Fix race condition: failed records must be discarded

Author: mgnsk

cache.go

@@ -122,7 +122,7 @@ loadOrStore:
 	defer func() {
 		if r := recover(); r != nil {
 			c.backend.Delete(key)
-			c.backend.Release(newElem)
+			c.backend.Discard(newElem)
 
 			panic(r)
 		}
@@ -131,7 +131,7 @@ loadOrStore:
 	value, ttl, err := f()
 	if err != nil {
 		c.backend.Delete(key)
-		c.backend.Release(newElem)
+		c.backend.Discard(newElem)
 
 		var zero V
 		return zero, err

internal/backend/backend.go

@@ -22,6 +22,7 @@ func (r *record[V]) Value() V {
 	return r.value
 }
 
+// Wait for the record to be either released or discarded.
 func (r *record[V]) Wait() {
 	r.wg.Wait()
 }
@@ -94,6 +95,11 @@ func (b *Backend[K, V]) Release(elem Element[V]) {
 	b.pool.Put(elem)
 }
 
+// Discard a reserved uninitialized element.
+func (*Backend[K, V]) Discard(elem Element[V]) {
+	elem.Value.wg.Done()
+}
+
 // Initialize a previously stored uninitialized element.
 func (b *Backend[K, V]) Initialize(key K, value V, ttl time.Duration) {
 	b.mu.Lock()