Implement database abstraction, error checking (ref #5907) (#6107)

This PR does two things, because one lead to the other:

- Move the leveldb specific stuff into a small "backend" package that
defines a backend interface and the leveldb implementation. This allows,
potentially, in the future, switching the db implementation so another
KV store should we wish to do so.

- Add proper error handling all along the way. The db and backend
packages are now errcheck clean. However, I drew the line at modifying
the FileSet API in order to keep this manageable and not continue
refactoring all of the rest of Syncthing. As such, the FileSet methods
still panic on database errors, except for the "database is closed"
error which is instead handled by silently returning as quickly as
possible, with the assumption that we're anyway "on the way out".

lib/db/instance.go

@@ -9,11 +9,9 @@ package db
 import (
 	"bytes"
 	"encoding/binary"
-	"fmt"
 
+	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/protocol"
-	"github.com/syndtr/goleveldb/leveldb"
-	"github.com/syndtr/goleveldb/leveldb/util"
 )
 
 type instance struct {
@@ -30,17 +28,26 @@ func newInstance(ll *Lowlevel) *instance {
 
 // updateRemoteFiles adds a list of fileinfos to the database and updates the
 // global versionlist and metadata.
-func (db *instance) updateRemoteFiles(folder, device []byte, fs []protocol.FileInfo, meta *metadataTracker) {
-	t := db.newReadWriteTransaction()
+func (db *instance) updateRemoteFiles(folder, device []byte, fs []protocol.FileInfo, meta *metadataTracker) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	var dk, gk, keyBuf []byte
 	devID := protocol.DeviceIDFromBytes(device)
 	for _, f := range fs {
 		name := []byte(f.Name)
-		dk = db.keyer.GenerateDeviceFileKey(dk, folder, device, name)
+		dk, err = db.keyer.GenerateDeviceFileKey(dk, folder, device, name)
+		if err != nil {
+			return err
+		}
 
-		ef, ok := t.getFileTrunc(dk, true)
+		ef, ok, err := t.getFileTrunc(dk, true)
+		if err != nil {
+			return err
+		}
 		if ok && unchanged(f, ef) {
 			continue
 		}
@@ -51,28 +58,49 @@ func (db *instance) updateRemoteFiles(folder, device []byte, fs []protocol.FileI
 		meta.addFile(devID, f)
 
 		l.Debugf("insert; folder=%q device=%v %v", folder, devID, f)
-		t.Put(dk, mustMarshal(&f))
+		if err := t.Put(dk, mustMarshal(&f)); err != nil {
+			return err
+		}
 
-		gk = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
-		keyBuf, _ = t.updateGlobal(gk, keyBuf, folder, device, f, meta)
+		gk, err = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
+		if err != nil {
+			return err
+		}
+		keyBuf, _, err = t.updateGlobal(gk, keyBuf, folder, device, f, meta)
+		if err != nil {
+			return err
+		}
 
-		t.checkFlush()
+		if err := t.Checkpoint(); err != nil {
+			return err
+		}
 	}
+
+	return t.commit()
 }
 
 // updateLocalFiles adds fileinfos to the db, and updates the global versionlist,
 // metadata, sequence and blockmap buckets.
-func (db *instance) updateLocalFiles(folder []byte, fs []protocol.FileInfo, meta *metadataTracker) {
-	t := db.newReadWriteTransaction()
+func (db *instance) updateLocalFiles(folder []byte, fs []protocol.FileInfo, meta *metadataTracker) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	var dk, gk, keyBuf []byte
 	blockBuf := make([]byte, 4)
 	for _, f := range fs {
 		name := []byte(f.Name)
-		dk = db.keyer.GenerateDeviceFileKey(dk, folder, protocol.LocalDeviceID[:], name)
+		dk, err = db.keyer.GenerateDeviceFileKey(dk, folder, protocol.LocalDeviceID[:], name)
+		if err != nil {
+			return err
+		}
 
