perf(resolver): replace slice-based LRU with container/list for O(1) operations

DNS resolver LRU cache used []string slice for access ordering:
- moveToFrontLocked: O(n) linear scan + slice重组 per cache update
- storeCache held exclusive write lock during entire O(n) operation,
  blocking all concurrent reads

Replace with container/list + map[string]*list.Element:
- storeCache: O(1) MoveToFront via list.Element pointer
- evictLRULocked: O(1) Back() + Remove()
- DeleteCacheEntry: O(1) instead of O(n) scan

This matches the LRU pattern already used by FileCache and
FileInfoCache. Write lock duration reduced from O(n) to O(1),
significantly reducing read contention under mixed workloads.

internal/resolver/cache.go

@@ -6,6 +6,7 @@
 package resolver
 
 import (
+	"container/list"
 	"time"
 )
 
@@ -59,23 +60,21 @@ func (r *DNSResolver) GetCacheEntry(host string) (*DNSCacheEntry, bool) {
 // DeleteCacheEntry 删除指定主机的缓存条目。
 func (r *DNSResolver) DeleteCacheEntry(host string) {
 	r.mu.Lock()
+	defer r.mu.Unlock()
 	delete(r.cache, host)
-	// 从 LRU 链表中移除
-	for i, h := range r.lruOrder {
-		if h == host {
-			r.lruOrder = append(r.lruOrder[:i], r.lruOrder[i+1:]...)
-			break
-		}
+	if elem, ok := r.lruIndex[host]; ok {
+		r.lruList.Remove(elem)
+		delete(r.lruIndex, host)
 	}
 	delete(r.refreshHosts, host)
-	r.mu.Unlock()
 }
 
 // ClearCache 清空所有缓存。
 func (r *DNSResolver) ClearCache() {
 	r.mu.Lock()
 	r.cache = make(map[string]*DNSCacheEntry)
-	r.lruOrder = make([]string, 0, r.config.CacheSize)
+	r.lruList = list.New()
+	r.lruIndex = make(map[string]*list.Element)
 	r.refreshHosts = make(map[string]struct{})
 	r.mu.Unlock()
 }

internal/resolver/mock_dns_test.go

@@ -421,22 +421,28 @@ func TestMockDNSMoveToFrontLocked(t *testing.T) {
 
 	resolver := New(cfg).(*DNSResolver)
 
-	// 设置缓存
 	resolver.cache = map[string]*DNSCacheEntry{
 		"a": {},
 		"b": {},
 		"c": {},
 	}
-	resolver.lruOrder = []string{"a", "b", "c"}
+	elemA := resolver.lruList.PushFront("a")
+	elemB := resolver.lruList.PushFront("b")
+	elemC := resolver.lruList.PushFront("c")
+	resolver.lruIndex["a"] = elemA
+	resolver.lruIndex["b"] = elemB
+	resolver.lruIndex["c"] = elemC
 
-	// 移动 "a" 到前端
-	resolver.moveToFrontLocked("a")
+	resolver.lruList.MoveToFront(resolver.lruIndex["a"])
 
-	// 验证顺序
+	order := []string{}
+	for e := resolver.lruList.Back(); e != nil; e = e.Prev() {
+		order = append(order, e.Value.(string))
+	}
 	expected := []string{"b", "c", "a"}
 	for i, v := range expected {
-		if resolver.lruOrder[i] != v {
-			t.Errorf("LRU order[%d] = %s, want %s", i, resolver.lruOrder[i], v)
+		if order[i] != v {
+			t.Errorf("LRU order[%d] = %s, want %s", i, order[i], v)
 		}
 	}
 }

internal/resolver/resolver.go

@@ -16,6 +16,7 @@
 package resolver
 
 import (
+	"container/list"
 	"context"
 	"fmt"
 	"net"
@@ -61,41 +62,31 @@ func (r *DNSResolver) storeCache(host string, entry *DNSCacheEntry) {
 	r.mu.Lock()
 	defer r.mu.Unlock()
 
-	// 已存在则更新并移到头部
-	if _, ok := r.cache[host]; ok {
+	if elem, ok := r.lruIndex[host]; ok {
 		r.cache[host] = entry
-		r.moveToFrontLocked(host)
+		r.lruList.MoveToFront(elem)
 		return
 	}
 
-	// 检查是否需要淘汰
 	if r.config.CacheSize > 0 && len(r.cache) >= r.config.CacheSize {
 		r.evictLRULocked()
 	}
 
 	r.cache[host] = entry
-	r.lruOrder = append(r.lruOrder, host)
+	elem := r.lruList.PushFront(host)
+	r.lruIndex[host] = elem
 }
 
 // evictLRULocked 淘汰最久未使用的条目（需持有锁）。
 func (r *DNSResolver) evictLRULocked() {
-	if len(r.lruOrder) == 0 {
+	oldest := r.lruList.Back()
+	if oldest == nil {
 		return
 	}
-	oldest := r.lruOrder[0]
-	delete(r.cache, oldest)
-	r.lruOrder = r.lruOrder[1:]
-}
-
-// moveToFrontLocked 将条目移到 LRU 链表尾部（最新）（需持有锁）。
-func (r *DNSResolver) moveToFrontLocked(host string) {
-	for i, h := range r.lruOrder {
-		if h == host {
-			r.lruOrder = append(r.lruOrder[:i], r.lruOrder[i+1:]...)
-			r.lruOrder = append(r.lruOrder, host)
-			return
-		}
-	}
+	host := oldest.Value.(string)
+	delete(r.cache, host)
+	delete(r.lruIndex, host)
+	r.lruList.Remove(oldest)
 }
 
 // DNSResolver 实现 Resolver 接口的 DNS 解析器。
@@ -103,8 +94,9 @@ type DNSResolver struct {
 	config       *config.ResolverConfig
 	stopCh       chan struct{}
 	refreshHosts map[string]struct{}
-	cache        map[string]*DNSCacheEntry // DNS 缓存
-	lruOrder     []string                  // LRU 访问顺序（最旧在前）
+	cache        map[string]*DNSCacheEntry
+	lruList      *list.List
+	lruIndex     map[string]*list.Element
 	hits         atomic.Int64
 	misses       atomic.Int64
 	errors       atomic.Int64
@@ -166,7 +158,8 @@ func New(cfg *config.ResolverConfig) Resolver {
 		stopCh:       make(chan struct{}),
 		refreshHosts: make(map[string]struct{}),
 		cache:        make(map[string]*DNSCacheEntry),
-		lruOrder:     make([]string, 0, cfg.CacheSize),
+		lruList:      list.New(),
+		lruIndex:     make(map[string]*list.Element),
 	}
 }