avoid nil

I1010 03:10:25.220345 mount_std.go:281 This is SeaweedFS version 30GB 3.76 82ed61c655 linux amd64
^[OR^[OR2024/10/10 03:12:56 Unimplemented opcode POLL
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x3983b41]
goroutine 112 [running]:
github.com/seaweedfs/seaweedfs/weed/mount/meta_cache.SubscribeMetaEvents.func2.mergeProceesors.1(0xc000513540)
/github/workspace/weed/mount/meta_cache/meta_cache_subscribe.go:30 +0x41
github.com/seaweedfs/seaweedfs/weed/pb.WithFilerClientFollowMetadata.makeSubscribeMetadataFunc.func1({0x514a560, 0xc0009b06d0})
/github/workspace/weed/pb/filer_pb_tail.go:87 +0x293
github.com/seaweedfs/seaweedfs/weed/mount.(*WFS).WithFilerClient.func1.1(0xc000d9e008)
/github/workspace/weed/mount/wfs_filer_client.go:27 +0x68
github.com/seaweedfs/seaweedfs/weed/pb.WithGrpcClient(0xa9?, 0x9d1f90fb?, 0xc00098fbe0, {0xc000e1c078, 0x15}, 0x0?, {0xc000a76bd0?, 0x1?, 0x1?
})
/github/workspace/weed/pb/grpc_client_server.go:155 +0x33a
github.com/seaweedfs/seaweedfs/weed/mount.(*WFS).WithFilerClient.func1()
/github/workspace/weed/mount/wfs_filer_client.go:25 +0x14c
github.com/seaweedfs/seaweedfs/weed/util.Retry({0x489a10c, 0xa}, 0xc00098fcf8)
/github/workspace/weed/util/retry.go:16 +0xa7
github.com/seaweedfs/seaweedfs/weed/mount.(*WFS).WithFilerClient(0x0?, 0x30?, 0xc00056a568?)
/github/workspace/weed/mount/wfs_filer_client.go:18 +0x65
github.com/seaweedfs/seaweedfs/weed/pb.WithFilerClientFollowMetadata({0x50f5898, 0xc000e2c180}, 0xc00101e000, 0xc000d22030)
/github/workspace/weed/pb/filer_pb_tail.go:51 +0x87
github.com/seaweedfs/seaweedfs/weed/mount/meta_cache.SubscribeMetaEvents.func2()
/github/workspace/weed/mount/meta_cache/meta_cache_subscribe.go:115 +0xcb
github.com/seaweedfs/seaweedfs/weed/util.RetryUntil({0x48b8b35, 0x11}, 0xc00098fef0, 0x4af0048)
/github/workspace/weed/util/retry.go:64 +0x4e
github.com/seaweedfs/seaweedfs/weed/mount/meta_cache.SubscribeMetaEvents(0xc000d1d980, 0x9d1f90fb, {0x50f5898, 0xc000e2c180}, {0x7ffdf6f98dca,
0x4}, 0x17fcf8b7c7ecf8cf, {0xc0000ac208, 0x1, 0x1})
/github/workspace/weed/mount/meta_cache/meta_cache_subscribe.go:113 +0x39b
created by github.com/seaweedfs/seaweedfs/weed/mount.(*WFS).StartBackgroundTasks in goroutine 1
/github/workspace/weed/mount/weedfs.go:150 +0x146

weed/mount/meta_cache/meta_cache_subscribe.go

@@ -15,7 +15,7 @@ type MetadataFollower struct {
 	ProcessEventFn    func(resp *filer_pb.SubscribeMetadataResponse) error
 }
 
-func mergeProceesors(mainProcessor func(resp *filer_pb.SubscribeMetadataResponse) error, followers ...*MetadataFollower) func(resp *filer_pb.SubscribeMetadataResponse) error {
+func mergeProcessors(mainProcessor func(resp *filer_pb.SubscribeMetadataResponse) error, followers ...*MetadataFollower) func(resp *filer_pb.SubscribeMetadataResponse) error {
 	return func(resp *filer_pb.SubscribeMetadataResponse) error {
 
 		// build the full path
@@ -23,16 +23,18 @@ func mergeProceesors(mainProcessor func(resp *filer_pb.SubscribeMetadataResponse
 		if entry == nil {
 			entry = resp.EventNotification.OldEntry
 		}
-		dir := resp.Directory
-		if resp.EventNotification.NewParentPath != "" {
-			dir = resp.EventNotification.NewParentPath
-		}
-		fp := util.NewFullPath(dir, entry.Name)
+		if entry != nil {
+			dir := resp.Directory
+			if resp.EventNotification.NewParentPath != "" {
+				dir = resp.EventNotification.NewParentPath
+			}
+			fp := util.NewFullPath(dir, entry.Name)
 
-		for _, follower := range followers {
-			if strings.HasPrefix(string(fp), follower.PathPrefixToWatch) {
-				if err := follower.ProcessEventFn(resp); err != nil {
-					return err
+			for _, follower := range followers {
+				if strings.HasPrefix(string(fp), follower.PathPrefixToWatch) {
+					if err := follower.ProcessEventFn(resp); err != nil {
+						return err
+					}
 				}
 			}
 		}
@@ -112,7 +114,7 @@ func SubscribeMetaEvents(mc *MetaCache, selfSignature int32, client filer_pb.Fil
 	}
 	util.RetryUntil("followMetaUpdates", func() error {
 		metadataFollowOption.ClientEpoch++
-		return pb.WithFilerClientFollowMetadata(client, metadataFollowOption, mergeProceesors(processEventFn, followers...))
+		return pb.WithFilerClientFollowMetadata(client, metadataFollowOption, mergeProcessors(processEventFn, followers...))
 	}, func(err error) bool {
 		glog.Errorf("follow metadata updates: %v", err)
 		return true