-		ef, ok := t.getFileByKey(dk)
+		ef, ok, err := t.getFileByKey(dk)
+		if err != nil {
+			return err
+		}
 		if ok && unchanged(f, ef) {
 			continue
 		}
@@ -80,13 +108,23 @@ func (db *instance) updateLocalFiles(folder []byte, fs []protocol.FileInfo, meta
 		if ok {
 			if !ef.IsDirectory() && !ef.IsDeleted() && !ef.IsInvalid() {
 				for _, block := range ef.Blocks {
-					keyBuf = db.keyer.GenerateBlockMapKey(keyBuf, folder, block.Hash, name)
-					t.Delete(keyBuf)
+					keyBuf, err = db.keyer.GenerateBlockMapKey(keyBuf, folder, block.Hash, name)
+					if err != nil {
+						return err
+					}
+					if err := t.Delete(keyBuf); err != nil {
+						return err
+					}
 				}
 			}
 
-			keyBuf = db.keyer.GenerateSequenceKey(keyBuf, folder, ef.SequenceNo())
-			t.Delete(keyBuf)
+			keyBuf, err = db.keyer.GenerateSequenceKey(keyBuf, folder, ef.SequenceNo())
+			if err != nil {
+				return err
+			}
+			if err := t.Delete(keyBuf); err != nil {
+				return err
+			}
 			l.Debugf("removing sequence; folder=%q sequence=%v %v", folder, ef.SequenceNo(), ef.FileName())
 		}
 
@@ -98,29 +136,54 @@ func (db *instance) updateLocalFiles(folder []byte, fs []protocol.FileInfo, meta
 		meta.addFile(protocol.LocalDeviceID, f)
 
 		l.Debugf("insert (local); folder=%q %v", folder, f)
-		t.Put(dk, mustMarshal(&f))
+		if err := t.Put(dk, mustMarshal(&f)); err != nil {
+			return err
+		}
 
-		gk = db.keyer.GenerateGlobalVersionKey(gk, folder, []byte(f.Name))
-		keyBuf, _ = t.updateGlobal(gk, keyBuf, folder, protocol.LocalDeviceID[:], f, meta)
+		gk, err = db.keyer.GenerateGlobalVersionKey(gk, folder, []byte(f.Name))
+		if err != nil {
+			return err
+		}
+		keyBuf, _, err = t.updateGlobal(gk, keyBuf, folder, protocol.LocalDeviceID[:], f, meta)
+		if err != nil {
+			return err
+		}
 
-		keyBuf = db.keyer.GenerateSequenceKey(keyBuf, folder, f.Sequence)
-		t.Put(keyBuf, dk)
+		keyBuf, err = db.keyer.GenerateSequenceKey(keyBuf, folder, f.Sequence)
+		if err != nil {
+			return err
+		}
+		if err := t.Put(keyBuf, dk); err != nil {
+			return err
+		}
 		l.Debugf("adding sequence; folder=%q sequence=%v %v", folder, f.Sequence, f.Name)
 
 		if !f.IsDirectory() && !f.IsDeleted() && !f.IsInvalid() {
 			for i, block := range f.Blocks {
 				binary.BigEndian.PutUint32(blockBuf, uint32(i))
-				keyBuf = db.keyer.GenerateBlockMapKey(keyBuf, folder, block.Hash, name)
-				t.Put(keyBuf, blockBuf)
+				keyBuf, err = db.keyer.GenerateBlockMapKey(keyBuf, folder, block.Hash, name)
+				if err != nil {
+					return err
+				}
+				if err := t.Put(keyBuf, blockBuf); err != nil {
+					return err
+				}
 			}
 		}
 
-		t.checkFlush()
+		if err := t.Checkpoint(); err != nil {
+			return err
+		}
 	}
+
+	return t.commit()
 }
 
-func (db *instance) withHave(folder, device, prefix []byte, truncate bool, fn Iterator) {
-	t := db.newReadOnlyTransaction()
+func (db *instance) withHave(folder, device, prefix []byte, truncate bool, fn Iterator) error {
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	if len(prefix) > 0 {
@@ -131,18 +194,31 @@ func (db *instance) withHave(folder, device, prefix []byte, truncate bool, fn It
 			prefix = append(prefix, '/')
 		}
 
