fix deadlock hang when broadcast to clients (#6184)

fix deadlock when broadcast to clients

when master thransfer leader, the old master will disconnect with all
filers and volumeServers, if the cluster is a big , the broadcast
messages may be more big than the max of the channel len 100, then if the
KeepConnect was not listen on the channel in disconnect, it will
deadlock. and the whole cluster will not serve!

weed/server/master_grpc_server.go

@@ -4,12 +4,13 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"github.com/google/uuid"
-	"github.com/seaweedfs/seaweedfs/weed/cluster"
 	"net"
 	"sort"
 	"time"
 
+	"github.com/google/uuid"
+	"github.com/seaweedfs/seaweedfs/weed/cluster"
+
 	"github.com/seaweedfs/seaweedfs/weed/pb"
 	"github.com/seaweedfs/seaweedfs/weed/stats"
 	"github.com/seaweedfs/seaweedfs/weed/storage/backend"
@@ -89,7 +90,7 @@ func (ms *MasterServer) SendHeartbeat(stream master_pb.Seaweed_SendHeartbeatServ
 			glog.V(0).Infof("unregister disconnected volume server %s:%d", dn.Ip, dn.Port)
 			ms.UnRegisterUuids(dn.Ip, dn.Port)
 
-			if len(message.DeletedVids) > 0 || len(message.DeletedEcVids) > 0 {
+			if ms.Topo.IsLeader() && (len(message.DeletedVids) > 0 || len(message.DeletedEcVids) > 0) {
 				ms.broadcastToClients(&master_pb.KeepConnectedResponse{VolumeLocation: message})
 			}
 		}
@@ -338,8 +339,14 @@ func (ms *MasterServer) KeepConnected(stream master_pb.Seaweed_KeepConnectedServ
 
 func (ms *MasterServer) broadcastToClients(message *master_pb.KeepConnectedResponse) {
 	ms.clientChansLock.RLock()
-	for _, ch := range ms.clientChans {
-		ch <- message
+	for client, ch := range ms.clientChans {
+		select {
+		case ch <- message:
+			glog.V(4).Infof("send message to %s", client)
+		default:
+			stats.MasterBroadcastToFullErrorCounter.Inc()
+			glog.Errorf("broadcastToClients %s message full", client)
+		}
 	}
 	ms.clientChansLock.RUnlock()
 }