etcdserver,wal: fix inconsistencies in WAL and snapshot

etcdserver/*, wal/*: changes to snapshots and wal logic
etcdserver/*: changes to snapshots and wal logic to fix #10219
etcdserver/*, wal/*: add Sync method
etcdserver/*, wal/*: find valid snapshots by cross checking snap files and wal snap entries
etcdserver/*, wal/*:Add comments, clean up error messages and tests
etcdserver/*, wal/*: Remove orphaned .snap.db files during Release

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

etcdserver/api/snap/snapshotter.go

@@ -22,16 +22,17 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
+	"strconv"
 	"strings"
 	"time"
 
+	"github.com/coreos/pkg/capnslog"
 	"go.etcd.io/etcd/etcdserver/api/snap/snappb"
 	pioutil "go.etcd.io/etcd/pkg/ioutil"
 	"go.etcd.io/etcd/pkg/pbutil"
 	"go.etcd.io/etcd/raft"
 	"go.etcd.io/etcd/raft/raftpb"
-
-	"github.com/coreos/pkg/capnslog"
+	"go.etcd.io/etcd/wal/walpb"
 	"go.uber.org/zap"
 )
 
@@ -108,21 +109,37 @@ func (s *Snapshotter) save(snapshot *raftpb.Snapshot) error {
 	return nil
 }
 
+// Load returns the newest snapshot.
 func (s *Snapshotter) Load() (*raftpb.Snapshot, error) {
+	return s.loadMatching(func(*raftpb.Snapshot) bool { return true })
+}
+
+// LoadNewestAvailable loads the newest snapshot available that is in walSnaps.
+func (s *Snapshotter) LoadNewestAvailable(walSnaps []walpb.Snapshot) (*raftpb.Snapshot, error) {
+	return s.loadMatching(func(snapshot *raftpb.Snapshot) bool {
+		m := snapshot.Metadata
+		for i := len(walSnaps) - 1; i >= 0; i-- {
+			if m.Term == walSnaps[i].Term && m.Index == walSnaps[i].Index {
+				return true
+			}
+		}
+		return false
+	})
+}
+
+// loadMatching returns the newest snapshot where matchFn returns true.
+func (s *Snapshotter) loadMatching(matchFn func(*raftpb.Snapshot) bool) (*raftpb.Snapshot, error) {
 	names, err := s.snapNames()
 	if err != nil {
 		return nil, err
 	}
 	var snap *raftpb.Snapshot
 	for _, name := range names {
-		if snap, err = loadSnap(s.lg, s.dir, name); err == nil {
-			break
-		}
-	}
-	if err != nil {
-		return nil, ErrNoSnapshot
-	}
+		if snap, err = loadSnap(s.lg, s.dir, name); err == nil && matchFn(snap) {
 			return snap, nil
+		}
+	}
+	return nil, ErrNoSnapshot
 }
 
 func loadSnap(lg *zap.Logger, dir, name string) (*raftpb.Snapshot, error) {
@@ -274,3 +291,31 @@ func (s *Snapshotter) cleanupSnapdir(filenames []string) error {
 	}
 	return nil
 }
+
+func (s *Snapshotter) ReleaseSnapDBs(snap raftpb.Snapshot) error {
+	dir, err := os.Open(s.dir)
+	if err != nil {
+		return err
+	}
+	defer dir.Close()
+	filenames, err := dir.Readdirnames(-1)
+	if err != nil {
+		return err
+	}
+	for _, filename := range filenames {
+		if strings.HasSuffix(filename, ".snap.db") {
+			hexIndex := strings.TrimSuffix(filepath.Base(filename), ".snap.db")
+			index, err := strconv.ParseUint(hexIndex, 16, 64)
+			if err != nil {
+				return fmt.Errorf("failed to parse index from .snap.db filename '%s': %v", filename, err)
+			}
+			if index < snap.Metadata.Index {
+				s.lg.Info("found orphaned .snap.db file; deleting", zap.String("path", filename))
+				if rmErr := os.Remove(filepath.Join(s.dir, filename)); rmErr != nil && !os.IsNotExist(rmErr) {
+					return fmt.Errorf("failed to remove orphaned defragmentation file %s: %v", filename, rmErr)
+				}
+			}
+		}
+	}
+	return nil
+}

etcdserver/api/snap/snapshotter_test.go

@@ -23,7 +23,10 @@ import (
 	"reflect"
 	"testing"
 
