Add an atomic.Bool to singleflight prefetching (#7963)

Also updated plugin to document single-flighting

Signed-off-by: Ryan Brewster <rpb@anthropic.com>

plugin/cache/item.go

@@ -2,6 +2,7 @@ package cache
 
 import (
 	"strings"
+	"sync/atomic"
 	"time"
 
 	"github.com/coredns/coredns/plugin/cache/freq"
@@ -25,6 +26,14 @@ type item struct {
 	stored  time.Time
 
 	*freq.Freq
+
+	// refreshing is set via CAS when a prefetch goroutine is dispatched for
+	// this item and cleared when it returns, bounding in-flight prefetches
+	// per item to one. A successful prefetch replaces this item in the cache
+	// with a new one (zero-valued refreshing); the deferred clear matters
+	// only when the prefetch fails and this item remains cached, so the next
+	// hit can retry.
+	refreshing atomic.Bool
 }
 
 func newItem(m *dns.Msg, now time.Time, d time.Duration) *item {