-		if f, ok := t.getFileTrunc(db.keyer.GenerateDeviceFileKey(nil, folder, device, unslashedPrefix), true); ok && !fn(f) {
-			return
+		key, err := db.keyer.GenerateDeviceFileKey(nil, folder, device, unslashedPrefix)
+		if err != nil {
+			return err
+		}
+		if f, ok, err := t.getFileTrunc(key, true); err != nil {
+			return err
+		} else if ok && !fn(f) {
+			return nil
 		}
 	}
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateDeviceFileKey(nil, folder, device, prefix)), nil)
+	key, err := db.keyer.GenerateDeviceFileKey(nil, folder, device, prefix)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key)
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	for dbi.Next() {
 		name := db.keyer.NameFromDeviceFileKey(dbi.Key())
 		if len(prefix) > 0 && !bytes.HasPrefix(name, prefix) {
-			return
+			return nil
 		}
 
 		f, err := unmarshalTrunc(dbi.Value(), truncate)
@@ -151,20 +227,38 @@ func (db *instance) withHave(folder, device, prefix []byte, truncate bool, fn It
 			continue
 		}
 		if !fn(f) {
-			return
+			return nil
 		}
 	}
+	return dbi.Error()
 }
 
-func (db *instance) withHaveSequence(folder []byte, startSeq int64, fn Iterator) {
-	t := db.newReadOnlyTransaction()
+func (db *instance) withHaveSequence(folder []byte, startSeq int64, fn Iterator) error {
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	dbi := t.NewIterator(&util.Range{Start: db.keyer.GenerateSequenceKey(nil, folder, startSeq), Limit: db.keyer.GenerateSequenceKey(nil, folder, maxInt64)}, nil)
+	first, err := db.keyer.GenerateSequenceKey(nil, folder, startSeq)
+	if err != nil {
+		return err
+	}
+	last, err := db.keyer.GenerateSequenceKey(nil, folder, maxInt64)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewRangeIterator(first, last)
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	for dbi.Next() {
-		f, ok := t.getFileByKey(dbi.Value())
+		f, ok, err := t.getFileByKey(dbi.Value())
+		if err != nil {
+			return err
+		}
 		if !ok {
 			l.Debugln("missing file for sequence number", db.keyer.SequenceFromSequenceKey(dbi.Key()))
 			continue
@@ -177,16 +271,27 @@ func (db *instance) withHaveSequence(folder []byte, startSeq int64, fn Iterator)
 			}
 		}
 		if !fn(f) {
-			return
+			return nil
 		}
 	}
+	return dbi.Error()
 }
 
-func (db *instance) withAllFolderTruncated(folder []byte, fn func(device []byte, f FileInfoTruncated) bool) {
-	t := db.newReadWriteTransaction()
+func (db *instance) withAllFolderTruncated(folder []byte, fn func(device []byte, f FileInfoTruncated) bool) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateDeviceFileKey(nil, folder, nil, nil).WithoutNameAndDevice()), nil)
+	key, err := db.keyer.GenerateDeviceFileKey(nil, folder, nil, nil)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key.WithoutNameAndDevice())
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	var gk, keyBuf []byte
@@ -194,8 +299,9 @@ func (db *instance) withAllFolderTruncated(folder []byte, fn func(device []byte,
 		device, ok := db.keyer.DeviceFromDeviceFileKey(dbi.Key())
 		if !ok {
 			// Not having the device in the index is bad. Clear it.
-			t.Delete(dbi.Key())
-			t.checkFlush()
+			if err := t.Delete(dbi.Key()); err != nil {
+				return err
+			}
 			continue
 		}
 		var f FileInfoTruncated
@@ -205,42 +311,61 @@ func (db *instance) withAllFolderTruncated(folder []byte, fn func(device []byte,
 		// we need to copy it.
 		err := f.Unmarshal(append([]byte{}, dbi.Value()...))
 		if err != nil {
-			l.Debugln("unmarshal error:", err)
-			continue
+			return err
 		}
 
 		switch f.Name {
 		case "", ".", "..", "/": // A few obviously invalid filenames
 			l.Infof("Dropping invalid filename %q from database", f.Name)
 			name := []byte(f.Name)
-			gk = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
-			keyBuf = t.removeFromGlobal(gk, keyBuf, folder, device, name, nil)
-			t.Delete(dbi.Key())
-			t.checkFlush()
+			gk, err = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
+			if err != nil {
+				return err
+			}
+			keyBuf, err = t.removeFromGlobal(gk, keyBuf, folder, device, name, nil)
+			if err != nil {
+				return err
+			}
+			if err := t.Delete(dbi.Key()); err != nil {
+				return err
+			}
 			continue
 		}
 
 		if !fn(device, f) {
-			return
+			return nil
 		}
 	}
+	if err := dbi.Error(); err != nil {
+		return err
+	}
+	return t.commit()
 }
 
