plugin/file: Fix race condition in file plugin closer (#8583)

This PR fixes race condition in file plugin closer

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/file/closest.go

@@ -8,9 +8,14 @@ import (
 
 // ClosestEncloser returns the closest encloser for qname.
 func (z *Zone) ClosestEncloser(qname string) (*tree.Elem, bool) {
+	_, zoneTree := z.snapshot()
+	if zoneTree == nil {
+		return nil, false
+	}
+
 	offset, end := dns.NextLabel(qname, 0)
 	for !end {
-		elem, _ := z.Search(qname)
+		elem, _ := zoneTree.Search(qname)
 		if elem != nil {
 			return elem, true
 		}
@@ -19,5 +24,5 @@ func (z *Zone) ClosestEncloser(qname string) (*tree.Elem, bool) {
 		offset, end = dns.NextLabel(qname, 0)
 	}
 
-	return z.Search(z.origin)
+	return zoneTree.Search(z.origin)
 }

plugin/file/closest_test.go

@@ -37,3 +37,27 @@ func TestClosestEncloser(t *testing.T) {
 		}
 	}
 }
+
+func TestClosestEncloserConcurrentSetData(t *testing.T) {
+	z, err := Parse(strings.NewReader(dbMiekNL), testzone, "stdin", 0)
+	if err != nil {
+		t.Fatalf("Expect no error when reading zone, got %q", err)
+	}
+
+	apex, zoneTree := z.snapshot()
+	start := make(chan struct{})
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		<-start
+		for range 10000 {
+			z.setData(apex, zoneTree)
+		}
+	}()
+
+	close(start)
+	for range 10000 {
+		z.ClosestEncloser("blaat.www.miek.nl.")
+	}
+	<-done
+}