From b27ddbadb5ecc869959d1affbd2d99812a23475f Mon Sep 17 00:00:00 2001
From: Nikolaus Schuetz <nikolauspschuetz@gmail.com>
Date: Thu, 13 Aug 2026 15:38:13 -0400
Subject: [PATCH] fix: guard LRUCache against non-positive capacity

NewLRUCache and NewSyncLRUCache are exported, but calling Put on a cache
created with a non-positive capacity panicked with a nil pointer
dereference: the eviction path removes cache.tail.prev, which on an empty
list is the head sentinel whose prev pointer is nil.

Return early from Put when capacity is non-positive, since such a cache
holds nothing. Add a regression test that fails (panics) before this
change and passes after.
---
 util/util.go      |  5 +++++
 util/util_test.go | 12 ++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/util/util.go b/util/util.go
index d4e0f27d..57e98c5e 100644
--- a/util/util.go
+++ b/util/util.go
@@ -391,6 +391,11 @@ func (cache *LRUCache) Get(key interface{}) (value interface{}, ok bool) {
 }
 
 func (cache *LRUCache) Put(key interface{}, value interface{}) {
+	if cache.capacity <= 0 {
+		// A cache with non-positive capacity holds nothing, so there is nothing
+		// to store and no eviction to perform.
+		return
+	}
 	n, ok := cache.m[key]
 	if ok {
 		cache.remove(n, false)
diff --git a/util/util_test.go b/util/util_test.go
index 45082b52..b25f9b83 100644
--- a/util/util_test.go
+++ b/util/util_test.go
@@ -232,6 +232,18 @@ func TestLRUCache(t *testing.T) {
 	testCacheEqual(t, cache, []int{1, 3, 4})
 }
 
+func TestLRUCacheNonPositiveCapacity(t *testing.T) {
+	// A cache created with a non-positive capacity holds nothing and must not
+	// panic on Put. Previously the eviction path removed cache.tail.prev, which
+	// on an empty list is the head sentinel whose prev pointer is nil, causing a
+	// nil pointer dereference.
+	cache := NewLRUCache(0)
+	cache.Put("a", 1)
+	if _, ok := cache.Get("a"); ok {
+		t.Errorf("NewLRUCache(0): Get(\"a\") ok = true, want false (zero-capacity cache holds nothing)")
+	}
+}
+
 func testEscapeStringLiterals(t *testing.T, input string, expected string) {
 	t.Helper()
 	result := EscapeStringLiterals(input)