-func (db *instance) getFileDirty(folder, device, file []byte) (protocol.FileInfo, bool) {
-	t := db.newReadOnlyTransaction()
+func (db *instance) getFileDirty(folder, device, file []byte) (protocol.FileInfo, bool, error) {
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return protocol.FileInfo{}, false, err
+	}
 	defer t.close()
 	return t.getFile(folder, device, file)
 }
 
-func (db *instance) getGlobalDirty(folder, file []byte, truncate bool) (FileIntf, bool) {
-	t := db.newReadOnlyTransaction()
+func (db *instance) getGlobalDirty(folder, file []byte, truncate bool) (FileIntf, bool, error) {
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return nil, false, err
+	}
 	defer t.close()
-	_, f, ok := t.getGlobal(nil, folder, file, truncate)
-	return f, ok
+	_, f, ok, err := t.getGlobal(nil, folder, file, truncate)
+	return f, ok, err
 }
 
-func (db *instance) withGlobal(folder, prefix []byte, truncate bool, fn Iterator) {
-	t := db.newReadOnlyTransaction()
+func (db *instance) withGlobal(folder, prefix []byte, truncate bool, fn Iterator) error {
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
 	if len(prefix) > 0 {
@@ -251,19 +376,28 @@ func (db *instance) withGlobal(folder, prefix []byte, truncate bool, fn Iterator
 			prefix = append(prefix, '/')
 		}
 
-		if _, f, ok := t.getGlobal(nil, folder, unslashedPrefix, truncate); ok && !fn(f) {
-			return
+		if _, f, ok, err := t.getGlobal(nil, folder, unslashedPrefix, truncate); err != nil {
+			return err
+		} else if ok && !fn(f) {
+			return nil
 		}
 	}
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateGlobalVersionKey(nil, folder, prefix)), nil)
+	key, err := db.keyer.GenerateGlobalVersionKey(nil, folder, prefix)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key)
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	var dk []byte
 	for dbi.Next() {
 		name := db.keyer.NameFromGlobalVersionKey(dbi.Key())
 		if len(prefix) > 0 && !bytes.HasPrefix(name, prefix) {
-			return
+			return nil
 		}
 
 		vl, ok := unmarshalVersionList(dbi.Value())
@@ -271,33 +405,45 @@ func (db *instance) withGlobal(folder, prefix []byte, truncate bool, fn Iterator
 			continue
 		}
 
-		dk = db.keyer.GenerateDeviceFileKey(dk, folder, vl.Versions[0].Device, name)
+		dk, err = db.keyer.GenerateDeviceFileKey(dk, folder, vl.Versions[0].Device, name)
+		if err != nil {
+			return err
+		}
 
-		f, ok := t.getFileTrunc(dk, truncate)
+		f, ok, err := t.getFileTrunc(dk, truncate)
+		if err != nil {
+			return err
+		}
 		if !ok {
 			continue
 		}
 
 		if !fn(f) {
-			return
+			return nil
 		}
 	}
+	if err != nil {
+		return err
+	}
+	return dbi.Error()
 }
 
-func (db *instance) availability(folder, file []byte) []protocol.DeviceID {
-	k := db.keyer.GenerateGlobalVersionKey(nil, folder, file)
-	bs, err := db.Get(k, nil)
-	if err == leveldb.ErrNotFound {
-		return nil
+func (db *instance) availability(folder, file []byte) ([]protocol.DeviceID, error) {
+	k, err := db.keyer.GenerateGlobalVersionKey(nil, folder, file)
+	if err != nil {
+		return nil, err
+	}
+	bs, err := db.Get(k)
+	if backend.IsNotFound(err) {
+		return nil, nil
 	}
 	if err != nil {
-		l.Debugln("surprise error:", err)
-		return nil
+		return nil, err
 	}
 
 	vl, ok := unmarshalVersionList(bs)
 	if !ok {
-		return nil
+		return nil, nil
 	}
 
 	var devices []protocol.DeviceID
@@ -312,19 +458,28 @@ func (db *instance) availability(folder, file []byte) []protocol.DeviceID {
 		devices = append(devices, n)
 	}
 