+	"go.etcd.io/etcd/pkg/fileutil"
+
 	"go.etcd.io/etcd/raft/raftpb"
+	"go.etcd.io/etcd/wal/walpb"
 	"go.uber.org/zap"
 )
 
@@ -166,12 +169,47 @@ func TestLoadNewestSnap(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	g, err := ss.Load()
+	cases := []struct {
+		name              string
+		availableWalSnaps []walpb.Snapshot
+		expected          *raftpb.Snapshot
+	}{
+		{
+			name:     "load-newest",
+			expected: &newSnap,
+		},
+		{
+			name:              "loadnewestavailable-newest",
+			availableWalSnaps: []walpb.Snapshot{{Index: 0, Term: 0}, {Index: 1, Term: 1}, {Index: 5, Term: 1}},
+			expected:          &newSnap,
+		},
+		{
+			name:              "loadnewestavailable-newest-unsorted",
+			availableWalSnaps: []walpb.Snapshot{{Index: 5, Term: 1}, {Index: 1, Term: 1}, {Index: 0, Term: 0}},
+			expected:          &newSnap,
+		},
+		{
+			name:              "loadnewestavailable-previous",
+			availableWalSnaps: []walpb.Snapshot{{Index: 0, Term: 0}, {Index: 1, Term: 1}},
+			expected:          testSnap,
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			var err error
+			var g *raftpb.Snapshot
+			if tc.availableWalSnaps != nil {
+				g, err = ss.LoadNewestAvailable(tc.availableWalSnaps)
+			} else {
+				g, err = ss.Load()
+			}
 			if err != nil {
 				t.Errorf("err = %v, want nil", err)
 			}
-	if !reflect.DeepEqual(g, &newSnap) {
-		t.Errorf("snap = %#v, want %#v", g, &newSnap)
+			if !reflect.DeepEqual(g, tc.expected) {
+				t.Errorf("snap = %#v, want %#v", g, tc.expected)
+			}
+		})
 	}
 }
 
@@ -229,3 +267,42 @@ func TestAllSnapshotBroken(t *testing.T) {
 		t.Errorf("err = %v, want %v", err, ErrNoSnapshot)
 	}
 }
+
+func TestReleaseSnapDBs(t *testing.T) {
+	dir := filepath.Join(os.TempDir(), "snapshot")
+	err := os.Mkdir(dir, 0700)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	snapIndices := []uint64{100, 200, 300, 400}
+	for _, index := range snapIndices {
+		filename := filepath.Join(dir, fmt.Sprintf("%016x.snap.db", index))
+		if err := ioutil.WriteFile(filename, []byte("snap file\n"), 0644); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	ss := New(zap.NewExample(), dir)
+
+	if err := ss.ReleaseSnapDBs(raftpb.Snapshot{Metadata: raftpb.SnapshotMetadata{Index: 300}}); err != nil {
+		t.Fatal(err)
+	}
+
+	deleted := []uint64{100, 200}
+	for _, index := range deleted {
+		filename := filepath.Join(dir, fmt.Sprintf("%016x.snap.db", index))
+		if fileutil.Exist(filename) {
+			t.Errorf("expected %s (index: %d)  to be deleted, but it still exists", filename, index)
+		}
+	}
+
+	retained := []uint64{300, 400}
+	for _, index := range retained {
+		filename := filepath.Join(dir, fmt.Sprintf("%016x.snap.db", index))
+		if !fileutil.Exist(filename) {
+			t.Errorf("expected %s (index: %d) to be retained, but it no longer exists", filename, index)
+		}
+	}
+}

etcdserver/raft.go

@@ -231,12 +231,26 @@ func (r *raftNode) start(rh *raftReadyHandler) {
 					r.transport.Send(r.processMessages(rd.Messages))
 				}
 
+				// Must save the snapshot file and WAL snapshot entry before saving any other entries or hardstate to
+				// ensure that recovery after a snapshot restore is possible.
+				if !raft.IsEmptySnap(rd.Snapshot) {
+					// gofail: var raftBeforeSaveSnap struct{}
+					if err := r.storage.SaveSnap(rd.Snapshot); err != nil {
+						if r.lg != nil {
+							r.lg.Fatal("failed to save Raft snapshot", zap.Error(err))
+						} else {
+							plog.Fatalf("failed to save Raft snapshot %v", err)
+						}
+					}
+					// gofail: var raftAfterSaveSnap struct{}
+				}
+
 				// gofail: var raftBeforeSave struct{}
 				if err := r.storage.Save(rd.HardState, rd.Entries); err != nil {
 					if r.lg != nil {
 						r.lg.Fatal("failed to save Raft hard state and entries", zap.Error(err))
 					} else {
-						plog.Fatalf("raft save state and entries error: %v", err)
+						plog.Fatalf("failed to save state and entries error: %v", err)
 					}
 				}
 				if !raft.IsEmptyHardState(rd.HardState) {
@@ -245,18 +259,22 @@ func (r *raftNode) start(rh *raftReadyHandler) {
 				// gofail: var raftAfterSave struct{}
 
