Re: [PATCH 1/6] t/checkout-last: checkout - doesn't work after rebase

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 13, 2013, 20:38 UTC
Message-ID: <7vhah1u4o5.fsf@alter.siamese.dyndns.org>
In-Reply-To: <CALkWK0nH5Qjc-PNNH0cAhsgZje7VzuLGqXZwjRQESOxLt-T78g@mail.gmail.com>

Ramkumar Ramachandra <artagnon@gmail.com> writes:

Show 11 quoted lines

> Junio C Hamano wrote:
>> Why two?
>
> What breaks checkout - is the initial HEAD detachment (which writes
> that "checkout: " message), before anything else happens.  None of
> <onto>, <upstream>, and <branch> make any difference: I'm testing
> exactly the code that I patched.
>
> I have recently been told that I should be testing "end-user behavior"
> by treating the programs as black-boxes, instead of "implementation".
> What is your opinion on the issue?  Should I write more tests?

Of course, the behaviours that should be observable by end-users need be spelled out. Also, an impementation detail that cannot be observed or make any difference to the end-user experience should not be cast in stone.

The guideline is a good one, but you need to realize that there is a difference between "do not test implementation details" and "do not look at implementation when designing tests". Only the former is necessary and correct: it lets you avoid over-specifying the behaviour.

Sometimes you can trivially tell that some obvious implementations that may be different from what you have can break the expectation you are setting. For example, in v1.5.0, "rebase A B" literally ran "git-checkout B" as the first thing, and if you want the end-user to expect that @{-$n} does not resolve to B because such internal "checkout" is not an user action, covering "rebase A B", even if you know that the current implemention happens to be immune to such a form of breakage, would be a good way to future-proof your code.

That still is testing the behaviour. You are just taking advantage of the fact that you know the implementation and can anticipate how future changes by careless people could break the behaviour.