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/sqlite/util.go

@@ -0,0 +1,117 @@
+// Copyright (C) 2025 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 sqlite
+
+import (
+	"database/sql/driver"
+	"errors"
+	"iter"
+
+	"github.com/jmoiron/sqlx"
+	"github.com/syncthing/syncthing/internal/gen/bep"
+	"github.com/syncthing/syncthing/internal/gen/dbproto"
+	"github.com/syncthing/syncthing/lib/osutil"
+	"github.com/syncthing/syncthing/lib/protocol"
+	"google.golang.org/protobuf/proto"
+)
+
+// iterStructs returns an iterator over the given struct type by scanning
+// the SQL rows. `rows` is closed when the iterator exits.
+func iterStructs[T any](rows *sqlx.Rows, err error) (iter.Seq[T], func() error) {
+	if err != nil {
+		return func(_ func(T) bool) {}, func() error { return err }
+	}
+
+	var retErr error
+	return func(yield func(T) bool) {
+		defer rows.Close()
+		for rows.Next() {
+			v := new(T)
+			if err := rows.StructScan(v); err != nil {
+				retErr = err
+				break
+			}
+			if cleanuper, ok := any(v).(interface{ cleanup() }); ok {
+				cleanuper.cleanup()
+			}
+			if !yield(*v) {
+				return
+			}
+		}
+		if err := rows.Err(); err != nil && retErr == nil {
+			retErr = err
+		}
+	}, func() error { return retErr }
+}
+
+// dbVector is a wrapper that allows protocol.Vector values to be serialized
+// to and from the database.
+type dbVector struct { //nolint:recvcheck
+	protocol.Vector
+}
+
+func (v dbVector) Value() (driver.Value, error) {
+	return v.String(), nil
+}
+
+func (v *dbVector) Scan(value any) error {
+	str, ok := value.(string)
+	if !ok {
+		return errors.New("not a string")
+	}
+	if str == "" {
+		v.Vector = protocol.Vector{}
+		return nil
+	}
+	vec, err := protocol.VectorFromString(str)
+	if err != nil {
+		return wrap(err)
+	}
+	v.Vector = vec
+
+	return nil
+}
+
+// indirectFI constructs a FileInfo from separate marshalled FileInfo and
+// BlockList bytes.
+type indirectFI struct {
+	Name       string // not used, must be present as dest for Need iterator
+	FiProtobuf []byte
+	BlProtobuf []byte
+	Size       int64 // not used
+	Modified   int64 // not used
+}
+
+func (i indirectFI) FileInfo() (protocol.FileInfo, error) {
+	var fi bep.FileInfo
+	if err := proto.Unmarshal(i.FiProtobuf, &fi); err != nil {
+		return protocol.FileInfo{}, wrap(err, "unmarshal fileinfo")
+	}
+	if len(i.BlProtobuf) > 0 {
+		var bl dbproto.BlockList
+		if err := proto.Unmarshal(i.BlProtobuf, &bl); err != nil {
+			return protocol.FileInfo{}, wrap(err, "unmarshal blocklist")
+		}
+		fi.Blocks = bl.Blocks
+	}
+	fi.Name = osutil.NativeFilename(fi.Name)
+	return protocol.FileInfoFromDB(&fi), nil
+}
+
+func prefixEnd(s string) string {
+	if s == "" {
+		panic("bug: cannot represent end prefix for empty string")
+	}
+	bs := []byte(s)
+	for i := len(bs) - 1; i >= 0; i-- {
+		if bs[i] < 0xff {
+			bs[i]++
+			break
+		}
+	}
+	return string(bs)
+}