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

[Patrick Steinhardt <ps@pks.im>]

On Tue, Aug 18, 2026 at 10:59:47AM -0400, D. Ben Knoble wrote:
> 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

s/about it/about/

> diff --git a/Documentation/config/core.adoc b/Documentation/config/core.adoc
> index 340329edc3..33104444ab 100644
> --- a/Documentation/config/core.adoc
> +++ b/Documentation/config/core.adoc
> @@ -118,6 +118,12 @@ core.trustctime::
>  	crawlers and some backup systems).
>  	See linkgit:git-update-index[1]. True by default.
>  
> +core.useNanosec::
> +	If true, use nanosecond precision for ctime and mtime
> +	comparisions between the index and the working tree (if Git
> +	was compiled to store it).
> +	See link:technical/racy-git.html[Racy Git]. False by default.

Should we mentino here that this may not be safe on all platforms and/or
filesystems, in addition to linking to racy-hit?

And do we really want to link to the HTML page here? The user may be
reading a manpage, so doing so feels a bit weird to me.

> 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

Do we want to omit a warning in case the config is enabled and we have
NO_SEC set? Or would that be too obnoxious?

> @@ -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

Can't we set this unconditionally? The respective field exists
unconditionally, too.

> diff --git a/read-cache.c b/read-cache.c
> index 6c449f393d..31888f77ee 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -353,12 +353,18 @@ 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)
>  {
> +#ifndef NO_NSEC
> +	int use_nsec = repo_config_values(istate->repo)->use_nanosec;
> +#endif
> +
>  	return (istate->timestamp.sec &&
> -#ifdef USE_NSEC
> -		 /* nanosecond timestamped files can also be racy! */
> -		(istate->timestamp.sec < sd->sd_mtime.sec ||
> -		 (istate->timestamp.sec == sd->sd_mtime.sec &&
> -		  istate->timestamp.nsec <= sd->sd_mtime.nsec))
> +#ifndef NO_NSEC
> +		/* nanosecond timestamped files can also be racy! */
> +		use_nsec
> +		? (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
>  #else
>  		istate->timestamp.sec <= sd->sd_mtime.sec
>  #endif

I think this would be a bit more readable if we had a single NO_NSEC
block.

> diff --git a/statinfo.c b/statinfo.c
> index 5e00af127d..2f2cec6282 100644
> --- a/statinfo.c
> +++ b/statinfo.c
> @@ -72,12 +72,14 @@ int match_stat_data(const struct stat_data *sd, struct stat *st)
>  	    sd->sd_ctime.sec != (unsigned int)st->st_ctime)
>  		changed |= CTIME_CHANGED;
>  
> -#ifdef USE_NSEC
> -	if (cfg->check_stat && sd->sd_mtime.nsec != ST_MTIME_NSEC(*st))
> -		changed |= MTIME_CHANGED;
> -	if (cfg->trust_ctime && cfg->check_stat &&
> -	    sd->sd_ctime.nsec != ST_CTIME_NSEC(*st))
> -		changed |= CTIME_CHANGED;
> +#ifndef NO_NSEC
> +	if (cfg->use_nanosec) {
> +		if (cfg->check_stat && sd->sd_mtime.nsec != ST_MTIME_NSEC(*st))
> +			changed |= MTIME_CHANGED;
> +		if (cfg->trust_ctime && cfg->check_stat &&
> +		    sd->sd_ctime.nsec != ST_CTIME_NSEC(*st))
> +			changed |= CTIME_CHANGED;
> +	}
>  #endif

There's one more site in "builtin/update-index.c" where we mention
USE_NSEC that wasn't updated as part of this patch.

Thanks!

Patrick