-	return devices
+	return devices, nil
 }
 
-func (db *instance) withNeed(folder, device []byte, truncate bool, fn Iterator) {
+func (db *instance) withNeed(folder, device []byte, truncate bool, fn Iterator) error {
 	if bytes.Equal(device, protocol.LocalDeviceID[:]) {
-		db.withNeedLocal(folder, truncate, fn)
-		return
+		return db.withNeedLocal(folder, truncate, fn)
 	}
 
-	t := db.newReadOnlyTransaction()
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateGlobalVersionKey(nil, folder, nil).WithoutName()), nil)
+	key, err := db.keyer.GenerateGlobalVersionKey(nil, folder, nil)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key.WithoutName())
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	var dk []byte
@@ -358,8 +513,14 @@ func (db *instance) withNeed(folder, device []byte, truncate bool, fn Iterator)
 				continue
 			}
 
-			dk = db.keyer.GenerateDeviceFileKey(dk, folder, vl.Versions[i].Device, name)
-			gf, ok := t.getFileTrunc(dk, truncate)
+			dk, err = db.keyer.GenerateDeviceFileKey(dk, folder, vl.Versions[i].Device, name)
+			if err != nil {
+				return err
+			}
+			gf, ok, err := t.getFileTrunc(dk, truncate)
+			if err != nil {
+				return err
+			}
 			if !ok {
 				continue
 			}
@@ -372,81 +533,171 @@ func (db *instance) withNeed(folder, device []byte, truncate bool, fn Iterator)
 			l.Debugf("need folder=%q device=%v name=%q have=%v invalid=%v haveV=%v globalV=%v globalDev=%v", folder, devID, name, have, haveFV.Invalid, haveFV.Version, needVersion, needDevice)
 
 			if !fn(gf) {
-				return
+				return nil
 			}
 
 			// This file is handled, no need to look further in the version list
 			break
 		}
 	}
+	return dbi.Error()
 }
 
-func (db *instance) withNeedLocal(folder []byte, truncate bool, fn Iterator) {
-	t := db.newReadOnlyTransaction()
+func (db *instance) withNeedLocal(folder []byte, truncate bool, fn Iterator) error {
+	t, err := db.newReadOnlyTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateNeedFileKey(nil, folder, nil).WithoutName()), nil)
+	key, err := db.keyer.GenerateNeedFileKey(nil, folder, nil)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key.WithoutName())
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	var keyBuf []byte
 	var f FileIntf
 	var ok bool
 	for dbi.Next() {
-		keyBuf, f, ok = t.getGlobal(keyBuf, folder, db.keyer.NameFromGlobalVersionKey(dbi.Key()), truncate)
+		keyBuf, f, ok, err = t.getGlobal(keyBuf, folder, db.keyer.NameFromGlobalVersionKey(dbi.Key()), truncate)
+		if err != nil {
+			return err
+		}
 		if !ok {
 			continue
 		}
 		if !fn(f) {
-			return
+			return nil
 		}
 	}
+	return dbi.Error()
 }
 
-func (db *instance) dropFolder(folder []byte) {
-	t := db.newReadWriteTransaction()
-	defer t.close()
-
-	for _, key := range [][]byte{
-		// Remove all items related to the given folder from the device->file bucket
-		db.keyer.GenerateDeviceFileKey(nil, folder, nil, nil).WithoutNameAndDevice(),
-		// Remove all sequences related to the folder
-		db.keyer.GenerateSequenceKey(nil, []byte(folder), 0).WithoutSequence(),
-		// Remove all items related to the given folder from the global bucket
-		db.keyer.GenerateGlobalVersionKey(nil, folder, nil).WithoutName(),
-		// Remove all needs related to the folder
-		db.keyer.GenerateNeedFileKey(nil, folder, nil).WithoutName(),
-		// Remove the blockmap of the folder
-		db.keyer.GenerateBlockMapKey(nil, folder, nil, nil).WithoutHashAndName(),
-	} {
-		t.deleteKeyPrefix(key)
+func (db *instance) dropFolder(folder []byte) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
 	}
-}
-
-func (db *instance) dropDeviceFolder(device, folder []byte, meta *metadataTracker) {
-	t := db.newReadWriteTransaction()
 	defer t.close()
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateDeviceFileKey(nil, folder, device, nil)), nil)
-	defer dbi.Release()
+	// Remove all items related to the given folder from the device->file bucket
+	k0, err := db.keyer.GenerateDeviceFileKey(nil, folder, nil, nil)
+	if err != nil {
+		return err
+	}
+	if err := t.deleteKeyPrefix(k0.WithoutNameAndDevice()); err != nil {
+		return err
+	}
 