 				if !raft.IsEmptySnap(rd.Snapshot) {
-					// gofail: var raftBeforeSaveSnap struct{}
-					if err := r.storage.SaveSnap(rd.Snapshot); err != nil {
+					// Force WAL to fsync its hard state before Release() releases
+					// old data from the WAL. Otherwise could get an error like:
+					// panic: tocommit(107) is out of range [lastIndex(84)]. Was the raft log corrupted, truncated, or lost?
+					// See https://github.com/etcd-io/etcd/issues/10219 for more details.
+					if err := r.storage.Sync(); err != nil {
 						if r.lg != nil {
-							r.lg.Fatal("failed to save Raft snapshot", zap.Error(err))
+							r.lg.Fatal("failed to sync Raft snapshot", zap.Error(err))
 						} else {
-							plog.Fatalf("raft save snapshot error: %v", err)
+							plog.Fatalf("failed to sync Raft snapshot %v", err)
 						}
 					}
+
 					// etcdserver now claim the snapshot has been persisted onto the disk
 					notifyc <- struct{}{}
 
-					// gofail: var raftAfterSaveSnap struct{}
+					// gofail: var raftBeforeApplySnap struct{}
 					r.raftStorage.ApplySnapshot(rd.Snapshot)
 					if r.lg != nil {
 						r.lg.Info("applied incoming Raft snapshot", zap.Uint64("snapshot-index", rd.Snapshot.Metadata.Index))
@@ -264,6 +282,15 @@ func (r *raftNode) start(rh *raftReadyHandler) {
 						plog.Infof("raft applied incoming snapshot at index %d", rd.Snapshot.Metadata.Index)
 					}
 					// gofail: var raftAfterApplySnap struct{}
+
+					if err := r.storage.Release(rd.Snapshot); err != nil {
+						if r.lg != nil {
+							r.lg.Fatal("failed to release Raft wal", zap.Error(err))
+						} else {
+							plog.Fatalf("failed to release Raft wal %v", err)
+						}
+					}
+					// gofail: var raftAfterWALRelease struct{}
 				}
 
 				r.raftStorage.Append(rd.Entries)

etcdserver/server.go

@@ -29,6 +29,10 @@ import (
 	"sync/atomic"
 	"time"
 
+	"github.com/coreos/go-semver/semver"
+	"github.com/coreos/pkg/capnslog"
+	humanize "github.com/dustin/go-humanize"
+	"github.com/prometheus/client_golang/prometheus"
 	"go.etcd.io/etcd/auth"
 	"go.etcd.io/etcd/etcdserver/api"
 	"go.etcd.io/etcd/etcdserver/api/membership"
@@ -57,11 +61,6 @@ import (
 	"go.etcd.io/etcd/raft/raftpb"
 	"go.etcd.io/etcd/version"
 	"go.etcd.io/etcd/wal"
-
-	"github.com/coreos/go-semver/semver"
-	"github.com/coreos/pkg/capnslog"
-	humanize "github.com/dustin/go-humanize"
-	"github.com/prometheus/client_golang/prometheus"
 	"go.uber.org/zap"
 )
 
