Re: [PATCH v2 2/2] ref-filter: 'contents:trailers' show error if `:` is missing

From: Eric Sunshine <sunshine@sunshineco.com>
Date: Aug 24, 2020, 03:49 UTC
Message-ID: <CAPig+cScdV1ORSbqDuUiOEvCd6TYgkR=3GK8OCUu4yuoKVy5Pg@mail.gmail.com>
In-Reply-To: <CA+CkUQ8Gst2RTaXY6t+ytWu_9Pu7eqnRYRrnawRwYd_NN=u0Lg@mail.gmail.com>

On Sun, Aug 23, 2020 at 8:56 PM Hariom verma <hariom18599@gmail.com> wrote:

Show 26 quoted lines

> On Sat, Aug 22, 2020 at 12:47 AM Junio C Hamano <gitster@pobox.com> wrote:
> > Eric Sunshine <sunshine@sunshineco.com> writes:
> > > ...an alternative would have been something like:
> > >
> > >   else if (!strcmp(arg, "trailers")) {
> > >     if (trailers_atom_parser(format, atom, NULL, err))
> > >       return -1;
> > >   } else if (skip_prefix(arg, "trailers:", &arg)) {
> > >     if (trailers_atom_parser(format, atom, arg, err))
> > >       return -1;
> > >   }
> > >
> > > which is quite simple to reason about (though has the cost of a tiny
> > > bit of duplication).
> >
> > Yeah, that looks quite simple and straight-forward.
>
> Recently, I sent a patch series "Improvements to ref-filter"[1]. A
> patch in this patch series introduced "sanitize" modifier to "subject"
> atom. i.e "%(subject:sanitize)".
>
> What if in the future we also want "%(contents:subject:sanitize)" to work?
> We can use this helper any number of times, whenever there is a need.
>
> Sorry, I missed saying this earlier. But I don't prefer duplicating
> the code here.

Pushing back on a reviewer suggestion is fine. Explaining the reason for your position -- as you do here -- helps reviewers understand why you feel the way you do. My review suggestion about making it easier to reason about the code while avoiding a brand new function, at the cost of a minor amount of duplication, was made in the context of this one-off case in which the function increased cognitive load and was used just once (not knowing that you envisioned future callers). If you expect the new function to be re-used by upcoming changes, then that may be a good reason to keep it. Stating so in the commit message will help reviewers see beyond the immediate patch or patch series.

Aside from a couple minor style violations[1,2], I don't particularly oppose the helper function, though I have a quibble with the name check_format_field(), which I don't find helpful, and which (at least for me) increases the cognitive load. The increased cognitive load, I think, comes not only from the function name not spelling out what the function actually does, but also because the function is dual-purpose: it's both checking that the argument matches a particular token ("trailers", in this case) and extracting the sub-argument. Perhaps naming it match_and_extract_subarg() or something similar would help, though that's a mouthful.

But the observation about the function being dual-purpose (thus potentially confusing) brings up other questions. For instance, is it too special-purpose? If you foresee more callers in the future with multiple-token arguments such as `%(content:subject:sanitize)`, should the function provide more assistance by splitting out each of the sub-arguments rather than stopping at the first? Taking that even further, a generalized helper for "splitting" arguments like that might be useful at the top-level of contents_atom_parser() too, rather than only for specific arguments, such as "trailers". Of course, this may all be way too ambitious for this little bug fix series or even for whatever upcoming changes you're planning, thus not worth pursuing.

As for the helper's implementation, I might have written it like this:

    static int check_format_field(...)
    {
        const char *opt
        if (!strcmp(arg, field))
            *option = NULL;
        else if (skip_prefix(arg, field, opt) && *opt == ':')
            *option = opt + 1;
        else
            return 0;
        return 1;
    }

which is more compact and closer to what I suggested earlier for avoiding the helper function in the first place. But, of course, programming is quite subjective, and you may find your implementation easier to reason about. Plus, your version has the benefit of being slightly more optimal since it avoids an extra string scan, although that probably is mostly immaterial considering that contents_atom_parser() itself contains a long chain of potentially sub-optimal strcmp() and skip_prefix() calls.

Footnotes

[1]: use `if (!*opt)` rather than `if (*opt == '\0')`
[2]: cuddle the closing brace and `else` on the same line like this:
     `} else if (...) {`