fix: keep metadata subscriptions progressing (#8730) (#8746)

* fix: keep metadata subscriptions progressing (#8730)

* test: cancel slow metadata writers with parent context

* filer: ignore missing persisted log chunks

test/metadata_subscribe/metadata_subscribe_integration_test.go

@@ -3,6 +3,7 @@ package metadata_subscribe
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"mime/multipart"
@@ -27,6 +28,8 @@ import (
 	"google.golang.org/grpc/credentials/insecure"
 )
 
+const slowConsumerMetadataPayloadSize = 4096
+
 // TestMetadataSubscribeBasic tests basic metadata subscription functionality
 func TestMetadataSubscribeBasic(t *testing.T) {
 	if testing.Short() {
@@ -680,6 +683,108 @@ func TestMetadataSubscribeMillionUpdates(t *testing.T) {
 	})
 }
 
+func TestMetadataSubscribeSlowConsumerKeepsProgressing(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping integration test in short mode")
+	}
+
+	testDir, err := os.MkdirTemp("", "seaweedfs_slow_consumer_test_")
+	require.NoError(t, err)
+	defer os.RemoveAll(testDir)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Minute)
+	defer cancel()
+
+	cluster, err := startSeaweedFSCluster(ctx, testDir)
+	require.NoError(t, err)
+	defer cluster.Stop()
+
+	require.NoError(t, waitForHTTPServer("http://127.0.0.1:9333", 30*time.Second))
+	require.NoError(t, waitForHTTPServer("http://127.0.0.1:8080", 30*time.Second))
+	require.NoError(t, waitForHTTPServer("http://127.0.0.1:8888", 30*time.Second))
+
+	t.Logf("Cluster started for slow consumer regression test")
+
+	t.Run("single_filer_slow_consumer", func(t *testing.T) {
+		var receivedCount int64
+		phaseOneEntries := 6000
+		phaseTwoEntries := 14000
+		totalEntries := phaseOneEntries + phaseTwoEntries
+		minExpected := int64(12000)
+		errChan := make(chan error, 1)
+
+		subCtx, subCancel := context.WithCancel(ctx)
+		defer subCancel()
+
+		go func() {
+			err := followMetadataSlowly(
+				subCtx,
+				"127.0.0.1:8888",
+				"/slow-consumer/",
+				time.Now().Add(-5*time.Second).UnixNano(),
+				time.Millisecond,
+				func(resp *filer_pb.SubscribeMetadataResponse) {
+					if resp.GetEventNotification() == nil {
+						return
+					}
+					entry := resp.GetEventNotification().GetNewEntry()
+					if entry == nil || entry.IsDirectory {
+						return
+					}
+					atomic.AddInt64(&receivedCount, 1)
+				},
+			)
+			if err != nil && !errors.Is(err, context.Canceled) {
+				errChan <- err
+			}
+		}()
+
+		time.Sleep(2 * time.Second)
+
+		payload := bytes.Repeat([]byte("x"), slowConsumerMetadataPayloadSize)
+		startTime := time.Now()
+		require.NoError(t, createMetadataEntries(ctx, "127.0.0.1:8888", 0, phaseOneEntries, payload))
+		t.Logf("Created phase 1 with %d entries in %v", phaseOneEntries, time.Since(startTime))
+
+		time.Sleep(2 * time.Second)
+
+		require.NoError(t, createMetadataEntries(ctx, "127.0.0.1:8888", phaseOneEntries, phaseTwoEntries, payload))
+		t.Logf("Created phase 2 with %d entries", phaseTwoEntries)
+
+		checkTicker := time.NewTicker(2 * time.Second)
+		defer checkTicker.Stop()
+		deadline := time.NewTimer(45 * time.Second)
+		defer deadline.Stop()
+
+		lastReceived := int64(-1)
+		stableChecks := 0
+
+		for {
+			select {
+			case err := <-errChan:
+				t.Fatalf("slow consumer subscription error: %v", err)
+			case <-deadline.C:
+				t.Fatalf("timed out waiting for slow consumer progress, received %d/%d", atomic.LoadInt64(&receivedCount), totalEntries)
+			case <-checkTicker.C:
+				received := atomic.LoadInt64(&receivedCount)
+				t.Logf("Slow consumer progress: %d/%d", received, totalEntries)
+				if received >= minExpected {
+					return
+				}
+				if received == lastReceived {
+					stableChecks++
+					if stableChecks >= 4 {
+						t.Fatalf("slow consumer stalled at %d/%d after writes completed", received, totalEntries)
+					}
+				} else {
+					stableChecks = 0
+				}
+				lastReceived = received
+			}
+		}
+	})
+}
+
 // Helper types and functions
 