+	// Remove all sequences related to the folder
+	k1, err := db.keyer.GenerateSequenceKey(nil, folder, 0)
+	if err != nil {
+		return err
+	}
+	if err := t.deleteKeyPrefix(k1.WithoutSequence()); err != nil {
+		return err
+	}
+
+	// Remove all items related to the given folder from the global bucket
+	k2, err := db.keyer.GenerateGlobalVersionKey(nil, folder, nil)
+	if err != nil {
+		return err
+	}
+	if err := t.deleteKeyPrefix(k2.WithoutName()); err != nil {
+		return err
+	}
+
+	// Remove all needs related to the folder
+	k3, err := db.keyer.GenerateNeedFileKey(nil, folder, nil)
+	if err != nil {
+		return err
+	}
+	if err := t.deleteKeyPrefix(k3.WithoutName()); err != nil {
+		return err
+	}
+
+	// Remove the blockmap of the folder
+	k4, err := db.keyer.GenerateBlockMapKey(nil, folder, nil, nil)
+	if err != nil {
+		return err
+	}
+	if err := t.deleteKeyPrefix(k4.WithoutHashAndName()); err != nil {
+		return err
+	}
+
+	return t.commit()
+}
+
+func (db *instance) dropDeviceFolder(device, folder []byte, meta *metadataTracker) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
+	defer t.close()
+
+	key, err := db.keyer.GenerateDeviceFileKey(nil, folder, device, nil)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key)
+	if err != nil {
+		return err
+	}
 	var gk, keyBuf []byte
 	for dbi.Next() {
 		name := db.keyer.NameFromDeviceFileKey(dbi.Key())
-		gk = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
-		keyBuf = t.removeFromGlobal(gk, keyBuf, folder, device, name, meta)
-		t.Delete(dbi.Key())
-		t.checkFlush()
+		gk, err = db.keyer.GenerateGlobalVersionKey(gk, folder, name)
+		if err != nil {
+			return err
+		}
+		keyBuf, err = t.removeFromGlobal(gk, keyBuf, folder, device, name, meta)
+		if err != nil {
+			return err
+		}
+		if err := t.Delete(dbi.Key()); err != nil {
+			return err
+		}
+		if err := t.Checkpoint(); err != nil {
+			return err
+		}
 	}
+	if err := dbi.Error(); err != nil {
+		return err
+	}
+	dbi.Release()
+
 	if bytes.Equal(device, protocol.LocalDeviceID[:]) {
-		t.deleteKeyPrefix(db.keyer.GenerateBlockMapKey(nil, folder, nil, nil).WithoutHashAndName())
+		key, err := db.keyer.GenerateBlockMapKey(nil, folder, nil, nil)
+		if err != nil {
+			return err
+		}
+		if err := t.deleteKeyPrefix(key.WithoutHashAndName()); err != nil {
+			return err
+		}
 	}
+	return t.commit()
 }
 
-func (db *instance) checkGlobals(folder []byte, meta *metadataTracker) {
-	t := db.newReadWriteTransaction()
+func (db *instance) checkGlobals(folder []byte, meta *metadataTracker) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	dbi := t.NewIterator(util.BytesPrefix(db.keyer.GenerateGlobalVersionKey(nil, folder, nil).WithoutName()), nil)
+	key, err := db.keyer.GenerateGlobalVersionKey(nil, folder, nil)
+	if err != nil {
+		return err
+	}
+	dbi, err := t.NewPrefixIterator(key.WithoutName())
+	if err != nil {
+		return err
+	}
 	defer dbi.Release()
 
 	var dk []byte
@@ -464,66 +715,98 @@ func (db *instance) checkGlobals(folder []byte, meta *metadataTracker) {
 		name := db.keyer.NameFromGlobalVersionKey(dbi.Key())
 		var newVL VersionList
 		for i, version := range vl.Versions {
-			dk = db.keyer.GenerateDeviceFileKey(dk, folder, version.Device, name)
-			_, err := t.Get(dk, nil)
-			if err == leveldb.ErrNotFound {
+			dk, err = db.keyer.GenerateDeviceFileKey(dk, folder, version.Device, name)
+			if err != nil {
+				return err
+			}
+			_, err := t.Get(dk)
+			if backend.IsNotFound(err) {
 				continue
 			}
 			if err != nil {
-				l.Debugln("surprise error:", err)
-				return
+				return err
 			}
 			newVL.Versions = append(newVL.Versions, version)
 
