Avoid race conditions with current filer address (#3474)

When multiple filer requests are in-flight and the current filer
disappears and a new one is selected by the first goroutine, then
there can be a lot of race conditions while retrieving the current
filer.
Therefore, load/save the current filer index atomically.

weed/mount/weedfs.go

@@ -2,7 +2,16 @@ package mount
 
 import (
 	"context"
+	"math/rand"
+	"os"
+	"path"
+	"path/filepath"
+	"sync/atomic"
+	"time"
+
 	"github.com/hanwen/go-fuse/v2/fuse"
+	"google.golang.org/grpc"
+
 	"github.com/seaweedfs/seaweedfs/weed/filer"
 	"github.com/seaweedfs/seaweedfs/weed/mount/meta_cache"
 	"github.com/seaweedfs/seaweedfs/weed/pb"
@@ -13,12 +22,6 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/util/chunk_cache"
 	"github.com/seaweedfs/seaweedfs/weed/util/grace"
 	"github.com/seaweedfs/seaweedfs/weed/wdclient"
-	"google.golang.org/grpc"
-	"math/rand"
-	"os"
-	"path"
-	"path/filepath"
-	"time"
 
 	"github.com/hanwen/go-fuse/v2/fs"
 )
@@ -26,7 +29,7 @@ import (
 type Option struct {
 	MountDirectory     string
 	FilerAddresses     []pb.ServerAddress
-	filerIndex         int
+	filerIndex         int32
 	GrpcDialOption     grpc.DialOption
 	FilerMountRootPath string
 	Collection         string
@@ -86,7 +89,7 @@ func NewSeaweedFileSystem(option *Option) *WFS {
 		dhmap:         NewDirectoryHandleToInode(),
 	}
 
-	wfs.option.filerIndex = rand.Intn(len(option.FilerAddresses))
+	wfs.option.filerIndex = int32(rand.Intn(len(option.FilerAddresses)))
 	wfs.option.setupUniqueCacheDirectory()
 	if option.CacheSizeMB > 0 {
 		wfs.chunkCache = chunk_cache.NewTieredChunkCache(256, option.getUniqueCacheDir(), option.CacheSizeMB, 1024*1024)
@@ -181,7 +184,8 @@ func (wfs *WFS) LookupFn() wdclient.LookupFileIdFunctionType {
 }
 
 func (wfs *WFS) getCurrentFiler() pb.ServerAddress {
-	return wfs.option.FilerAddresses[wfs.option.filerIndex]
+	i := atomic.LoadInt32(&wfs.option.filerIndex)
+	return wfs.option.FilerAddresses[i]
 }
 
 func (option *Option) setupUniqueCacheDirectory() {