@@ -426,10 +425,19 @@ func NewServer(cfg ServerConfig) (srv *EtcdServer, err error) {
 				plog.Warningf("discovery token ignored since a cluster has already been initialized. Valid log found at %q", cfg.WALDir())
 			}
 		}
-		snapshot, err = ss.Load()
+
+		// Find a snapshot to start/restart a raft node
+		walSnaps, serr := wal.ValidSnapshotEntries(cfg.Logger, cfg.WALDir())
+		if serr != nil {
+			return nil, serr
+		}
+		// snapshot files can be orphaned if etcd crashes after writing them but before writing the corresponding
+		// wal log entries
+		snapshot, err = ss.LoadNewestAvailable(walSnaps)
 		if err != nil && err != snap.ErrNoSnapshot {
 			return nil, err
 		}
+
 		if snapshot != nil {
 			if err = st.Recovery(snapshot.Data); err != nil {
 				if cfg.Logger != nil {
@@ -2370,8 +2378,7 @@ func (s *EtcdServer) snapshot(snapi uint64, confState raftpb.ConfState) {
 				plog.Panicf("unexpected create snapshot error %v", err)
 			}
 		}
-		// SaveSnap saves the snapshot and releases the locked wal files
-		// to the snapshot index.
+		// SaveSnap saves the snapshot to file and appends the corresponding WAL entry.
 		if err = s.r.storage.SaveSnap(snap); err != nil {
 			if lg != nil {
 				lg.Panic("failed to save snapshot", zap.Error(err))
@@ -2387,6 +2394,13 @@ func (s *EtcdServer) snapshot(snapi uint64, confState raftpb.ConfState) {
 		} else {
 			plog.Infof("saved snapshot at index %d", snap.Metadata.Index)
 		}
+		if err = s.r.storage.Release(snap); err != nil {
+			if lg != nil {
+				lg.Panic("failed to release wal", zap.Error(err))
+			} else {
+				plog.Panicf("failed to release wal %v", err)
+			}
+		}
 
 		// When sending a snapshot, etcd will pause compaction.
 		// After receives a snapshot, the slow follower needs to get all the entries right after

etcdserver/server_test.go

@@ -990,15 +990,19 @@ func TestSnapshot(t *testing.T) {
 	ch := make(chan struct{}, 2)
 
 	go func() {
-		gaction, _ := p.Wait(1)
+		gaction, _ := p.Wait(2)
 		defer func() { ch <- struct{}{} }()
 
-		if len(gaction) != 1 {
-			t.Errorf("len(action) = %d, want 1", len(gaction))
+		if len(gaction) != 2 {
+			t.Fatalf("len(action) = %d, want 2", len(gaction))
 		}
 		if !reflect.DeepEqual(gaction[0], testutil.Action{Name: "SaveSnap"}) {
 			t.Errorf("action = %s, want SaveSnap", gaction[0])
 		}
+
+		if !reflect.DeepEqual(gaction[1], testutil.Action{Name: "Release"}) {
+			t.Errorf("action = %s, want Release", gaction[1])
+		}
 	}()
 
 	go func() {
@@ -1081,20 +1085,32 @@ func TestSnapshotOrdering(t *testing.T) {
 		n.readyc <- raft.Ready{Snapshot: snapMsg.Snapshot}
 	}()
 
+	ac := <-p.Chan()
+	if ac.Name != "Save" {
+		t.Fatalf("expected Save, got %+v", ac)
+	}
+
+	if ac := <-p.Chan(); ac.Name != "SaveSnap" {
+		t.Fatalf("expected SaveSnap, got %+v", ac)
+	}
+
 	if ac := <-p.Chan(); ac.Name != "Save" {
 		t.Fatalf("expected Save, got %+v", ac)
 	}
-	if ac := <-p.Chan(); ac.Name != "Save" {
-		t.Fatalf("expected Save, got %+v", ac)
-	}
+
 	// confirm snapshot file still present before calling SaveSnap
 	snapPath := filepath.Join(snapdir, fmt.Sprintf("%016x.snap.db", 1))
 	if !fileutil.Exist(snapPath) {
 		t.Fatalf("expected file %q, got missing", snapPath)
 	}
+
 	// unblock SaveSnapshot, etcdserver now permitted to move snapshot file
-	if ac := <-p.Chan(); ac.Name != "SaveSnap" {
-		t.Fatalf("expected SaveSnap, got %+v", ac)
+	if ac := <-p.Chan(); ac.Name != "Sync" {
+		t.Fatalf("expected Sync, got %+v", ac)
+	}
+
+	if ac := <-p.Chan(); ac.Name != "Release" {
+		t.Fatalf("expected Release, got %+v", ac)
 	}
 }
 
@@ -1133,16 +1149,22 @@ func TestTriggerSnap(t *testing.T) {
 
 	donec := make(chan struct{})
 	go func() {
-		wcnt := 2 + snapc
+		wcnt := 3 + snapc
 		gaction, _ := p.Wait(wcnt)
 
