fix: FilerClient supports multiple filer addresses for high availability

Critical fix: FilerClient now accepts []ServerAddress instead of single address
- Prevents mount failure when first filer is down (regression fix)
- Implements automatic failover to remaining filers
- Uses round-robin with atomic index tracking (same pattern as WFS.WithFilerClient)
- Retries all configured filers before giving up
- Updates successful filer index for future requests

Changes:
- NewFilerClient([]pb.ServerAddress, ...) instead of (pb.ServerAddress, ...)
- filerVolumeProvider references FilerClient for failover access
- LookupVolumeIds tries all filers with util.Retry pattern
- Mount passes all option.FilerAddresses for HA
- S3 wraps single filer in slice for API consistency

This restores the high availability that existed in the old implementation
where mount would automatically failover between configured filers.

weed/mount/weedfs.go

@@ -102,6 +102,7 @@ type WFS struct {
 
 func NewSeaweedFileSystem(option *Option) *WFS {
 	// Create FilerClient for efficient volume location caching
+	// Pass all filer addresses for high availability with automatic failover
 	// Configure URL preference based on VolumeServerAccess option
 	var opts *wdclient.FilerClientOption
 	if option.VolumeServerAccess == "publicUrl" {
@@ -111,7 +112,7 @@ func NewSeaweedFileSystem(option *Option) *WFS {
 	}
 
 	filerClient := wdclient.NewFilerClient(
-		option.FilerAddresses[0],
+		option.FilerAddresses, // Pass all filer addresses for HA
 		option.GrpcDialOption,
 		option.DataCenter,
 		opts,

weed/s3api/s3api_server.go

@@ -95,7 +95,8 @@ func NewS3ApiServerWithStore(router *mux.Router, option *S3ApiServerOption, expl
 
 	// Initialize FilerClient for volume location caching
 	// Uses the battle-tested vidMap with filer-based lookups
-	filerClient := wdclient.NewFilerClient(option.Filer, option.GrpcDialOption, option.DataCenter)
+	// S3 API typically connects to a single filer, but wrap in slice for consistency
+	filerClient := wdclient.NewFilerClient([]pb.ServerAddress{option.Filer}, option.GrpcDialOption, option.DataCenter)
 	glog.V(0).Infof("S3 API initialized FilerClient for volume location caching")
 
 	s3ApiServer = &S3ApiServer{

weed/wdclient/filer_client.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"strings"
+	"sync/atomic"
 	"time"
 
 	"google.golang.org/grpc"
@@ -11,6 +12,7 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/glog"
 	"github.com/seaweedfs/seaweedfs/weed/pb"
 	"github.com/seaweedfs/seaweedfs/weed/pb/filer_pb"
+	"github.com/seaweedfs/seaweedfs/weed/util"
 )
 
 // UrlPreference controls which URL to use for volume access
@@ -23,9 +25,11 @@ const (
 
 // FilerClient provides volume location services by querying a filer
 // It uses the shared vidMap cache for efficient lookups
+// Supports multiple filer addresses with automatic failover for high availability
 type FilerClient struct {
 	*vidMapClient
-	filerAddress   pb.ServerAddress
+	filerAddresses []pb.ServerAddress
+	filerIndex     int32 // atomic: current filer index for round-robin
 	grpcDialOption grpc.DialOption
 	urlPreference  UrlPreference
 	grpcTimeout    time.Duration
@@ -33,10 +37,9 @@ type FilerClient struct {
 }
 
 // filerVolumeProvider implements VolumeLocationProvider by querying filer
+// Supports multiple filer addresses with automatic failover
 type filerVolumeProvider struct {
-	filerAddress   pb.ServerAddress
-	grpcDialOption grpc.DialOption
-	grpcTimeout    time.Duration
+	filerClient *FilerClient
 }
 
 // FilerClientOption holds optional configuration for FilerClient
@@ -46,9 +49,14 @@ type FilerClientOption struct {
 	CacheSize     int // Number of historical vidMap snapshots (0 = use default)
 }
 
-// NewFilerClient creates a new client that queries filer for volume locations
+// NewFilerClient creates a new client that queries filer(s) for volume locations
+// Supports multiple filer addresses for high availability with automatic failover
 // Uses sensible defaults: 5-second gRPC timeout, PreferUrl, DefaultVidMapCacheSize
-func NewFilerClient(filerAddress pb.ServerAddress, grpcDialOption grpc.DialOption, dataCenter string, opts ...*FilerClientOption) *FilerClient {
+func NewFilerClient(filerAddresses []pb.ServerAddress, grpcDialOption grpc.DialOption, dataCenter string, opts ...*FilerClientOption) *FilerClient {
+	if len(filerAddresses) == 0 {
+		glog.Fatal("NewFilerClient requires at least one filer address")
+	}
+
 	// Apply defaults
 	grpcTimeout := 5 * time.Second
 	urlPref := PreferUrl
@@ -68,20 +76,23 @@ func NewFilerClient(filerAddress pb.ServerAddress, grpcDialOption grpc.DialOptio
 		}
 	}
 
