lib/db: Fix iterating sequence index (fixes #5340) (#5462)

There was a problem in iterating the sequence index that could result
in missing updates. The issue is that while the index was (correctly)
iterated in a snapshot, the actual file infos were read dirty outside of
the snapshot. This fixes this by doing the reads inside the snapshot,
and also updates a couple of other places that did the same thing more
or less harmfully (I didn't investigate).

To avoid similar issues in the future I did some renaming of the
getFile* methods - the ones in a transaction are just getFile, while the
ones directly on the database are variants of getFileDirty to highlight
what's going on.

lib/db/db_test.go

@@ -149,7 +149,7 @@ func TestUpdate0to3(t *testing.T) {
 
 	updater.updateSchema0to1()
 
-	if _, ok := db.getFile(db.keyer.GenerateDeviceFileKey(nil, folder, protocol.LocalDeviceID[:], []byte(slashPrefixed))); ok {
+	if _, ok := db.getFileDirty(folder, protocol.LocalDeviceID[:], []byte(slashPrefixed)); ok {
 		t.Error("File prefixed by '/' was not removed during transition to schema 1")
 	}
 

lib/db/instance.go

@@ -99,6 +99,9 @@ func (db *instance) removeSequences(folder []byte, fs []protocol.FileInfo) {
 }
 
 func (db *instance) withHave(folder, device, prefix []byte, truncate bool, fn Iterator) {
+	t := db.newReadOnlyTransaction()
+	defer t.close()
+
 	if len(prefix) > 0 {
 		unslashedPrefix := prefix
 		if bytes.HasSuffix(prefix, []byte{'/'}) {
@@ -107,14 +110,11 @@ func (db *instance) withHave(folder, device, prefix []byte, truncate bool, fn It
 			prefix = append(prefix, '/')
 		}
 
-		if f, ok := db.getFileTrunc(db.keyer.GenerateDeviceFileKey(nil, folder, device, unslashedPrefix), true); ok && !fn(f) {
+		if f, ok := t.getFileTrunc(db.keyer.GenerateDeviceFileKey(nil, folder, device, unslashedPrefix), true); ok && !fn(f) {
 			return
 		}
 	}
 
-	t := db.newReadOnlyTransaction()
-	defer t.close()
-
 	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateDeviceFileKey(nil, folder, device, prefix)), nil)
 	defer dbi.Release()
 
@@ -124,11 +124,7 @@ func (db *instance) withHave(folder, device, prefix []byte, truncate bool, fn It
 			return
 		}
 
-		// The iterator function may keep a reference to the unmarshalled
-		// struct, which in turn references the buffer it was unmarshalled
-		// from. dbi.Value() just returns an internal slice that it reuses, so
-		// we need to copy it.
-		f, err := unmarshalTrunc(append([]byte{}, dbi.Value()...), truncate)
+		f, err := unmarshalTrunc(dbi.Value(), truncate)
 		if err != nil {
 			l.Debugln("unmarshal error:", err)
 			continue
@@ -147,7 +143,7 @@ func (db *instance) withHaveSequence(folder []byte, startSeq int64, fn Iterator)
 	defer dbi.Release()
 
 	for dbi.Next() {
-		f, ok := db.getFile(dbi.Value())
+		f, ok := t.getFileByKey(dbi.Value())
 		if !ok {
 			l.Debugln("missing file for sequence number", db.keyer.SequenceFromSequenceKey(dbi.Key()))
 			continue
@@ -209,27 +205,21 @@ func (db *instance) withAllFolderTruncated(folder []byte, fn func(device []byte,
 	}
 }
 
