Re: [PATCH v3 3/3] core: convert build-time USE_NSEC into runtime core.useNanosec

From: D. Ben Knoble <ben.knoble@gmail.com>
Date: Aug 19, 2026, 12:53 UTC
Message-ID: <CALnO6CAZ-_k=+xTZwi-+s2aeKwgkoY5Z_iJjF6_sBDreKEsTaw@mail.gmail.com>
In-Reply-To: <xmqqh5krxnwd.fsf@gitster.g>

[Patrick, the below probably helps answer some of your questions as well.]

On Tue, Aug 18, 2026 at 2:51 PM Junio C Hamano <gitster@pobox.com> wrote:

Show 48 quoted lines

>
> "D. Ben Knoble" <ben.knoble@gmail.com> writes:
>
> > diff --git a/environment.c b/environment.c
> > index 6676e6f5ae..c7f6b801f4 100644
> > --- a/environment.c
> > +++ b/environment.c
> > @@ -571,6 +571,13 @@ int git_default_core_config(const char *var, const char *value,
> >               return 0;
> >       }
> >
> > +#ifndef NO_NSEC
> > +     if (!strcmp(var, "core.usenanosec")) {
> > +             cfg->use_nanosec = git_config_bool(var, value);
> > +             return 0;
> > +     }
> > +#endif
>
> What this hunk tells us: At build time, you could choose to ignore
> core.usenanosec configuration variable, preventing cfg->use_nanosec
> from getting flipped to true by the configured value.
>
> > @@ -769,6 +776,9 @@ void repo_config_values_init(struct repo_config_values *cfg)
> >       cfg->ignore_case = 0;
> >       cfg->trust_executable_bit = 1;
> >       cfg->has_symlinks = platform_has_symlinks();
> > +#ifndef NO_NSEC
> > +     cfg->use_nanosec = 0;
> > +#endif
>
> I think we want to unconditionally initialize it to 0, unless the
> definition of the .use_nanosec member itself in the structure is
> conditional on NO_NSEC.  And ...
>
> >
> >       /* section "sparse" config values */
> >       cfg->sparse_expect_files_outside_of_patterns = 0;
> > diff --git a/environment.h b/environment.h
> > index e7ec5b0437..a35534afe5 100644
> > --- a/environment.h
> > +++ b/environment.h
> > @@ -139,6 +139,7 @@ struct repo_config_values {
> >       int ignore_case;
> >       int trust_executable_bit;
> >       int has_symlinks;
> > +     int use_nanosec;
>
> ... that is not the case.

Doh! I actually intended to send this version with a compiled-out member when NO_NSEC, since that was the only path I had come up with. No point in running around with code that's been asked to be ignored, eh? However…

> Which means that git_default_core_config() does keep the initial
> value of the member without getting affected by the configuration,
> but it does not necessarily be keeping "false".  It may be keeping
> the uninitialized state instead ;-).

[ugly #ifdef trimmed]

Show 19 quoted lines

> Ugly.  How about getting rid of the latter #ifndef/#else/#endif and
> instead keeping the "if use_nsec, pay attention to nsec, otherwise
> only the seconds part" ternary?  As to the early part, as you can
> arrange cfg's '.use_nanosec' to always hold a sensible value, the
> function can become
>
>         return (istate->timestamp.sec &&
>                 (repo_config_values(istate->repo)->use_nanosec
>                  ? (istate->timestamp.sec < sd->sd_mtime.sec ||
>                    (istate->timestamp.sec == sd->sd_mtime.sec &&
>                     istate->timestamp.nsec <= sd->sd_mtime.nsec))
>                  : istate->timestamp.sec <= sd->sd_mtime.sec));
>
> I think.
>
> The code you presented here for is_racy_stat() sprinkled with
> #ifndef/#else/#endif would be sensible if repo_config_values struct
> defined the '.use_nanosec' member conditionally.  But that is not
> what is happening here.

…I now see a world where we could avoid quite a bit of headache:

- use #if[n]def NO_NSEC to ignore the config variable, but otherwise
- unconditionally compile the cfg->use_nanosec checks

That is, future readers/writers won't have to remember that they can only use the use_nanosec member under compiler conditionals; it will always be initialized to a safe value (either always false or from config). If we're lucky, the compiler will optimize the checks away in NO_NSEC builds ;)

I think this is what you are suggesting Junio, so let me see what I can come up with.

-- 
D. Ben Knoble