried if
we ever saw that case in an actual submitted test. People might see it
a bit while debugging, though: particularly if they're using
KUNIT_EXPECT_EQ(test, 1, 2) as a way of forcing a test to fail. (I've
done this while testing tooling, for instance.)

>
> It didn't feel worth adding a check up-front to see if both sides are
> literals to handle this better.
>
> *A quick grep suggests 100+ comparisons to an integer literal as the
> right hand side.
>
> Signed-off-by: Daniel Latypov <dlatypov@...gle.com>
> ---

I tested this, and it works well: the results are definitely more
human readable. I could see it making things slightly more complicated
for people who wanted to automatically parse assertion errors, but
no-one is doing that, and the extra complexity is pretty minimal
anyway.

One thing which might be worth doing is expanding this to
KUNIT_EXPECT_STREQ() and/or KUNIT_EXPECT_PTR_EQ(). These have slightly
more complicated formatting (quotes, leading 0s, etc), though.
Comparing pointer literals is pretty unlikely to show up, though, so I
don't think it's as important. (I thought that maybe the KASAN shadow
memory tests might use them, but a quick look didn't reveal any.)

For the record, this is what STREQ()/PTR_EQ()/ failures with literals look like:
# example_simple_test: EXPECTATION FAILED at lib/kunit/kunit-example-test.c:31
Expected "abc" == "abd", but
    "abc" == abc
    "abd" == abd
# example_simple_test: EXPECTATION FAILED at lib/kunit/kunit-example-test.c:33
Expected 0x124 == 0x1234, but
    0x124 == 0000000000000124
    0x1234 == 0000000000001234

Either way, though, this is:

Tested-by: David Gow <davidgow@...gle.com>

Cheers,
-- David

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