avoid race conditions access to MasterClient.currentMaster (#3538)

https://github.com/seaweedfs/seaweedfs/issues/3510

weed/wdclient/masterclient.go

@@ -27,10 +27,10 @@ type MasterClient struct {
 	grpcDialOption grpc.DialOption
 
 	vidMap
-	vidMapCacheSize int
-
+	vidMapCacheSize  int
 	OnPeerUpdate     func(update *master_pb.ClusterNodeUpdate, startFrom time.Time)
 	OnPeerUpdateLock sync.RWMutex
+	accessLock       sync.RWMutex
 }
 
 func NewMasterClient(grpcDialOption grpc.DialOption, filerGroup string, clientType string, clientHost pb.ServerAddress, clientDataCenter string, rack string, masters map[string]pb.ServerAddress) *MasterClient {
@@ -61,7 +61,7 @@ func (mc *MasterClient) LookupFileIdWithFallback(fileId string) (fullUrls []stri
 	if err == nil && len(fullUrls) > 0 {
 		return
 	}
-	err = pb.WithMasterClient(false, mc.currentMaster, mc.grpcDialOption, false, func(client master_pb.SeaweedClient) error {
+	err = pb.WithMasterClient(false, mc.getCurrentMaster(), mc.grpcDialOption, false, func(client master_pb.SeaweedClient) error {
 		resp, err := client.LookupVolume(context.Background(), &master_pb.LookupVolumeRequest{
 			VolumeOrFileIds: []string{fileId},
 		})
@@ -91,9 +91,21 @@ func (mc *MasterClient) LookupFileIdWithFallback(fileId string) (fullUrls []stri
 	return
 }
 
+func (mc *MasterClient) getCurrentMaster() pb.ServerAddress {
+	mc.accessLock.RLock()
+	defer mc.accessLock.RUnlock()
+	return mc.currentMaster
+}
+
+func (mc *MasterClient) setCurrentMaster(master pb.ServerAddress) {
+	mc.accessLock.Lock()
+	mc.currentMaster = master
+	mc.accessLock.Unlock()
+}
+
 func (mc *MasterClient) GetMaster() pb.ServerAddress {
 	mc.WaitUntilConnected()
-	return mc.currentMaster
+	return mc.getCurrentMaster()
 }
 
 func (mc *MasterClient) GetMasters() map[string]pb.ServerAddress {
@@ -103,7 +115,7 @@ func (mc *MasterClient) GetMasters() map[string]pb.ServerAddress {
 
 func (mc *MasterClient) WaitUntilConnected() {
 	for {
-		if mc.currentMaster != "" {
+		if mc.getCurrentMaster() != "" {
 			return
 		}
 		time.Sleep(time.Duration(rand.Int31n(200)) * time.Millisecond)
@@ -151,8 +163,7 @@ func (mc *MasterClient) tryAllMasters() {
 		for nextHintedLeader != "" {
 			nextHintedLeader = mc.tryConnectToMaster(nextHintedLeader)
 		}
-
-		mc.currentMaster = ""
+		mc.setCurrentMaster("")
 	}
 }
 
@@ -204,7 +215,7 @@ func (mc *MasterClient) tryConnectToMaster(master pb.ServerAddress) (nextHintedL
 		} else {
 			mc.resetVidMap()
 		}
-		mc.currentMaster = master
+		mc.setCurrentMaster(master)
 
 		for {
 			resp, err := stream.Recv()
@@ -216,8 +227,8 @@ func (mc *MasterClient) tryConnectToMaster(master pb.ServerAddress) (nextHintedL
 
 			if resp.VolumeLocation != nil {
 				// maybe the leader is changed
-				if resp.VolumeLocation.Leader != "" && string(mc.currentMaster) != resp.VolumeLocation.Leader {
-					glog.V(0).Infof("currentMaster %v redirected to leader %v", mc.currentMaster, resp.VolumeLocation.Leader)
+				if resp.VolumeLocation.Leader != "" && string(mc.getCurrentMaster()) != resp.VolumeLocation.Leader {
+					glog.V(0).Infof("currentMaster %v redirected to leader %v", mc.getCurrentMaster(), resp.VolumeLocation.Leader)
 					nextHintedLeader = pb.ServerAddress(resp.VolumeLocation.Leader)
 					stats.MasterClientConnectCounter.WithLabelValues(stats.RedirectedToLeader).Inc()
 					return nil
@@ -279,10 +290,10 @@ func (mc *MasterClient) updateVidMap(resp *master_pb.KeepConnectedResponse) {
 
 func (mc *MasterClient) WithClient(streamingMode bool, fn func(client master_pb.SeaweedClient) error) error {
 	return util.Retry("master grpc", func() error {
-		for mc.currentMaster == "" {
+		for mc.getCurrentMaster() == "" {
 			time.Sleep(3 * time.Second)
 		}
-		return pb.WithMasterClient(streamingMode, mc.currentMaster, mc.grpcDialOption, false, func(client master_pb.SeaweedClient) error {
+		return pb.WithMasterClient(streamingMode, mc.getCurrentMaster(), mc.grpcDialOption, false, func(client master_pb.SeaweedClient) error {
 			return fn(client)
 		})
 	})