From c0f9107ac571e25087dc3956d9ada5c06d6d71a4 Mon Sep 17 00:00:00 2001
From: Rupayon Haldar <80724680+rupayon123@users.noreply.github.com>
Date: Wed, 23 Sep 2026 08:33:49 -0400
Subject: [PATCH] Stop recursive key references from overflowing stack

---
 key.go      | 15 ++++++++++++++-
 key_test.go | 13 +++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/key.go b/key.go
index 800d04a..89a27e3 100644
--- a/key.go
+++ b/key.go
@@ -140,6 +140,10 @@ func (k *Key) NestedValues() []string {
 
 // transformValue takes a raw value and transforms to its final string.
 func (k *Key) transformValue(val string) string {
+	return k.transformValueWithVisited(val, map[*Key]bool{k: true})
+}
+
+func (k *Key) transformValueWithVisited(val string, visited map[*Key]bool) string {
 	if k.s.f.ValueMapper != nil {
 		val = k.s.f.ValueMapper(val)
 	}
@@ -169,8 +173,17 @@ func (k *Key) transformValue(val string) string {
 			}
 		}
 
+		// A reference back to a key already being resolved is left intact,
+		// just like a direct self-reference.
+		if visited[nk] {
+			break
+		}
+		visited[nk] = true
+		replacement := nk.transformValueWithVisited(nk.value, visited)
+		delete(visited, nk)
+
 		// Substitute by new value and take off leading '%(' and trailing ')s'.
-		val = strings.ReplaceAll(val, vr, nk.String())
+		val = strings.ReplaceAll(val, vr, replacement)
 	}
 	return val
 }
diff --git a/key_test.go b/key_test.go
index e57a7bd..e013abe 100644
--- a/key_test.go
+++ b/key_test.go
@@ -626,6 +626,19 @@ expires = %(expires)s`))
 		assert.Equal(t, "yes", f.Section("package").Key("expires").String())
 	})
 
+	t.Run("mutually recursive keys remain unresolved", func(t *testing.T) {
+		f, err := Load([]byte("[s]\na = %(b)s\nb = %(a)s\n"))
+		require.NoError(t, err)
+		assert.Equal(t, "%(a)s", f.Section("s").Key("a").String())
+		assert.Equal(t, "%(b)s", f.Section("s").Key("b").String())
+	})
+
+	t.Run("longer reference cycle remains unresolved", func(t *testing.T) {
+		f, err := Load([]byte("[s]\na = %(b)s\nb = %(c)s\nc = %(a)s\n"))
+		require.NoError(t, err)
+		assert.Equal(t, "%(a)s", f.Section("s").Key("a").String())
+	})
+
 	t.Run("recursive value with no target found", func(t *testing.T) {
 		f, err := Load([]byte(`
 [foo]