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From:   Eric Sunshine <sunshine@sunshineco.com>
Date:   Sun, 9 Aug 2020 15:34:57 -0400
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Subject: Re: [PATCH v2] test_cmp: diagnose incorrect arguments
To:     Junio C Hamano <gitster@pobox.com>
Cc:     Git List <git@vger.kernel.org>, Elijah Newren <newren@gmail.com>,
        Shourya Shukla <shouryashukla.oo@gmail.com>
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On Sun, Aug 9, 2020 at 3:12 PM Junio C Hamano <gitster@pobox.com> wrote:
> The motivation makes quite a lot of sense.
>
> I suspect that it is a bug to use "-", i.e. "read from the standard
> input", for "$2", because
>
>  (1) if it is used for the expected output, we got the comparison in
>      the wrong direction;

I had the same concern that it didn't necessarily make sense to allow
"-" for $2, but eventually thought of a case such as:

    sed '...' actual | test_cmp expect - &&

which massages 'actual' before test_cmp() sees it. True, we don't
actually have such callers, but it theoretically is legitimate.

>  (2) if it is used for the actual output, we have a command whose
>      output we are interested in on the upstream side of a pipe to
>      discard its exit status.

If the command upstream is a Git command, that could indeed be a
reason for concern, but, as illustrated above, it could just be a
system command.