cmd/cli: add loop guard for LAN/PTR queries

cmd/cli/loop_test.go

@@ -0,0 +1,38 @@
+package cli
+
+import (
+	"sync"
+	"testing"
+)
+
+func Test_loopGuard(t *testing.T) {
+	lg := newLoopGuard()
+	key := "foo"
+
+	var mu sync.Mutex
+	i := 0
+	n := 1000
+	do := func() {
+		locked := lg.TryLock(key)
+		defer lg.Unlock(key)
+		if locked {
+			mu.Lock()
+			i++
+			mu.Unlock()
+		}
+	}
+
+	var wg sync.WaitGroup
+	wg.Add(n)
+	for i := 0; i < n; i++ {
+		go func() {
+			defer wg.Done()
+			do()
+		}()
+	}
+	wg.Wait()
+
+	if i == n {
+		t.Fatalf("i must not be increased %d times", n)
+	}
+}