chore: switch database engine to sqlite (fixes #9954) (#9965)

Switch the database from LevelDB to SQLite, for greater stability and
simpler code.

Co-authored-by: Tommy van der Vorst <tommy@pixelspark.nl>
Co-authored-by: bt90 <btom1990@googlemail.com>

internal/db/olddb/lowlevel.go

@@ -0,0 +1,70 @@
+// Copyright (C) 2014 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package olddb
+
+import (
+	"encoding/binary"
+	"time"
+
+	"github.com/syncthing/syncthing/internal/db/olddb/backend"
+)
+
+// deprecatedLowlevel is the lowest level database interface. It has a very simple
+// purpose: hold the actual backend database, and the in-memory state
+// that belong to that database. In the same way that a single on disk
+// database can only be opened once, there should be only one deprecatedLowlevel for
+// any given backend.
+type deprecatedLowlevel struct {
+	backend.Backend
+	folderIdx *smallIndex
+	deviceIdx *smallIndex
+	keyer     keyer
+}
+
+func NewLowlevel(backend backend.Backend) (*deprecatedLowlevel, error) {
+	// Only log restarts in debug mode.
+	db := &deprecatedLowlevel{
+		Backend:   backend,
+		folderIdx: newSmallIndex(backend, []byte{KeyTypeFolderIdx}),
+		deviceIdx: newSmallIndex(backend, []byte{KeyTypeDeviceIdx}),
+	}
+	db.keyer = newDefaultKeyer(db.folderIdx, db.deviceIdx)
+	return db, nil
+}
+
+// ListFolders returns the list of folders currently in the database
+func (db *deprecatedLowlevel) ListFolders() []string {
+	return db.folderIdx.Values()
+}
+
+func (db *deprecatedLowlevel) IterateMtimes(fn func(folder, name string, ondisk, virtual time.Time) error) error {
+	it, err := db.NewPrefixIterator([]byte{KeyTypeVirtualMtime})
+	if err != nil {
+		return err
+	}
+	defer it.Release()
+	for it.Next() {
+		key := it.Key()[1:]
+		folderID, ok := db.folderIdx.Val(binary.BigEndian.Uint32(key))
+		if !ok {
+			continue
+		}
+		name := key[4:]
+		val := it.Value()
+		var ondisk, virtual time.Time
+		if err := ondisk.UnmarshalBinary(val[:len(val)/2]); err != nil {
+			continue
+		}
+		if err := virtual.UnmarshalBinary(val[len(val)/2:]); err != nil {
+			continue
+		}
+		if err := fn(string(folderID), string(name), ondisk, virtual); err != nil {
+			return err
+		}
+	}
+	return it.Error()
+}