Subject: Re: test-lib.sh musings: test_expect_failure considered harmful

From: Junio C Hamano <[email-masked]>
Date: Wed, 13 Oct 2021 10:16:18 -0700

Derrick Stolee <[email-masked]> writes:

>> But even with the shortcomings of expect_failure, it still is much
>> better than claiming that we expect a bogus outcome.
>> 
>> Improving the shortcomings of expect_failure would be a much better
>> use of our time than advocating an abuse of expect_sucess, I would
>> think.
>
> I agree that test_expect_failure has these drawbacks. I've recently
> been using _expect_success to document "bad" behavior so we can verify
> that behavior changes when that behavior is fixed. But it does have
> the drawback of looking like we claim the result is by design.

Yeah, I think I saw (and I think I used the same technique myself)
people expect a bad output with test_expect_success with an in-code
(not in-log) comment that explicitly says "This documents the
current behaviour, which is wrong", and that is a very acceptable
solution, I would think.

> One possible way to correct this is to create a "test_expected_failure"
> helper that could be placed on the step(s) of the &&-chain that are
> expected to fail. The helper could set some variable to true if the
> failure is hit, and false otherwise. It can also convert a failure
> into a positive result. Then, test_expect_failure could look for that
> variable's value (after verifying that the &&-chain returns success)
> to show that all expected failures completed correctly.

Yup, I would very much like the direction, and further imagine that
the above approach can be extended to ...

> This could have the side-effect of having a "fixed" test_expect_failure
> show as a failed test, not a "TODO" message.

... avoid such downside.  Perhaps call that magic "we know this step
fails currently" test_known_breakage and declare that we deprecate
the use of test_expect_failure in new tests.  Such a test might look
like this:

    test_expect_success 'commit error message should not duplicate' '
	test_when_finished "chmod -R u+rwx ." &&
	chmod u-rwx .git/objects/ &&
	orig_head=$(git rev-parse HEAD) &&
	test_must_fail git commit --allow-empty -m "read-only" 2>rawerr &&
	grep "insufficient permission" rawerr >err &&
	test_known_breakage test_line_count = 1 err &&
	new_head=$(git rev-parse HEAD) &&
	test "$orig_head" = "$new_head"
    '

which may use your trick to turn both failure and success to OK (to
let the remainder of the test to continue) but signal the
surrounding test_expect_success to say either "TODO know breakage"
or "Fixed".

Thanks.