Fix single-item cache bug by adding explicit inList flag

The intrinsic linked list change used next==nil && prev==nil to detect items not in the list.
This fails for the first/only item, which has both pointers nil while legitimately being in the list.

Add an inList boolean to Item, set by Insert/Remove, and use it in doPromote/doDelete instead of checking pointer state.

Fixes size double-counting on promotion and missing cleanup on delete.

The item struct already uses two cache-lines, adding the inList field does not have any significant performance impact.

Author: pior

cache.go

@@ -331,7 +331,7 @@ doAllDeletes:
 }
 
 func (c *Cache[T]) doDelete(item *Item[T]) {
-	if item.next == nil && item.prev == nil {
+	if !item.inList {
 		item.promotions = -2
 	} else {
 		c.size -= item.size
@@ -344,12 +344,11 @@ func (c *Cache[T]) doDelete(item *Item[T]) {
 }
 
 func (c *Cache[T]) doPromote(item *Item[T]) bool {
-	//already deleted
 	if item.promotions == -2 {
 		return false
 	}
 
-	if item.next != nil || item.prev != nil { // not a new item
+	if item.inList {
 		if item.shouldPromote(c.getsPerPromote) {
 			c.list.MoveToFront(item)
 			item.promotions = 0

item.go

@@ -29,6 +29,7 @@ type Item[T any] struct {
 	value      T
 	next       *Item[T]
 	prev       *Item[T]
+	inList     bool
 }
 
 func newItem[T any](key string, value T, expires int64, track bool) *Item[T] {

layeredcache.go

@@ -293,7 +293,7 @@ drain:
 }
 
 func (c *LayeredCache[T]) doDelete(item *Item[T]) {
-	if item.prev == nil && item.next == nil {
+	if !item.inList {
 		item.promotions = -2
 	} else {
 		c.size -= item.size
@@ -306,12 +306,11 @@ func (c *LayeredCache[T]) doDelete(item *Item[T]) {
 }
 
 func (c *LayeredCache[T]) doPromote(item *Item[T]) bool {
-	// deleted before it ever got promoted
 	if item.promotions == -2 {
 		return false
 	}
 
-	if item.next != nil || item.prev != nil { // not a new item
+	if item.inList {
 		if item.shouldPromote(c.getsPerPromote) {
 			c.list.MoveToFront(item)
 			item.promotions = 0

list.go

@@ -26,6 +26,7 @@ func (l *List[T]) Remove(item *Item[T]) {
 	}
 	item.next = nil
 	item.prev = nil
+	item.inList = false
 }
 
 func (l *List[T]) MoveToFront(item *Item[T]) {
@@ -36,6 +37,7 @@ func (l *List[T]) MoveToFront(item *Item[T]) {
 func (l *List[T]) Insert(item *Item[T]) {
 	head := l.Head
 	l.Head = item
+	item.inList = true
 	if head == nil {
 		l.Tail = item
 		return