From 598f40c4b3de30d8f7c0666823a9d90884b78bee Mon Sep 17 00:00:00 2001 From: Jeff King Date: Tue, 17 Mar 2026 19:02:23 -0400 Subject: [PATCH] contrib/diff-highlight: do not highlight identical pairs We pair lines for highlighting based on their position in the hunk. So we should never see two identical lines paired, like: -one -two +one +something else which would pair -one/+one, because that implies that the diff could easily be shrunk by turning line "one" into context. But there is (at least) one exception: removing a newline at the end of a file will produce a diff like: -foo +foo \No newline at end of file And we will pair those two lines. As a result, we end up marking the whole line, including the newline, as the shared prefix. And there's an empty suffix. The most obvious bug here is that when we try to print the highlighted lines, we remove the trailing newline from the suffix, but do not bother with the prefix (under the assumption that there had to be a difference _somewhere_ in the line, and thus the prefix would not eat all the way up to the newline). And so you get an extra line like: -foo +foo \No newline at end of file This is obviously ugly, but also causes interactive.diffFilter to (rightly) complain that the input and output do not match their lines 1-to-1. This could easily be fixed by chomping the prefix, too, but I think the problem is deeper. For one, I suspect some of the other logic gets confused by forming an array with zero-indexed element "3" in a 3-element array. But more importantly, we try not to highlight whole lines, as there's nothing interesting to show there. So let's catch this early in is_pair_interesting() and bail to our usual passthrough strategy. Reported-by: Scott Baker Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- contrib/diff-highlight/DiffHighlight.pm | 12 ++++++++++++ contrib/diff-highlight/t/t9400-diff-highlight.sh | 11 +++++++++++ 2 files changed, 23 insertions(+) diff --git a/contrib/diff-highlight/DiffHighlight.pm b/contrib/diff-highlight/DiffHighlight.pm index 3d061bc0b7..f0607a4b68 100644 --- a/contrib/diff-highlight/DiffHighlight.pm +++ b/contrib/diff-highlight/DiffHighlight.pm @@ -273,6 +273,18 @@ sub highlight_line { # or suffix (disregarding boring bits like whitespace and colorization). sub is_pair_interesting { my ($a, $pa, $sa, $b, $pb, $sb) = @_; + + # We hit this case if the prefix consumed the entire line, meaning + # that two lines are identical. This generally shouldn't happen, + # since it implies the diff isn't minimal (you could shrink the hunk by + # making this a context line). But you can see it when the line + # content is the same, but the trailing newline is dropped, like: + # + # -foo + # +foo + # \No newline at end of file + return 0 if $pa == @$a || $pb == @$b; + my $prefix_a = join('', @$a[0..($pa-1)]); my $prefix_b = join('', @$b[0..($pb-1)]); my $suffix_a = join('', @$a[($sa+1)..$#$a]); diff --git a/contrib/diff-highlight/t/t9400-diff-highlight.sh b/contrib/diff-highlight/t/t9400-diff-highlight.sh index dee296739c..2a9b68cf3b 100755 --- a/contrib/diff-highlight/t/t9400-diff-highlight.sh +++ b/contrib/diff-highlight/t/t9400-diff-highlight.sh @@ -340,4 +340,15 @@ test_expect_success 'diff-highlight handles --graph with leading dash' ' test_cmp expect actual ' +test_expect_success 'highlight diff that removes final newline' ' + printf "content\n" >a && + printf "content" >b && + dh_test a b <<-\EOF + @@ -1 +1 @@ + -content + +content + \ No newline at end of file + EOF +' + test_done