From dfceb3d27be25dce3f9b727922771a86d08e3d4e Mon Sep 17 00:00:00 2001
From: Bryce Kwon <bryce@brycekwon.com>
Date: Sat, 7 Nov 2026 13:35:00 -1000
Subject: Keep a followed diff to the path it names

A followed diff kept every path that started with the requested one,
so following small.c took small.cpp in as well. A path now has to be
the requested one or lie below it, as the pathspec of an unfollowed
diff matches.
---
 source/ui-diff.c    | 15 +++++++++++++--
 tests/t0107-diff.sh | 27 +++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/source/ui-diff.c b/source/ui-diff.c
index c696ab1..2974744 100644
--- a/source/ui-diff.c
+++ b/source/ui-diff.c
@@ -178,12 +178,23 @@ static void count_diff_lines(char *line, int len)
 	render_line_fn(line, len);
 }
 
+/*
+ * A path is kept when it is the prefix itself or lies below it, as the
+ * pathspec of a diff that does not follow would match it.
+ */
+static int path_in_prefix(const char *path, const char *prefix)
+{
+	const char *rest;
+
+	return skip_prefix(path, prefix, &rest) && (!*rest || *rest == '/');
+}
+
 static int show_filepair(struct diff_filepair *pair)
 {
 	if (!current_prefix)
 		return 1;
-	return starts_with(pair->one->path, current_prefix) ||
-		starts_with(pair->two->path, current_prefix);
+	return path_in_prefix(pair->one->path, current_prefix) ||
+		path_in_prefix(pair->two->path, current_prefix);
 }
 
 static void print_line(char *line, int len)
diff --git a/tests/t0107-diff.sh b/tests/t0107-diff.sh
index 2560c42..6caf539 100755
--- a/tests/t0107-diff.sh
+++ b/tests/t0107-diff.sh
@@ -368,6 +368,33 @@ test_expect_success 'a followed file is diffed as the rename alone' '
 	! grep "a/file-1 b/file-1" tmp
 '
 
+# A followed path is matched as a pathspec would match it, the file itself or
+# whatever lies below it, rather than every path starting with the same
+# characters.
+test_expect_success 'a followed path takes in no file it only prefixes' '
+	mkrepo repos/prefixed 1 &&
+	(
+		cd repos/prefixed &&
+		mkdir small.d &&
+		echo c >small.c &&
+		echo cpp >small.cpp &&
+		echo d >small.d/f &&
+		git add . &&
+		git commit -m "add the files" &&
+		echo C >small.c &&
+		echo CPP >small.cpp &&
+		echo D >small.d/f &&
+		git commit -am "change the files"
+	) &&
+	cgitrc_repo prefixedrc prefixed enable-follow-links=1 &&
+	cgit_ok prefixedrc "url=prefixed/diff/small.c&follow=1" >tmp &&
+	grep "diff --git a/small.c b/small.c" tmp &&
+	! grep "small.cpp" tmp &&
+	cgit_ok prefixedrc "url=prefixed/diff/small.d&follow=1" >tmp &&
+	grep "diff --git a/small.d/f b/small.d/f" tmp &&
+	! grep "small.c" tmp
+'
+
 test_expect_success 'following with no path limits nothing' '
 	cgit_ok followrc "url=moved/commit/&follow=1" >tmp &&
 	grep "diff --git a/file-1 b/file-1" tmp &&
-- 
cgit 2.12.1