Re: Why does git-grep appear to treat exclude pathspecs differently?

From: D. Ben Knoble <ben.knoble@gmail.com>
Date: Jul 30, 2025, 21:49 UTC
Message-ID: <CALnO6CDNDfx6L7CmPwE0eBZFkd_JkZf6hDwrADccAb14QhJGDg@mail.gmail.com>
In-Reply-To: <xmqqv7nezdw2.fsf@gitster.g>

On Sat, Jul 26, 2025 at 8:16 PM Junio C Hamano <gitster@pobox.com> wrote:

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>
> "D. Ben Knoble" <ben.knoble@gmail.com> writes:
>
> > With Git 2.48.1, I observe the following behavior:
> >
> > - "git ls-files :^:Documentation/RelNotes | grep Rel" yields
> > "RelNotes", as expected
>
> It is deliberately confusing to spell ":(exclude)" as ":^:".

What makes you say that? It's documented in "git help revisions":

           A pathspec that begins with a colon : has special meaning. In the
           short form, the leading colon : is followed by zero or more "magic
           signature" letters (which optionally is terminated by another colon
           :), and the remainder is the pattern to match against the path.

and

           exclude
               After a path matches any non-exclude pathspec, it will be run
               through all exclude pathspecs (magic signature: ! or its synonym
               ^).

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>
> > - "git grep squash :^:Documentation/RelNotes" yields the error
> >
> > fatal: ambiguous argument ':^:Documentation/RelNotes': unknown
> > revision or path not in the working tree.
>
> I think if you write it in longhand,
>
>     $ git grep squash ':(exclude)Documentation/RelNotes'
>
> you would not see such an error.

Indeed, I left this syntax out of my original, but it works.

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> The error message comes from setup.c:die_verify_filename(), I think,
> and setup.c:looks_like_pathspec() allows the control flow to avoid
> calling that filename verification code path.  It knows to let the
> longhand magic pathspec go, and it may be trivial to teach it a
> shorthand magic too, but I offhand do not know the implications of
> such a change---there might be unintended consequences.

Hm. Running a debugger, this looks accurate. We are in the "!seen_dashdash" case of builtin/grep.c, with the call

    verify_filename(prefix=0x0000000000000000,
arg=":^:Documentation/RelNotes", diagnose_misspelt_rev=1)

which eventually dies as noted. However:

- looks_like_pathspec() only checks for long magic, as you noted
- setup.c:check_filename() looks for short-magic, too, but only
considers ":^" to work like a pathname if we're excluding everything?

I think what I find confusing is that, while this is definitely a DWIM case for git-grep, it doesn't seem to do DWIM :) We are verifying that the remaining arguments are filenames, but couldn't they really be full pathspecs, as long as they aren't revisions?

The difference for "^:<path>" from ":^:<path>", just to complete the
story, is that in setup.c:check_filename() we try to stat whatever
comes after ":^":
- for the former, that's <path> and we are ok
- for the latter, that's :<path>, fail.

So it seems like the places that check for short-magic should also consider the optional trailing colon?

-- 
D. Ben Knoble