 type TestCluster struct {
@@ -915,3 +1020,106 @@ func subscribeToMetadataWithOptions(ctx context.Context, filerGrpcAddress, pathP
 		}
 	})
 }
+
+func followMetadataSlowly(
+	ctx context.Context,
+	filerGrpcAddress, pathPrefix string,
+	sinceNs int64,
+	delay time.Duration,
+	onEvent func(resp *filer_pb.SubscribeMetadataResponse),
+) error {
+	grpcDialOption := security.LoadClientTLS(util.GetViper(), "grpc.client")
+	if grpcDialOption == nil {
+		grpcDialOption = grpc.WithTransportCredentials(insecure.NewCredentials())
+	}
+
+	option := &pb.MetadataFollowOption{
+		ClientName:     "slow_consumer_test",
+		ClientId:       util.RandomInt32(),
+		ClientEpoch:    int32(time.Now().Unix()),
+		PathPrefix:     pathPrefix,
+		StartTsNs:      sinceNs,
+		EventErrorType: pb.DontLogError,
+	}
+
+	return pb.FollowMetadata(pb.ServerAddress(filerGrpcAddress), grpcDialOption, option, func(resp *filer_pb.SubscribeMetadataResponse) error {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
+		}
+
+		onEvent(resp)
+
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case <-time.After(delay):
+			return nil
+		}
+	})
+}
+
+func createMetadataEntries(ctx context.Context, filerGrpcAddress string, startIndex, total int, payload []byte) error {
+	const workers = 10
+
+	grpcDialOption := grpc.WithTransportCredentials(insecure.NewCredentials())
+	errCh := make(chan error, workers)
+	var wg sync.WaitGroup
+
+	for workerID := 0; workerID < workers; workerID++ {
+		wg.Add(1)
+		go func(workerID int) {
+			defer wg.Done()
+
+			err := pb.WithFilerClient(false, 0, pb.ServerAddress(filerGrpcAddress), grpcDialOption, func(client filer_pb.SeaweedFilerClient) error {
+				for idx := startIndex + workerID; idx < startIndex+total; idx += workers {
+					select {
+					case <-ctx.Done():
+						return ctx.Err()
+					default:
+					}
+
+					dir := fmt.Sprintf("/slow-consumer/bucket-%02d", idx%6)
+					name := fmt.Sprintf("entry-%05d", idx)
+
+					_, err := client.CreateEntry(ctx, &filer_pb.CreateEntryRequest{
+						Directory: dir,
+						Entry: &filer_pb.Entry{
+							Name:        name,
+							IsDirectory: false,
+							Attributes: &filer_pb.FuseAttributes{
+								FileSize: uint64(len(payload)),
+								Mtime:    time.Now().Unix(),
+								FileMode: 0644,
+								Uid:      1000,
+								Gid:      1000,
+							},
+							Extended: map[string][]byte{
+								"payload": payload,
+							},
+						},
+					})
+					if err != nil {
+						return err
+					}
+				}
+				return nil
+			})
+			if err != nil {
+				errCh <- err
+			}
+		}(workerID)
+	}
+
+	wg.Wait()
+	close(errCh)
+
+	for err := range errCh {
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}

weed/filer/filer.go

@@ -81,7 +81,7 @@ func NewFiler(masters pb.ServerDiscovery, grpcDialOption grpc.DialOption, filerH
 		f.UniqueFilerId = -f.UniqueFilerId
 	}
 
