Fix: upstream dns ExchangeContext workaround (#468)

common/picker/picker.go

@@ -59,6 +59,7 @@ func (p *Picker) Wait() interface{} {
 }
 
 // WaitWithoutCancel blocks until the first result return, if timeout will return nil.
+// The return of this function will not wait for the cancel of context.
 func (p *Picker) WaitWithoutCancel() interface{} {
 	select {
 	case <-p.firstDone:

dns/client.go

@@ -16,6 +16,7 @@ func (c *client) Exchange(m *D.Msg) (msg *D.Msg, err error) {
 }
 
 func (c *client) ExchangeContext(ctx context.Context, m *D.Msg) (msg *D.Msg, err error) {
+	// Please note that miekg/dns ExchangeContext doesn't respond to context cancel.
 	msg, _, err = c.Client.ExchangeContext(ctx, m, c.Address)
 	return
 }

dns/resolver.go

@@ -192,7 +192,7 @@ func (r *Resolver) batchExchange(clients []resolver, m *D.Msg) (msg *D.Msg, err
 		})
 	}
 
-	elm := fast.Wait()
+	elm := fast.WaitWithoutCancel()
 	if elm == nil {
 		return nil, errors.New("All DNS requests failed")
 	}

dns/util.go

@@ -134,6 +134,7 @@ func transform(servers []NameServer) []resolver {
 					NextProtos: []string{"dns"},
 				},
 				UDPSize: 4096,
+				Timeout: 5 * time.Second,
 			},
 			Address: s.Addr,
 		})