Http: when caching allow single inflight GET request only (#22959)

plugin/http_limit.go

@@ -0,0 +1,21 @@
+package plugin
+
+import "sync"
+
+var (
+	httpMu      sync.Mutex
+	httpMutexes = map[string]*sync.Mutex{}
+)
+
+func muForKey(key string) *sync.Mutex {
+	httpMu.Lock()
+	defer httpMu.Unlock()
+
+	if mu, ok := httpMutexes[key]; ok {
+		return mu
+	}
+
+	mu := new(sync.Mutex)
+	httpMutexes[key] = mu
+	return mu
+}