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/set.go

@@ -0,0 +1,67 @@
+// 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 db provides a set type to track local/remote files with newness
+// checks. We must do a certain amount of normalization in here. We will get
+// fed paths with either native or wire-format separators and encodings
+// depending on who calls us. We transform paths to wire-format (NFC and
+// slashes) on the way to the database, and transform to native format
+// (varying separator and encoding) on the way back out.
+package olddb
+
+import (
+	"github.com/syncthing/syncthing/lib/osutil"
+	"github.com/syncthing/syncthing/lib/protocol"
+)
+
+type deprecatedFileSet struct {
+	folder string
+	db     *deprecatedLowlevel
+}
+
+// The Iterator is called with either a protocol.FileInfo or a
+// FileInfoTruncated (depending on the method) and returns true to
+// continue iteration, false to stop.
+type Iterator func(f protocol.FileInfo) bool
+
+func NewFileSet(folder string, db *deprecatedLowlevel) (*deprecatedFileSet, error) {
+	s := &deprecatedFileSet{
+		folder: folder,
+		db:     db,
+	}
+	return s, nil
+}
+
+type Snapshot struct {
+	folder string
+	t      readOnlyTransaction
+}
+
+func (s *deprecatedFileSet) Snapshot() (*Snapshot, error) {
+	t, err := s.db.newReadOnlyTransaction()
+	if err != nil {
+		return nil, err
+	}
+	return &Snapshot{
+		folder: s.folder,
+		t:      t,
+	}, nil
+}
+
+func (s *Snapshot) Release() {
+	s.t.close()
+}
+
+func (s *Snapshot) WithHaveSequence(startSeq int64, fn Iterator) error {
+	return s.t.withHaveSequence([]byte(s.folder), startSeq, nativeFileIterator(fn))
+}
+
+func nativeFileIterator(fn Iterator) Iterator {
+	return func(fi protocol.FileInfo) bool {
+		fi.Name = osutil.NativeFilename(fi.Name)
+		return fn(fi)
+	}
+}