adding locking to avoid nil VolumeLocationList

fix panic: runtime error: invalid memory address or nil pointer dereference
Oct 22 00:53:44 bedb-master1 weed[8055]: [signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x17658da]
Oct 22 00:53:44 bedb-master1 weed[8055]: goroutine 310 [running]:
Oct 22 00:53:44 bedb-master1 weed[8055]: github.com/chrislusf/seaweedfs/weed/topology.(*VolumeLocationList).Length(...)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/seaweedfs/weed/topology/volume_location_list.go:35
Oct 22 00:53:44 bedb-master1 weed[8055]: github.com/chrislusf/seaweedfs/weed/topology.(*VolumeLayout).enoughCopies(...)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/seaweedfs/weed/topology/volume_layout.go:376
Oct 22 00:53:44 bedb-master1 weed[8055]: github.com/chrislusf/seaweedfs/weed/topology.(*VolumeLayout).ensureCorrectWritables(0xc000111d50, 0xc000b55438)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/seaweedfs/weed/topology/volume_layout.go:202 +0x5a
Oct 22 00:53:44 bedb-master1 weed[8055]: github.com/chrislusf/seaweedfs/weed/topology.(*Topology).SyncDataNodeRegistration(0xc00042ac60, 0xc001454d30, 0x1, 0x1, 0xc0005fc000, 0xc00135de40, 0x4, 0xc00135de50, 0x10, 0x10d, ...)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/seaweedfs/weed/topology/topology.go:224 +0x616
Oct 22 00:53:44 bedb-master1 weed[8055]: github.com/chrislusf/seaweedfs/weed/server.(*MasterServer).SendHeartbeat(0xc000162700, 0x23b97c0, 0xc000ae2c90, 0x0, 0x0)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/seaweedfs/weed/server/master_grpc_server.go:106 +0x325
Oct 22 00:53:44 bedb-master1 weed[8055]: github.com/chrislusf/seaweedfs/weed/pb/master_pb._Seaweed_SendHeartbeat_Handler(0x1f8e7c0, 0xc000162700, 0x23b0a60, 0xc00024b440, 0x3172c38, 0xc000ab7100)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/seaweedfs/weed/pb/master_pb/master.pb.go:4250 +0xad
Oct 22 00:53:44 bedb-master1 weed[8055]: google.golang.org/grpc.(*Server).processStreamingRPC(0xc0001f31e0, 0x23bb800, 0xc000ac5500, 0xc000ab7100, 0xc0001fea80, 0x311fec0, 0x0, 0x0, 0x0)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/go/pkg/mod/google.golang.org/grpc@v1.29.1/server.go:1329 +0xcd8
Oct 22 00:53:44 bedb-master1 weed[8055]: google.golang.org/grpc.(*Server).handleStream(0xc0001f31e0, 0x23bb800, 0xc000ac5500, 0xc000ab7100, 0x0)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/go/pkg/mod/google.golang.org/grpc@v1.29.1/server.go:1409 +0xc5c
Oct 22 00:53:44 bedb-master1 weed[8055]: google.golang.org/grpc.(*Server).serveStreams.func1.1(0xc0001ce8b0, 0xc0001f31e0, 0x23bb800, 0xc000ac5500, 0xc000ab7100)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/go/pkg/mod/google.golang.org/grpc@v1.29.1/server.go:746 +0xa5
Oct 22 00:53:44 bedb-master1 weed[8055]: created by google.golang.org/grpc.(*Server).serveStreams.func1
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/go/pkg/mod/google.golang.org/grpc@v1.29.1/server.go:744 +0xa5
Oct 22 00:53:44 bedb-master1 systemd[1]: weedmaster.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
Oct 22 00:53:44 bedb-master1 systemd[1]: weedmaster.service: Failed with result 'exit-code'.

weed/topology/topology.go

@@ -221,7 +221,7 @@ func (t *Topology) SyncDataNodeRegistration(volumes []*master_pb.VolumeInformati
 	}
 	for _, v := range changedVolumes {
 		vl := t.GetVolumeLayout(v.Collection, v.ReplicaPlacement, v.Ttl)
-		vl.ensureCorrectWritables(&v)
+		vl.EnsureCorrectWritables(&v)
 	}
 	return
 }

weed/topology/volume_layout.go

@@ -198,6 +198,13 @@ func (vl *VolumeLayout) UnRegisterVolume(v *storage.VolumeInfo, dn *DataNode) {
 	}
 }
 
+func (vl *VolumeLayout) EnsureCorrectWritables(v *storage.VolumeInfo) {
+	vl.accessLock.Lock()
+	defer vl.accessLock.Unlock()
+
+	vl.ensureCorrectWritables(v)
+}
+
 func (vl *VolumeLayout) ensureCorrectWritables(v *storage.VolumeInfo) {
 	if vl.enoughCopies(v.Id) && vl.isWritable(v) {
 		if !vl.oversizedVolumes.IsTrue(v.Id) {

weed/topology/volume_location_list.go

@@ -32,6 +32,9 @@ func (dnll *VolumeLocationList) Head() *DataNode {
 }
 
 func (dnll *VolumeLocationList) Length() int {
+	if dnll == nil {
+		return 0
+	}
 	return len(dnll.list)
 }