-	f.LocalMetaLogBuffer = log_buffer.NewLogBuffer("local", LogFlushInterval, f.logFlushFunc, nil, notifyFn)
+	f.LocalMetaLogBuffer = log_buffer.NewLogBuffer("local", LogFlushInterval, f.logFlushFunc, f.readPersistedLogBufferPosition, notifyFn)
 	f.metaLogCollection = collection
 	f.metaLogReplication = replication
 

weed/filer/filer_notify.go

@@ -2,8 +2,10 @@ package filer
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
+	nethttp "net/http"
 	"regexp"
 	"strings"
 	"time"
@@ -16,6 +18,7 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/notification"
 	"github.com/seaweedfs/seaweedfs/weed/pb/filer_pb"
 	"github.com/seaweedfs/seaweedfs/weed/util"
+	util_http "github.com/seaweedfs/seaweedfs/weed/util/http"
 )
 
 func (f *Filer) NotifyUpdateEvent(ctx context.Context, oldEntry, newEntry *Entry, deleteChunks, isFromOtherCluster bool, signatures []int32) {
@@ -174,6 +177,7 @@ func (f *Filer) logFlushFunc(logBuffer *log_buffer.LogBuffer, startTime, stopTim
 var (
 	volumeNotFoundPattern = regexp.MustCompile(`volume \d+? not found`)
 	chunkNotFoundPattern  = regexp.MustCompile(`(urls not found|File Not Found)`)
+	httpNotFoundPattern   = regexp.MustCompile(`404 Not Found: not found`)
 )
 
 // isChunkNotFoundError checks if the error indicates that a volume or chunk
@@ -183,8 +187,13 @@ func isChunkNotFoundError(err error) bool {
 	if err == nil {
 		return false
 	}
+	if errors.Is(err, util_http.ErrNotFound) || errors.Is(err, nethttp.ErrMissingFile) {
+		return true
+	}
 	errMsg := err.Error()
-	return volumeNotFoundPattern.MatchString(errMsg) || chunkNotFoundPattern.MatchString(errMsg)
+	return volumeNotFoundPattern.MatchString(errMsg) ||
+		chunkNotFoundPattern.MatchString(errMsg) ||
+		httpNotFoundPattern.MatchString(errMsg)
 }
 
 func (f *Filer) ReadPersistedLogBuffer(startPosition log_buffer.MessagePosition, stopTsNs int64, eachLogEntryFn log_buffer.EachLogEntryFuncType) (lastTsNs int64, isDone bool, err error) {
@@ -220,3 +229,17 @@ func (f *Filer) ReadPersistedLogBuffer(startPosition log_buffer.MessagePosition,
 
 	return
 }
+
+func (f *Filer) readPersistedLogBufferPosition(startPosition log_buffer.MessagePosition, stopTsNs int64, eachLogEntryFn log_buffer.EachLogEntryFuncType) (lastReadPosition log_buffer.MessagePosition, isDone bool, err error) {
+	lastReadPosition = startPosition
+
+	lastTsNs, isDone, err := f.ReadPersistedLogBuffer(startPosition, stopTsNs, eachLogEntryFn)
+	if err != nil {
+		return lastReadPosition, isDone, err
+	}
+	if lastTsNs != 0 {
+		lastReadPosition = log_buffer.NewMessagePosition(lastTsNs, 1)
+	}
+
+	return lastReadPosition, isDone, nil
+}

weed/filer/meta_aggregator.go

@@ -74,6 +74,18 @@ func (ma *MetaAggregator) OnPeerUpdate(update *master_pb.ClusterNodeUpdate, star
 	}
 }
 
+func (ma *MetaAggregator) HasRemotePeers() bool {
+	ma.peerChansLock.Lock()
+	defer ma.peerChansLock.Unlock()
+
+	for address := range ma.peerChans {
+		if address != ma.self {
+			return true
+		}
+	}
+	return false
+}
+
 func (ma *MetaAggregator) loopSubscribeToOneFiler(f *Filer, self pb.ServerAddress, peer pb.ServerAddress, startFrom time.Time, stopChan chan struct{}) {
 	lastTsNs := startFrom.UnixNano()
 	for {

weed/server/filer_grpc_server_sub_meta.go

@@ -24,6 +24,9 @@ const (
 )
 
 func (fs *FilerServer) SubscribeMetadata(req *filer_pb.SubscribeMetadataRequest, stream filer_pb.SeaweedFiler_SubscribeMetadataServer) error {
+	if fs.filer.MetaAggregator == nil || !fs.filer.MetaAggregator.HasRemotePeers() {
+		return fs.SubscribeLocalMetadata(req, stream)
+	}
 
