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

> Racy Git problems persist today, manifesting themselves in the
> performance of commands like "git diff" in new worktrees [1]. We have
> long had a build knob "USE_NSEC" to tell Git to use in-core nanosecond
> precision when available, which mitigates most if not all racy issues,
> but most builds we know about it don't use it. In part, that's because
> someone distributing Git can't safely enable it at compile-time if they
> don't know exactly what platforms their distribution will be used on.
>
> [1]: https://lore.kernel.org/git/CALnO6CADMJSixqYvL1Yo8qKX5rWhKQ+2OoSEuPUh-yoeK9TseQ@mail.gmail.com
>
> These days, most platforms are likely to be safe for the USE_NSEC code.
> Regardless, we want to give users the ability to benefit from it. This
> requires exposing the compile-time gated code as a runtime option.
>
> In addition, update the Racy Git documentation and other mentions of
> USE_NSEC in the code.
>
> Best-viewed-with: --ignore-space-change

> diff --git a/environment.c b/environment.c
> index 6676e6f5ae..e6a50060e8 100644
> --- a/environment.c
> +++ b/environment.c
> @@ -571,6 +571,11 @@ int git_default_core_config(const char *var, const char *value,
>  		return 0;
>  	}
>  
> +	if (!strcmp(var, "core.usenanosec")) {
> +		cfg->use_nanosec = git_config_bool(var, value);
> +		return 0;
> +	}

> diff --git a/read-cache.c b/read-cache.c
> index 6c449f393d..297646c357 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -353,15 +353,16 @@ static int ce_match_stat_basic(const struct cache_entry *ce, struct stat *st)
>  static int is_racy_stat(const struct index_state *istate,
>  			const struct stat_data *sd)
>  {
> +	int use_nsec = 0;
> +	repo_config_get_bool(the_repository, "core.useNanosec", &use_nsec);