Re: [PATCH] log: make --follow following renames for non-linear history optional

[Patrick Steinhardt <ps@pks.im>]

On Thu, Nov 12, 2026 at 09:05:07AM +0000, Miklos Vajna wrote:
> diff --git a/Documentation/config/log.adoc b/Documentation/config/log.adoc
> index f7dfce69b5..72a3486a73 100644
> --- a/Documentation/config/log.adoc
> +++ b/Documentation/config/log.adoc
> @@ -53,7 +53,9 @@ This is the same as the `--decorate` option of the `git log`.
>  `log.follow`::
>  	If `true`, `git log` will act as if the `--follow` option was used when
>  	a single <path> is given.  This has the same limitations as `--follow`,
> -	i.e. it cannot be used to follow multiple files.
> +	i.e. it cannot be used to follow multiple files and does not work well
> +	on non-linear history.  If `merges`, `git log` will act as if the
> +	`--follow=merges` option was used.
>  
>  `log.graphColors`::
>  	A list of colors, separated by commas, that can be used to draw

I'd think we should also document here that "merges" is expensive, same
as you do for "--follow[=merges]".

> diff --git a/builtin/log.c b/builtin/log.c
> index 350b35c556..db707877f6 100644
> --- a/builtin/log.c
> +++ b/builtin/log.c
> @@ -505,7 +508,10 @@ static int git_log_config(const char *var, const char *value,
>  		return 0;
>  	}
>  	if (!strcmp(var, "log.follow")) {
> -		cfg->default_follow = git_config_bool(var, value);
> +		if (value && !strcmp(value, "merges"))
> +			cfg->default_follow = 2;
> +		else
> +			cfg->default_follow = git_config_bool(var, value);
>  		return 0;
>  	}
>  	if (skip_prefix(var, "color.decorate.", &slot_name))

I think we should use `git_parse_maybe_bool()` here, as we otherwise die
with "fatal: bad boolean config value 'bogus' for 'log.follow'" in case
it's not a boolean. Then we can die with our own error message.

> @@ -814,9 +820,14 @@ int cmd_log_reflog(int argc,
>  
>  static void log_setup_revisions_tweak(struct rev_info *rev)
>  {
> +	/* If --follow is not used, use the log.follow option. */
>  	if (rev->diffopt.flags.default_follow_renames &&
> -	    diff_check_follow_pathspec(&rev->prune_data, 0))
> +	    !rev->diffopt.flags.follow_renames &&
> +	    diff_check_follow_pathspec(&rev->prune_data, 0)) {
>  		rev->diffopt.flags.follow_renames = 1;
> +		rev->diffopt.flags.follow_merges =
> +			rev->diffopt.flags.default_follow_merges;
> +	}
>  
>  	if (rev->first_parent_only)
>  		diff_merges_default_to_first_parent(rev);

I assume that this change here is done so that "--follow" overrides
"log.follow=merge". Makes sense.

> diff --git a/log-tree.c b/log-tree.c
> index 83a3c4bf9b..93cc61af2d 100644
> --- a/log-tree.c
> +++ b/log-tree.c
> @@ -1277,7 +1277,8 @@ int log_tree_commit(struct rev_info *opt, struct commit *commit)
>  	opt->diffopt.no_free = 1;
>  
>  	/* Any recorded path for this commit? If so, restore it */
> -	if (opt->diffopt.flags.follow_renames) {
> +	if (opt->diffopt.flags.follow_renames &&
> +	    opt->diffopt.flags.follow_merges) {
>  		const char *stored = recall_follow_pathspec(opt, commit);
>  		if (stored) {
>  			const char *current = pathspec_single_path(&opt->diffopt.pathspec);
> @@ -1300,7 +1301,8 @@ int log_tree_commit(struct rev_info *opt, struct commit *commit)
>  		show_diff_of_diff(opt);
>  
>  	/* Record what path each parent of this commit should use */
> -	if (opt->diffopt.flags.follow_renames) {
> +	if (opt->diffopt.flags.follow_renames &&
> +	    opt->diffopt.flags.follow_merges) {
>  		struct commit_list *parents = get_saved_parents(opt, commit);
>  		if (parents && parents->next) {
>  			struct commit_list *p;

Okay, and these two changes here disable the logic where we record the
changed paths on all parents, and that should get us back to previous
speed by default. Good.

As far as I've seen we only ever set `follow_merges` when
`follow_renames = 1`, so it feels redundant to check for both. But on
the other hand that's a bit more defensive, so maybe that's not a bad
idea.

Other than those nits this patch looks good to me, thanks!

Patrick