 	ctx := stream.Context()
 	peerAddress := findClientAddress(ctx, 0)
@@ -99,18 +102,11 @@ func (fs *FilerServer) SubscribeMetadata(req *filer_pb.SubscribeMetadataRequest,
 		glog.V(4).Infof("read in memory %v aggregated subscribe %s from %+v", clientName, req.PathPrefix, lastReadTime)
 
 		lastReadTime, isDone, readInMemoryLogErr = fs.filer.MetaAggregator.MetaLogBuffer.LoopProcessLogData("aggMeta:"+clientName, lastReadTime, req.UntilNs, func() bool {
-			// Check if the client has disconnected by monitoring the context
 			select {
 			case <-ctx.Done():
 				return false
 			default:
 			}
-
-			fs.filer.MetaAggregator.ListenersLock.Lock()
-			atomic.AddInt64(&fs.filer.MetaAggregator.ListenersWaits, 1)
-			fs.filer.MetaAggregator.ListenersCond.Wait()
-			atomic.AddInt64(&fs.filer.MetaAggregator.ListenersWaits, -1)
-			fs.filer.MetaAggregator.ListenersLock.Unlock()
 			return fs.hasClient(req.ClientId, req.ClientEpoch)
 		}, eachLogEntryFn)
 		if readInMemoryLogErr != nil {
@@ -237,23 +233,12 @@ func (fs *FilerServer) SubscribeLocalMetadata(req *filer_pb.SubscribeMetadataReq
 		glog.V(3).Infof("read in memory %v local subscribe %s from %+v", clientName, req.PathPrefix, lastReadTime)
 
 		lastReadTime, isDone, readInMemoryLogErr = fs.filer.LocalMetaLogBuffer.LoopProcessLogData("localMeta:"+clientName, lastReadTime, req.UntilNs, func() bool {
-
-			// Check if the client has disconnected by monitoring the context
 			select {
 			case <-ctx.Done():
 				return false
 			default:
 			}
-
-			fs.listenersLock.Lock()
-			atomic.AddInt64(&fs.listenersWaits, 1)
-			fs.listenersCond.Wait()
-			atomic.AddInt64(&fs.listenersWaits, -1)
-			fs.listenersLock.Unlock()
-			if !fs.hasClient(req.ClientId, req.ClientEpoch) {
-				return false
-			}
-			return true
+			return fs.hasClient(req.ClientId, req.ClientEpoch)
 		}, eachLogEntryFn)
 		if readInMemoryLogErr != nil {
 			if readInMemoryLogErr == log_buffer.ResumeFromDiskError {

weed/util/log_buffer/log_buffer.go

@@ -606,6 +606,22 @@ func (logBuffer *LogBuffer) invalidateAllDiskCacheChunks() {
 func (logBuffer *LogBuffer) GetEarliestTime() time.Time {
 	return logBuffer.startTime
 }
+
+func (logBuffer *LogBuffer) HasData() bool {
+	logBuffer.RLock()
+	defer logBuffer.RUnlock()
+
+	if logBuffer.pos > 0 {
+		return true
+	}
+	for _, buf := range logBuffer.prevBuffers.buffers {
+		if buf.size > 0 {
+			return true
+		}
+	}
+	return false
+}
+
 func (logBuffer *LogBuffer) GetEarliestPosition() MessagePosition {
 	return MessagePosition{
 		Time:   logBuffer.startTime,
@@ -771,7 +787,9 @@ func (logBuffer *LogBuffer) ReadFromBuffer(lastReadPosition MessagePosition) (bu
 					glog.Errorf("ReadFromBuffer: buffer corruption in prevBuffer: %v", err)
 					return nil, -1, fmt.Errorf("%w: %v", ErrBufferCorrupted, err)
 				}
-				return copiedBytes(buf.buf[pos:buf.size]), buf.offset, nil
+				if pos < buf.size {
+					return copiedBytes(buf.buf[pos:buf.size]), buf.offset, nil
+				}
 			}
 		}
 		// If current buffer is not empty, return it

weed/util/log_buffer/log_buffer_test.go

@@ -1,6 +1,7 @@
 package log_buffer
 
 import (
+	"bytes"
 	"crypto/rand"
 	"fmt"
 	"io"
@@ -67,6 +68,48 @@ func TestNewLogBufferFirstBuffer(t *testing.T) {
 	}
 }
 