 		// each operation is recorded as a Save
-		// (SnapshotCount+1) * Puts + SaveSnap = (SnapshotCount+1) * Save + SaveSnap
+		// (SnapshotCount+1) * Puts + SaveSnap = (SnapshotCount+1) * Save + SaveSnap + Release
 		if len(gaction) != wcnt {
-			t.Errorf("len(action) = %d, want %d", len(gaction), wcnt)
+			t.Logf("gaction: %v", gaction)
+			t.Fatalf("len(action) = %d, want %d", len(gaction), wcnt)
 		}
-		if !reflect.DeepEqual(gaction[wcnt-1], testutil.Action{Name: "SaveSnap"}) {
-			t.Errorf("action = %s, want SaveSnap", gaction[wcnt-1])
+
+		if !reflect.DeepEqual(gaction[wcnt-2], testutil.Action{Name: "SaveSnap"}) {
+			t.Errorf("action = %s, want SaveSnap", gaction[wcnt-2])
+		}
+
+		if !reflect.DeepEqual(gaction[wcnt-1], testutil.Action{Name: "Release"}) {
+			t.Errorf("action = %s, want Release", gaction[wcnt-1])
 		}
 		close(donec)
 	}()

etcdserver/storage.go

@@ -36,6 +36,10 @@ type Storage interface {
 	SaveSnap(snap raftpb.Snapshot) error
 	// Close closes the Storage and performs finalization.
 	Close() error
+	// Release releases the locked wal files older than the provided snapshot.
+	Release(snap raftpb.Snapshot) error
+	// Sync WAL
+	Sync() error
 }
 
 type storage struct {
@@ -47,24 +51,37 @@ func NewStorage(w *wal.WAL, s *snap.Snapshotter) Storage {
 	return &storage{w, s}
 }
 
-// SaveSnap saves the snapshot to disk and release the locked
-// wal files since they will not be used.
+// SaveSnap saves the snapshot file to disk and writes the WAL snapshot entry.
 func (st *storage) SaveSnap(snap raftpb.Snapshot) error {
 	walsnap := walpb.Snapshot{
 		Index: snap.Metadata.Index,
 		Term:  snap.Metadata.Term,
 	}
-	err := st.WAL.SaveSnapshot(walsnap)
+	// save the snapshot file before writing the snapshot to the wal.
+	// This makes it possible for the snapshot file to become orphaned, but prevents
+	// a WAL snapshot entry from having no corresponding snapshot file.
+	err := st.Snapshotter.SaveSnap(snap)
 	if err != nil {
 		return err
 	}
-	err = st.Snapshotter.SaveSnap(snap)
-	if err != nil {
-		return err
-	}
-	return st.WAL.ReleaseLockTo(snap.Metadata.Index)
+	// gofail: var raftBeforeWALSaveSnaphot struct{}
+
+	return st.WAL.SaveSnapshot(walsnap)
 }
 
+// Release releases resources older than the given snap and are no longer needed:
+// - releases the locks to the wal files that are older than the provided wal for the given snap.
+// - deletes any .snap.db files that are older than the given snap.
+func (st *storage) Release(snap raftpb.Snapshot) error {
+	if err := st.WAL.ReleaseLockTo(snap.Metadata.Index); err != nil {
+		return err
+	}
+	return st.Snapshotter.ReleaseSnapDBs(snap)
+}
+
+// readWAL reads the WAL at the given snap and returns the wal, its latest HardState and cluster ID, and all entries that appear
+// after the position of the given snap in the WAL.
+// The snap must have been previously saved to the WAL, or this call will panic.
 func readWAL(lg *zap.Logger, waldir string, snap walpb.Snapshot) (w *wal.WAL, id, cid types.ID, st raftpb.HardState, ents []raftpb.Entry) {
 	var (
 		err       error

pkg/mock/mockstorage/storage_recorder.go

@@ -45,4 +45,16 @@ func (p *storageRecorder) SaveSnap(st raftpb.Snapshot) error {
 	return nil
 }
 
+func (p *storageRecorder) Release(st raftpb.Snapshot) error {
+	if !raft.IsEmptySnap(st) {
+		p.Record(testutil.Action{Name: "Release"})
+	}
+	return nil
+}
+
+func (p *storageRecorder) Sync() error {
+	p.Record(testutil.Action{Name: "Sync"})
+	return nil
+}
+
 func (p *storageRecorder) Close() error { return nil }

wal/wal.go

@@ -532,6 +532,63 @@ func (w *WAL) ReadAll() (metadata []byte, state raftpb.HardState, ents []raftpb.
 	return metadata, state, ents, err
 }
 