 			if i == 0 {
-				if fi, ok := t.getFileByKey(dk); ok {
+				if fi, ok, err := t.getFileByKey(dk); err != nil {
+					return err
+				} else if ok {
 					meta.addFile(protocol.GlobalDeviceID, fi)
 				}
 			}
 		}
 
 		if len(newVL.Versions) != len(vl.Versions) {
-			t.Put(dbi.Key(), mustMarshal(&newVL))
-			t.checkFlush()
+			if err := t.Put(dbi.Key(), mustMarshal(&newVL)); err != nil {
+				return err
+			}
 		}
 	}
+	if err := dbi.Error(); err != nil {
+		return err
+	}
+
 	l.Debugf("db check completed for %q", folder)
+	return t.commit()
 }
 
-func (db *instance) getIndexID(device, folder []byte) protocol.IndexID {
-	cur, err := db.Get(db.keyer.GenerateIndexIDKey(nil, device, folder), nil)
+func (db *instance) getIndexID(device, folder []byte) (protocol.IndexID, error) {
+	key, err := db.keyer.GenerateIndexIDKey(nil, device, folder)
 	if err != nil {
-		return 0
+		return 0, err
+	}
+	cur, err := db.Get(key)
+	if backend.IsNotFound(err) {
+		return 0, nil
+	} else if err != nil {
+		return 0, err
 	}
 
 	var id protocol.IndexID
 	if err := id.Unmarshal(cur); err != nil {
-		return 0
+		return 0, nil
 	}
 
-	return id
+	return id, nil
 }
 
-func (db *instance) setIndexID(device, folder []byte, id protocol.IndexID) {
+func (db *instance) setIndexID(device, folder []byte, id protocol.IndexID) error {
 	bs, _ := id.Marshal() // marshalling can't fail
-	if err := db.Put(db.keyer.GenerateIndexIDKey(nil, device, folder), bs, nil); err != nil && err != leveldb.ErrClosed {
-		panic("storing index ID: " + err.Error())
+	key, err := db.keyer.GenerateIndexIDKey(nil, device, folder)
+	if err != nil {
+		return err
 	}
+	return db.Put(key, bs)
 }
 
-func (db *instance) dropMtimes(folder []byte) {
-	db.dropPrefix(db.keyer.GenerateMtimesKey(nil, folder))
+func (db *instance) dropMtimes(folder []byte) error {
+	key, err := db.keyer.GenerateMtimesKey(nil, folder)
+	if err != nil {
+		return err
+	}
+	return db.dropPrefix(key)
 }
 
-func (db *instance) dropFolderMeta(folder []byte) {
-	db.dropPrefix(db.keyer.GenerateFolderMetaKey(nil, folder))
+func (db *instance) dropFolderMeta(folder []byte) error {
+	key, err := db.keyer.GenerateFolderMetaKey(nil, folder)
+	if err != nil {
+		return err
+	}
+	return db.dropPrefix(key)
 }
 
-func (db *instance) dropPrefix(prefix []byte) {
-	t := db.newReadWriteTransaction()
+func (db *instance) dropPrefix(prefix []byte) error {
+	t, err := db.newReadWriteTransaction()
+	if err != nil {
+		return err
+	}
 	defer t.close()
 
-	t.deleteKeyPrefix(prefix)
+	if err := t.deleteKeyPrefix(prefix); err != nil {
+		return err
+	}
+	return t.commit()
 }
 
 func unmarshalTrunc(bs []byte, truncate bool) (FileIntf, error) {
@@ -551,15 +834,6 @@ func unmarshalVersionList(data []byte) (VersionList, bool) {
 	return vl, true
 }
 
-type errorSuggestion struct {
-	inner      error
-	suggestion string
-}
-
-func (e errorSuggestion) Error() string {
-	return fmt.Sprintf("%s (%s)", e.inner.Error(), e.suggestion)
-}
-
 // unchanged checks if two files are the same and thus don't need to be updated.
 // Local flags or the invalid bit might change without the version
 // being bumped.