-	provider := &filerVolumeProvider{
-		filerAddress:   filerAddress,
-		grpcDialOption: grpcDialOption,
-		grpcTimeout:    grpcTimeout,
-	}
-
-	return &FilerClient{
-		vidMapClient:   newVidMapClient(provider, dataCenter, cacheSize),
-		filerAddress:   filerAddress,
+	fc := &FilerClient{
+		filerAddresses: filerAddresses,
+		filerIndex:     0,
 		grpcDialOption: grpcDialOption,
 		urlPreference:  urlPref,
 		grpcTimeout:    grpcTimeout,
 		cacheSize:      cacheSize,
 	}
+
+	// Create provider that references this FilerClient for failover support
+	provider := &filerVolumeProvider{
+		filerClient: fc,
+	}
+
+	fc.vidMapClient = newVidMapClient(provider, dataCenter, cacheSize)
+
+	return fc
 }
 
 // GetLookupFileIdFunction returns a lookup function with URL preference handling
@@ -123,21 +134,32 @@ func (fc *FilerClient) GetLookupFileIdFunction() LookupFileIdFunctionType {
 	}
 }
 
-// LookupVolumeIds queries the filer for volume locations
+// LookupVolumeIds queries the filer for volume locations with automatic failover
+// Tries all configured filer addresses until one succeeds (high availability)
 // Note: Unlike master's VolumeIdLocation, filer's Locations message doesn't currently have
 // an Error field. This implementation handles the current structure while being prepared
 // for future error reporting enhancements.
 func (p *filerVolumeProvider) LookupVolumeIds(ctx context.Context, volumeIds []string) (map[string][]Location, error) {
+	fc := p.filerClient
 	result := make(map[string][]Location)
 
 	// Create a timeout context for the gRPC call
-	timeoutCtx, cancel := context.WithTimeout(ctx, p.grpcTimeout)
+	timeoutCtx, cancel := context.WithTimeout(ctx, fc.grpcTimeout)
 	defer cancel()
 
 	// Convert grpcTimeout to milliseconds for the signature parameter
-	timeoutMs := int32(p.grpcTimeout.Milliseconds())
+	timeoutMs := int32(fc.grpcTimeout.Milliseconds())
+
+	// Try all filer addresses with round-robin starting from current index
+	var lastErr error
+	err := util.Retry("filer volume lookup", func() error {
+		i := atomic.LoadInt32(&fc.filerIndex)
+		n := int32(len(fc.filerAddresses))
+
+		for x := int32(0); x < n; x++ {
+			filerAddress := fc.filerAddresses[i]
 
-	err := pb.WithGrpcFilerClient(false, timeoutMs, p.filerAddress, p.grpcDialOption, func(client filer_pb.SeaweedFilerClient) error {
+			err := pb.WithGrpcFilerClient(false, timeoutMs, filerAddress, fc.grpcDialOption, func(client filer_pb.SeaweedFilerClient) error {
 				resp, err := client.LookupVolume(timeoutCtx, &filer_pb.LookupVolumeRequest{
 					VolumeIds: volumeIds,
 				})
@@ -180,10 +202,28 @@ func (p *filerVolumeProvider) LookupVolumeIds(ctx context.Context, volumeIds []s
 			})
 
 			if err != nil {
-		// gRPC error - this is a communication or server failure
+				glog.V(1).Infof("FilerClient: filer %s lookup failed (attempt %d/%d): %v", filerAddress, x+1, n, err)
+				lastErr = err
+				i++
+				if i >= n {
+					i = 0
+				}
+				continue
+			}
+
+			// Success - update the preferred filer index for next time
+			atomic.StoreInt32(&fc.filerIndex, i)
+			glog.V(3).Infof("FilerClient: looked up %d volumes on %s, found %d", len(volumeIds), filerAddress, len(result))
+			return nil
+		}
+
+		// All filers failed
+		return fmt.Errorf("all %d filer(s) failed, last error: %w", n, lastErr)
+	})
+
+	if err != nil {
 		return nil, fmt.Errorf("filer volume lookup failed for %d volume(s): %w", len(volumeIds), err)
 	}
 
-	glog.V(3).Infof("FilerClient: looked up %d volumes, found %d", len(volumeIds), len(result))
 	return result, nil
 }