+func TestReadFromBufferTimestampBased_AfterFlushReturnsNewerData(t *testing.T) {
+	lb := NewLogBuffer("test", time.Hour, nil, nil, nil)
+	defer lb.ShutdownLogBuffer()
+
+	payload := bytes.Repeat([]byte("x"), 4096)
+	var sealed *MemBuffer
+
+	for i := 0; i < 5000; i++ {
+		if err := lb.AddDataToBuffer([]byte("k"), payload, int64(i+1)); err != nil {
+			t.Fatalf("AddDataToBuffer(%d): %v", i, err)
+		}
+		candidate := lb.prevBuffers.buffers[len(lb.prevBuffers.buffers)-1]
+		if candidate.size > 0 {
+			sealed = &MemBuffer{
+				size:      candidate.size,
+				startTime: candidate.startTime,
+				stopTime:  candidate.stopTime,
+				offset:    candidate.offset,
+			}
+			break
+		}
+	}
+
+	if sealed == nil {
+		t.Fatal("expected first buffer flush to produce a sealed buffer")
+	}
+
+	for i := 5000; i < 5100; i++ {
+		if err := lb.AddDataToBuffer([]byte("k"), payload, int64(i+1)); err != nil {
+			t.Fatalf("AddDataToBuffer(%d): %v", i, err)
+		}
+	}
+
+	buf, _, err := lb.ReadFromBuffer(NewMessagePosition(sealed.stopTime.UnixNano(), sealed.offset))
+	if err != nil {
+		t.Fatalf("ReadFromBuffer returned error: %v", err)
+	}
+	if buf == nil || buf.Len() == 0 {
+		t.Fatalf("expected newer data after the first sealed buffer, got %v", buf)
+	}
+}
+
 // TestReadFromBuffer_OldOffsetReturnsResumeFromDiskError tests that requesting an old offset
 // that has been flushed to disk properly returns ResumeFromDiskError instead of hanging forever.
 // This reproduces the bug where Schema Registry couldn't read the _schemas topic.

weed/util/log_buffer/log_read.go

@@ -77,6 +77,7 @@ func (logBuffer *LogBuffer) LoopProcessLogData(readerName string, startPosition
 		if err == ResumeFromDiskError {
 			// Try to read from disk if readFromDiskFn is available
 			if logBuffer.ReadFromDiskFn != nil {
+				prevReadPosition := lastReadPosition
 				lastReadPosition, isDone, err = logBuffer.ReadFromDiskFn(lastReadPosition, stopTsNs, eachLogDataFn)
 				if err != nil {
 					return lastReadPosition, isDone, err
@@ -84,6 +85,11 @@ func (logBuffer *LogBuffer) LoopProcessLogData(readerName string, startPosition
 				if isDone {
 					return lastReadPosition, isDone, nil
 				}
+				if lastReadPosition != prevReadPosition {
+					continue
+				}
+			} else if logBuffer.HasData() {
+				return lastReadPosition, isDone, ResumeFromDiskError
 			}
 
 			// CRITICAL: Check if client is still connected
@@ -261,6 +267,7 @@ func (logBuffer *LogBuffer) LoopProcessLogDataWithOffset(readerName string, star
 		if err == ResumeFromDiskError {
 			// Try to read from disk if readFromDiskFn is available
 			if logBuffer.ReadFromDiskFn != nil {
+				prevReadPosition := lastReadPosition
 				// Wrap eachLogDataFn to match the expected signature
 				diskReadFn := func(logEntry *filer_pb.LogEntry) (bool, error) {
 					return eachLogDataFn(logEntry, logEntry.Offset)
@@ -272,7 +279,11 @@ func (logBuffer *LogBuffer) LoopProcessLogDataWithOffset(readerName string, star
 				if isDone {
 					return lastReadPosition, isDone, nil
 				}
-				// Continue to next iteration after disk read
+				if lastReadPosition != prevReadPosition {
+					continue
+				}
+			} else if logBuffer.HasData() {
+				return lastReadPosition, isDone, ResumeFromDiskError
 			}
 