-func (db *instance) getFile(key []byte) (protocol.FileInfo, bool) {
-	if f, ok := db.getFileTrunc(key, false); ok {
-		return f.(protocol.FileInfo), true
-	}
-	return protocol.FileInfo{}, false
-}
-
-func (db *instance) getFileTrunc(key []byte, trunc bool) (FileIntf, bool) {
+func (db *instance) getFileDirty(folder, device, file []byte) (protocol.FileInfo, bool) {
+	key := db.keyer.GenerateDeviceFileKey(nil, folder, device, file)
 	bs, err := db.Get(key, nil)
 	if err == leveldb.ErrNotFound {
-		return nil, false
+		return protocol.FileInfo{}, false
 	}
 	if err != nil {
 		l.Debugln("surprise error:", err)
-		return nil, false
+		return protocol.FileInfo{}, false
 	}
 
-	f, err := unmarshalTrunc(bs, trunc)
-	if err != nil {
+	var f protocol.FileInfo
+	if err := f.Unmarshal(bs); err != nil {
 		l.Debugln("unmarshal error:", err)
-		return nil, false
+		return protocol.FileInfo{}, false
 	}
 	return f, true
 }
@@ -256,7 +246,7 @@ func (db *instance) getGlobalInto(t readOnlyTransaction, gk, dk, folder, file []
 	}
 
 	dk = db.keyer.GenerateDeviceFileKey(dk, folder, vl.Versions[0].Device, file)
-	if fi, ok := db.getFileTrunc(dk, truncate); ok {
+	if fi, ok := t.getFileTrunc(dk, truncate); ok {
 		return gk, dk, fi, true
 	}
 
@@ -264,6 +254,9 @@ func (db *instance) getGlobalInto(t readOnlyTransaction, gk, dk, folder, file []
 }
 
 func (db *instance) withGlobal(folder, prefix []byte, truncate bool, fn Iterator) {
+	t := db.newReadOnlyTransaction()
+	defer t.close()
+
 	if len(prefix) > 0 {
 		unslashedPrefix := prefix
 		if bytes.HasSuffix(prefix, []byte{'/'}) {
@@ -272,14 +265,11 @@ func (db *instance) withGlobal(folder, prefix []byte, truncate bool, fn Iterator
 			prefix = append(prefix, '/')
 		}
 
-		if f, ok := db.getGlobal(folder, unslashedPrefix, truncate); ok && !fn(f) {
+		if _, _, f, ok := db.getGlobalInto(t, nil, nil, folder, unslashedPrefix, truncate); ok && !fn(f) {
 			return
 		}
 	}
 
-	t := db.newReadOnlyTransaction()
-	defer t.close()
-
 	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateGlobalVersionKey(nil, folder, prefix)), nil)
 	defer dbi.Release()
 
@@ -297,7 +287,7 @@ func (db *instance) withGlobal(folder, prefix []byte, truncate bool, fn Iterator
 
 		fk = db.keyer.GenerateDeviceFileKey(fk, folder, vl.Versions[0].Device, name)
 
-		f, ok := db.getFileTrunc(fk, truncate)
+		f, ok := t.getFileTrunc(fk, truncate)
 		if !ok {
 			continue
 		}
@@ -504,7 +494,7 @@ func (db *instance) checkGlobals(folder []byte, meta *metadataTracker) {
 			newVL.Versions = append(newVL.Versions, version)
 
 			if i == 0 {
-				if fi, ok := db.getFile(fk); ok {
+				if fi, ok := t.getFileByKey(fk); ok {
 					meta.addFile(protocol.GlobalDeviceID, fi)
 				}
 			}

lib/db/schemaupdater.go

@@ -203,7 +203,7 @@ func (db *schemaUpdater) updateSchema2to3() {
 			name := []byte(f.FileName())
 			dk = db.keyer.GenerateDeviceFileKey(dk, folder, protocol.LocalDeviceID[:], name)
 			var v protocol.Vector
-			haveFile, ok := db.getFileTrunc(dk, true)
+			haveFile, ok := t.getFileTrunc(dk, true)
 			if ok {
 				v = haveFile.FileVersion()
 			}

