Re: [PATCH v4 6/6] log: fix --follow following an edit on a side the merge did not take

[Patrick Steinhardt <ps@pks.im>]

On Fri, Oct 30, 2026 at 09:10:14PM +0100, Miklos Vajna wrote:
> Take a history where F is renamed to G on master, F is edited on a side
> branch, and the side branch is merged with '-s ours', so the merge keeps
> G and drops F. Here 'git log --follow G' doesn't show the edit of F on
> the side branch, while it did before the previous commit.
> 
> The problem is that follow_needs_renames() only searches for renames in
> a parent without the path if the merge base still has the path, which
> covers the case when the parent renamed the path. Here the other side
> of the merge did the rename, so the merge base only has the old name.
> The parent is then told to follow G, which it doesn't have, and the
> history of F on its side is lost. Searching for renames in each parent
> that lacks the path would fix this, but that is the expensive search
> that the previous commit avoids, e.g. 148 merges hit this case for
> include/linux/rsc_table.h in linux.git.
> 
> Fix the problem by also searching for renames when the merge base
> doesn't have the path, but the parent changed a file since the merge
> base that the merge no longer has: only such a file can be the old name
> of the path that still needs to be followed in the parent.

This isn't true though in the case where you copy a file though, is it?
E.g. the following test case is exactly the same as the one you add in
this commit, but instead of using a rename we use a copy:

diff --git a/t/t4219-log-follow-merge.sh b/t/t4219-log-follow-merge.sh
index a85fb78ca28..c67d9481ab5 100755
--- a/t/t4219-log-follow-merge.sh
+++ b/t/t4219-log-follow-merge.sh
@@ -297,4 +297,36 @@ test_expect_success '--follow follows an edit on the side the merge did not take
 	test_cmp expect actual
 '
 
+test_expect_success 'setup merge keeping the copied path, other side edited the original' '
+	git init keep-copied &&
+	printf "line 1\nline 2\nline 3\nline 4\nline 5\n" >keep-copied/F &&
+	git -C keep-copied add F &&
+	test_tick &&
+	git -C keep-copied commit -m "O: add F" &&
+	git -C keep-copied branch B &&
+	cp keep-copied/F keep-copied/G &&
+	git -C keep-copied add G &&
+	test_tick &&
+	git -C keep-copied commit -m "A1: copy F to G on master" &&
+
+	git -C keep-copied checkout B &&
+	printf "line 1\nline 2\nline 3\nline 4 T\nline 5\n" >keep-copied/F &&
+	test_tick &&
+	git -C keep-copied commit -am "B2: update F on B" &&
+
+	git -C keep-copied checkout master &&
+	test_tick &&
+	git -C keep-copied merge -s ours -m "M: merge B, keep F and G" B
+'
+
+test_expect_success '--follow follows an edit of the copy source on the side the merge did not take' '
+	git -C keep-copied log --follow --pretty=tformat:%s G >actual &&
+	cat >expect <<-\EOF &&
+	B2: update F on B
+	A1: copy F to G on master
+	O: add F
+	EOF
+	test_cmp expect actual
+'
+
 test_done

This test passes on master, but fails with your patch series.

Patrick