 			// CRITICAL: Check if client is still connected after disk read

weed/util/log_buffer/log_read_test.go

@@ -307,6 +307,47 @@ func TestLoopProcessLogDataWithOffset_StopTime(t *testing.T) {
 	t.Logf("Loop correctly exited for past stopTsNs in %v (waitForDataFn called %d times)", elapsed, callCount)
 }
 
+func TestLoopProcessLogData_SlowConsumerFallsBehind(t *testing.T) {
+	flushFn := func(logBuffer *LogBuffer, startTime, stopTime time.Time, buf []byte, minOffset, maxOffset int64) {}
+	logBuffer := NewLogBuffer("test", 1*time.Minute, flushFn, nil, nil)
+	defer logBuffer.ShutdownLogBuffer()
+
+	baseTime := time.Now()
+	for i := 0; i < 1000; i++ {
+		ts := baseTime.Add(time.Duration(i) * time.Millisecond)
+		if err := logBuffer.AddDataToBuffer([]byte("key"), []byte("value"), ts.UnixNano()); err != nil {
+			t.Fatalf("AddDataToBuffer(%d): %v", i, err)
+		}
+	}
+
+	oldPosition := NewMessagePosition(baseTime.Add(-10*time.Second).UnixNano(), 1)
+
+	waitForDataFn := func() bool {
+		t.Errorf("waitForDataFn should not be called for a slow consumer that has fallen behind")
+		return false
+	}
+
+	eachLogEntryFn := func(logEntry *filer_pb.LogEntry) (bool, error) {
+		return false, nil
+	}
+
+	done := make(chan struct{})
+	var err error
+	go func() {
+		_, _, err = logBuffer.LoopProcessLogData("slow-consumer", oldPosition, 0, waitForDataFn, eachLogEntryFn)
+		close(done)
+	}()
+
+	select {
+	case <-done:
+		if err != ResumeFromDiskError {
+			t.Fatalf("expected ResumeFromDiskError, got %v", err)
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("LoopProcessLogData blocked instead of returning ResumeFromDiskError")
+	}
+}
+
 // BenchmarkLoopProcessLogDataWithOffset_EmptyBuffer benchmarks the performance
 // of the loop with an empty buffer to ensure no busy-waiting
 func BenchmarkLoopProcessLogDataWithOffset_EmptyBuffer(b *testing.B) {

weed/util/log_buffer/sealed_buffer.go

@@ -32,7 +32,7 @@ func newSealedBuffers(size int) *SealedBuffers {
 }
 
 func (sbs *SealedBuffers) SealBuffer(startTime, stopTime time.Time, buf []byte, pos int, startOffset int64, endOffset int64) (newBuf []byte) {
-	oldMemBuffer := sbs.buffers[0]
+	oldBuf := sbs.buffers[0].buf
 	size := len(sbs.buffers)
 	for i := 0; i < size-1; i++ {
 		sbs.buffers[i].buf = sbs.buffers[i+1].buf
@@ -48,12 +48,12 @@ func (sbs *SealedBuffers) SealBuffer(startTime, stopTime time.Time, buf []byte,
 	sbs.buffers[size-1].stopTime = stopTime
 	sbs.buffers[size-1].startOffset = startOffset
 	sbs.buffers[size-1].offset = endOffset
-	return oldMemBuffer.buf
+	return oldBuf
 }
 
 func (mb *MemBuffer) locateByTs(lastReadTime time.Time) (pos int, err error) {
 	lastReadTs := lastReadTime.UnixNano()
-	for pos < len(mb.buf) {
+	for pos < mb.size {
 		size, t, readErr := readTs(mb.buf, pos)
 		if readErr != nil {
 			// Return error if buffer is corrupted
@@ -64,7 +64,7 @@ func (mb *MemBuffer) locateByTs(lastReadTime time.Time) (pos int, err error) {
 		}
 		pos += size + 4
 	}
-	return len(mb.buf), nil
+	return mb.size, nil
 }
 
 func (mb *MemBuffer) String() string {