lib/db/set.go

@@ -161,11 +161,9 @@ func (s *FileSet) Update(device protocol.DeviceID, fs []protocol.FileInfo) {
 	// filter slice according to https://github.com/golang/go/wiki/SliceTricks#filtering-without-allocating
 	oldFs := fs
 	fs = fs[:0]
-	var dk []byte
 	folder := []byte(s.folder)
 	for _, nf := range oldFs {
-		dk = s.db.keyer.GenerateDeviceFileKey(dk, folder, device[:], []byte(osutil.NormalizedFilename(nf.Name)))
-		ef, ok := s.db.getFile(dk)
+		ef, ok := s.db.getFileDirty(folder, device[:], []byte(osutil.NormalizedFilename(nf.Name)))
 		if ok && ef.Version.Equal(nf.Version) && ef.IsInvalid() == nf.IsInvalid() {
 			continue
 		}
@@ -246,7 +244,7 @@ func (s *FileSet) WithPrefixedGlobalTruncated(prefix string, fn Iterator) {
 }
 
 func (s *FileSet) Get(device protocol.DeviceID, file string) (protocol.FileInfo, bool) {
-	f, ok := s.db.getFile(s.db.keyer.GenerateDeviceFileKey(nil, []byte(s.folder), device[:], []byte(osutil.NormalizedFilename(file))))
+	f, ok := s.db.getFileDirty([]byte(s.folder), device[:], []byte(osutil.NormalizedFilename(file)))
 	f.Name = osutil.NativeFilename(f.Name)
 	return f, ok
 }

lib/db/set_test.go

@@ -1363,6 +1363,108 @@ func TestCaseSensitive(t *testing.T) {
 	}
 }
 
+func TestSequenceIndex(t *testing.T) {
+	// This test attempts to verify correct operation of the sequence index.
+
+	// It's a stress test and needs to run for a long time, but we don't
+	// really have time for that in normal builds.
+	runtime := time.Minute
+	if testing.Short() {
+		runtime = time.Second
+	}
+
+	// Set up a db and a few files that we will manipulate.
+
+	ldb := db.OpenMemory()
+	s := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeBasic, "."), ldb)
+
+	local := []protocol.FileInfo{
+		{Name: filepath.FromSlash("banana"), Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}},
+		{Name: filepath.FromSlash("pineapple"), Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}},
+		{Name: filepath.FromSlash("orange"), Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}},
+		{Name: filepath.FromSlash("apple"), Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}},
+		{Name: filepath.FromSlash("jackfruit"), Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}},
+	}
+
+	// Start a  background routine that makes updates to these files as fast
+	// as it can. We always update the same files in the same order.
+
+	done := make(chan struct{})
+	defer close(done)
+
+	go func() {
+		for {
+			select {
+			case <-done:
+				return
+			default:
+			}
+
+			for i := range local {
+				local[i].Version = local[i].Version.Update(42)
+			}
+			s.Update(protocol.LocalDeviceID, local)
+		}
+	}()
+
+	// Start a routine to walk the sequence index and inspect the result.
+
+	seen := make(map[string]db.FileIntf)
+	latest := make([]db.FileIntf, 0, len(local))
+	var seq int64
+	t0 := time.Now()
+
+	for time.Since(t0) < runtime {
+		// Walk the changes since our last iteration. This should give is
+		// one instance each of the files that are changed all the time, or
+		// a subset of those files if we manage to run before a complete
+		// update has happened since our last iteration.
+		latest = latest[:0]
+		s.WithHaveSequence(seq+1, func(f db.FileIntf) bool {
+			seen[f.FileName()] = f
+			latest = append(latest, f)
+			seq = f.SequenceNo()
+			return true
+		})
+
+		// Calculate the spread in sequence number.
+		var max, min int64
+		for _, v := range seen {
+			s := v.SequenceNo()
+			if max == 0 || max < s {
+				max = s
+			}
+			if min == 0 || min > s {
+				min = s
+			}
+		}
+
+		// We shouldn't see a spread larger than the number of files, as
+		// that would mean we have missed updates. For example, if we were
+		// to see the following:
+		//
+		// banana    N
+		// pineapple N+1
+		// orange    N+2
+		// apple     N+10
+		// jackfruit N+11
+		//
+		// that would mean that there have been updates to banana, pineapple
+		// and orange that we didn't see in this pass. If those files aren't
+		// updated again, those updates are permanently lost.
+		if max-min > int64(len(local)) {
+			for _, v := range seen {
+				t.Log("seen", v.FileName(), v.SequenceNo())
+			}
+			for _, v := range latest {
+				t.Log("latest", v.FileName(), v.SequenceNo())
+			}
+			t.Fatal("large spread")
+		}
+		time.Sleep(time.Millisecond)
+	}
+}
+
 func replace(fs *db.FileSet, device protocol.DeviceID, files []protocol.FileInfo) {
 	fs.Drop(device)
 	fs.Update(device, files)

