Re: [Bug?] "git show -s" still worries about renameLimit?

[Jeff King <peff@peff.net>]

On Sun, May 04, 2025 at 01:00:16AM -0700, Junio C Hamano wrote:

> $ git show -s | cat
> warning: exhaustive rename detection was skipped due to too many files.
> warning: you may want to set your diff.renameLimit variable to at least 6123 and retry the command.
> commit a3a9dd8be6b8767e690b014715aefa2ba39672e2 (HEAD -> master)
> Author: Junio C Hamano <gitster@pobox.com>
> Date:   Sat Apr 19 14:27:03 2025 -0700
> 
>     Something something something
> 
> As we have -M (rename detection) on by default these days, and this
> particular commit has very many deletions and creations, if we were
> asking to show some diff (not necessarily patch text output, but
> just "--stat" or even "--raw") it is fair to warn about rename
> detection being limited by diff.renameLimit.
> 
> But the command knows that with "-s" the user declined to show any
> diff computation, so it feels wrong to even _count_ how many
> diff_filepairs there are and comparing with the renameLimit, in
> order to warn about busting the limit.

This seemed eerily familiar. See this thread:

  https://lore.kernel.org/git/87h750q1b9.fsf@gnu.org/

and in particular this proposal:

  https://lore.kernel.org/git/YqI%2FTcZyXomxtXtN@coredump.intra.peff.net/

I've been carrying that patch in my tree (reproduced below), but I don't
remember why I never polished it. I wonder if it was the question about
--exit-code below. Or maybe I was just nervous about other corner cases.

-- >8 --
Subject: [PATCH] show: skip diff when possible

Running:

  git show -s $commit

will still compute a diff for $commit, even though we aren't going to
show it. This is wasted computation, since it cannot affect the output
or exit code of the program.

In the more general case:

  - if the requested diff format is NO_OUTPUT, then we won't change the
    output of the diff itself

  - if rev_info.always_show_header is set, then we will show the commit
    regardless of whether the diff is empty (which is true for git-show,
    for example, but not git-log)

  - we don't use --exit-code here (should check?)

Signed-off-by: Jeff King <peff@peff.net>
---
 log-tree.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/log-tree.c b/log-tree.c
index a4d4ab59ca..740219ce99 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -1105,6 +1105,10 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 	if (!all_need_diff && !opt->merges_need_diff)
 		return 0;
 
+	if (opt->diffopt.output_format == DIFF_FORMAT_NO_OUTPUT &&
+	    opt->always_show_header)
+		return 0;
+
 	parse_commit_or_die(commit);
 	oid = get_commit_tree_oid(commit);
 
-- 
2.49.0.754.gd827f9aa09