Re: [PATCH] diff: fix behaviour of the "-s" option

> Junio C Hamano <gitster@pobox.com> writes:
>
>> I haven't run any tests (not just your new one, but existing ones)
>> but ...
>
> And of course, not writing tests fails to even realize that the bug
> has two components, "-s" failing to clear the bits previously set,
> and other options not clearing the bit set by "-s".
>
> This version may still be rough, but at least the full test suite
> has been run with it, so I have a bit more confidence than the
> earlier one (which may not mean much).
>
> ------- >8 ------------- >8 ------------- >8 -------------
> Sergey Organov noticed and reported "--patch --no-patch --raw"
> behaves differently from "--raw".  It turns out there are a few
> interesting bugs in the implementation and documentation.
>
>  * First, the documentation for "--no-patch" was unclear that it
>    could be read to mean "--no-patch" countermands an earlier
>    "--patch" but not other things.  The intention of "--no-patch"
>    ever since it was introduced at d09cd15d (diff: allow --no-patch
>    as synonym for -s, 2013-07-16) was to serve as a synonym for
>    "-s", so "--raw --patch --no-patch" should have produced no
>    output, but it can be (mis)read to allow showing only "--raw"
>    output.
>
>  * Then the interaction between "-s" and other format options were
>    poorly implemented.  Modern versions of Git uses one bit each to
>    represent formatting options like "--patch", "--stat" in a single
>    output_format word, but for historical reasons, "-s" also is
>    represented as another bit in the same word.  This allows two
>    interesting bugs to happen, and we have both.
>
>    (1) After setting a format bit, then setting NO_OUTPUT with "-s",
>        the code to process another "--<format>" option drops the
>        NO_OUTPUT bit to allow output to be shown again.  However,
>        the code to handle "-s" only set NO_OUTPUT without unsetting
>        format bits set earlier, so the earlier format bit got
>        revealed upon seeing the second "--<format>" option.  THis is
>        the problem Sergey observed.
>
>    (2) After setting NO_OUTPUT with "-s", code to process
>        "--<format>" option can forget to unset NO_OUTPUT, leaving
>        the command still silent.
>
> It is tempting to change the meaning of "--no-patch" to mean
> "disable only the patch format output" and reimplement "-s" as "not
> showing anything", but it would be an end-user visible change in
> behaviour.  Let's fix the interactions of these bits to first make
> "-s" work as intended.
>
> The fix is conceptually very simple.
>
>  * Whenever we set DIFF_FORMAT_FOO becasuse we saw the "--foo"
>    option (e.g. DIFF_FORMAT_RAW is set when the "--raw" option is
>    given), we make sure we drop DIFF_FORMAT_NO_OUTPUT.  We forgot to
>    do so in some of the options and caused (2) above.
>
>  * When processing "-s" option, we should not just set
>    DIFF_FORMAT_NO_OUTPUT bit, but clear other DIFF_FORMAT_* bits.
>    We didn't do so and retained format bits set by options
>    previously seen, causing (1) above.