lib/db/structs.go

@@ -164,7 +164,7 @@ func (vl VersionList) String() string {
 // update brings the VersionList up to date with file. It returns the updated
 // VersionList, a potentially removed old FileVersion and its index, as well as
 // the index where the new FileVersion was inserted.
-func (vl VersionList) update(folder, device []byte, file protocol.FileInfo, db *instance) (_ VersionList, removedFV FileVersion, removedAt int, insertedAt int) {
+func (vl VersionList) update(folder, device []byte, file protocol.FileInfo, t readOnlyTransaction) (_ VersionList, removedFV FileVersion, removedAt int, insertedAt int) {
 	vl, removedFV, removedAt = vl.pop(device)
 
 	nv := FileVersion{
@@ -198,7 +198,7 @@ func (vl VersionList) update(folder, device []byte, file protocol.FileInfo, db *
 			// to determine the winner.)
 			//
 			// A surprise missing file entry here is counted as a win for us.
-			if of, ok := db.getFile(db.keyer.GenerateDeviceFileKey(nil, folder, vl.Versions[i].Device, []byte(file.Name))); !ok || file.WinsConflict(of) {
+			if of, ok := t.getFile(folder, vl.Versions[i].Device, []byte(file.Name)); !ok || file.WinsConflict(of) {
 				vl = vl.insertAt(i, nv)
 				return vl, removedFV, removedAt, i
 			}

lib/db/transactions.go

@@ -37,7 +37,32 @@ func (t readOnlyTransaction) close() {
 }
 
 func (t readOnlyTransaction) getFile(folder, device, file []byte) (protocol.FileInfo, bool) {
-	return t.db.getFile(t.db.keyer.GenerateDeviceFileKey(nil, folder, device, file))
+	return t.getFileByKey(t.db.keyer.GenerateDeviceFileKey(nil, folder, device, file))
+}
+
+func (t readOnlyTransaction) getFileByKey(key []byte) (protocol.FileInfo, bool) {
+	if f, ok := t.getFileTrunc(key, false); ok {
+		return f.(protocol.FileInfo), true
+	}
+	return protocol.FileInfo{}, false
+}
+
+func (t readOnlyTransaction) getFileTrunc(key []byte, trunc bool) (FileIntf, bool) {
+	bs, err := t.Get(key, nil)
+	if err == leveldb.ErrNotFound {
+		return nil, false
+	}
+	if err != nil {
+		l.Debugln("surprise error:", err)
+		return nil, false
+	}
+
+	f, err := unmarshalTrunc(bs, trunc)
+	if err != nil {
+		l.Debugln("unmarshal error:", err)
+		return nil, false
+	}
+	return f, true
 }
 
 // A readWriteTransaction is a readOnlyTransaction plus a batch for writes.
@@ -90,7 +115,7 @@ func (t readWriteTransaction) updateGlobal(gk, folder, device []byte, file proto
 	if svl, err := t.Get(gk, nil); err == nil {
 		fl.Unmarshal(svl) // Ignore error, continue with empty fl
 	}
-	fl, removedFV, removedAt, insertedAt := fl.update(folder, device, file, t.db)
+	fl, removedFV, removedAt, insertedAt := fl.update(folder, device, file, t.readOnlyTransaction)
 	if insertedAt == -1 {
 		l.Debugln("update global; same version, global unchanged")
 		return false