+// ValidSnapshotEntries returns all the valid snapshot entries in the wal logs in the given directory.
+// Snapshot entries are valid if their index is less than or equal to the most recent committed hardstate.
+func ValidSnapshotEntries(lg *zap.Logger, walDir string) ([]walpb.Snapshot, error) {
+	var snaps []walpb.Snapshot
+	var state raftpb.HardState
+	var err error
+
+	rec := &walpb.Record{}
+	names, err := readWALNames(lg, walDir)
+	if err != nil {
+		return nil, err
+	}
+
+	// open wal files in read mode, so that there is no conflict
+	// when the same WAL is opened elsewhere in write mode
+	rs, _, closer, err := openWALFiles(lg, walDir, names, 0, false)
+	if err != nil {
+		return nil, err
+	}
+	defer func() {
+		if closer != nil {
+			closer()
+		}
+	}()
+
+	// create a new decoder from the readers on the WAL files
+	decoder := newDecoder(rs...)
+
+	for err = decoder.decode(rec); err == nil; err = decoder.decode(rec) {
+		switch rec.Type {
+		case snapshotType:
+			var loadedSnap walpb.Snapshot
+			pbutil.MustUnmarshal(&loadedSnap, rec.Data)
+			snaps = append(snaps, loadedSnap)
+		case stateType:
+			state = mustUnmarshalState(rec.Data)
+		}
+	}
+	// We do not have to read out all the WAL entries
+	// as the decoder is opened in read mode.
+	if err != io.EOF && err != io.ErrUnexpectedEOF {
+		return nil, err
+	}
+
+	// filter out any snaps that are newer than the committed hardstate
+	n := 0
+	for _, s := range snaps {
+		if s.Index <= state.Commit {
+			snaps[n] = s
+			n++
+		}
+	}
+	snaps = snaps[:n:n]
+
+	return snaps, nil
+}
+
 // Verify reads through the given WAL and verifies that it is not corrupted.
 // It creates a new decoder to read through the records of the given WAL.
 // It does not conflict with any open WAL, but it is recommended not to
@@ -728,6 +785,10 @@ func (w *WAL) sync() error {
 	return err
 }
 
+func (w *WAL) Sync() error {
+	return w.sync()
+}
+
 // ReleaseLockTo releases the locks, which has smaller index than the given index
 // except the largest one among them.
 // For example, if WAL is holding lock 1,2,3,4,5,6, ReleaseLockTo(4) will release

wal/wal_test.go

@@ -950,3 +950,56 @@ func TestRenameFail(t *testing.T) {
 		t.Fatalf("expected error, got %v", werr)
 	}
 }
+
+// TestValidSnapshotEntries ensures ValidSnapshotEntries returns all valid wal snapshot entries, accounting
+// for hardstate
+func TestValidSnapshotEntries(t *testing.T) {
+	p, err := ioutil.TempDir(os.TempDir(), "waltest")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(p)
+	snap0 := walpb.Snapshot{Index: 0, Term: 0}
+	snap1 := walpb.Snapshot{Index: 1, Term: 1}
+	state1 := raftpb.HardState{Commit: 1, Term: 1}
+	snap2 := walpb.Snapshot{Index: 2, Term: 1}
+	snap3 := walpb.Snapshot{Index: 3, Term: 2}
+	state2 := raftpb.HardState{Commit: 3, Term: 2}
+	snap4 := walpb.Snapshot{Index: 4, Term: 2} // will be orphaned since the last committed entry will be snap3
+	func() {
+		var w *WAL
+		w, err = Create(zap.NewExample(), p, nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer w.Close()
+
+		// snap0 is implicitly created at index 0, term 0
+		if err = w.SaveSnapshot(snap1); err != nil {
+			t.Fatal(err)
+		}
+		if err = w.Save(state1, nil); err != nil {
+			t.Fatal(err)
+		}
+		if err = w.SaveSnapshot(snap2); err != nil {
+			t.Fatal(err)
+		}
+		if err = w.SaveSnapshot(snap3); err != nil {
+			t.Fatal(err)
+		}
+		if err = w.Save(state2, nil); err != nil {
+			t.Fatal(err)
+		}
+		if err = w.SaveSnapshot(snap4); err != nil {
+			t.Fatal(err)
+		}
+	}()
+	walSnaps, serr := ValidSnapshotEntries(zap.NewExample(), p)
+	if serr != nil {
+		t.Fatal(serr)
+	}
+	expected := []walpb.Snapshot{snap0, snap1, snap2, snap3}
+	if !reflect.DeepEqual(walSnaps, expected) {
+		t.Errorf("expected walSnaps %+v, got %+v", expected, walSnaps)
+	}
+}