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/interface.go

@@ -0,0 +1,123 @@
+// 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 db // import "github.com/syncthing/syncthing/internal/db/sqlite"
+
+import (
+	"iter"
+	"time"
+
+	"github.com/syncthing/syncthing/lib/config"
+	"github.com/syncthing/syncthing/lib/protocol"
+	"github.com/thejerf/suture/v4"
+)
+
+type DB interface {
+	Service(maintenanceInterval time.Duration) suture.Service
+
+	// Basics
+	Update(folder string, device protocol.DeviceID, fs []protocol.FileInfo) error
+	Close() error
+
+	// Single files
+	GetDeviceFile(folder string, device protocol.DeviceID, file string) (protocol.FileInfo, bool, error)
+	GetGlobalAvailability(folder, file string) ([]protocol.DeviceID, error)
+	GetGlobalFile(folder string, file string) (protocol.FileInfo, bool, error)
+
+	// File iterators
+	//
+	// n.b. there is a slight inconsistency in the return types where some
+	// return a FileInfo iterator and some a FileMetadata iterator. The
+	// latter is more lightweight, and the discrepancy depends on how the
+	// functions tend to be used. We can introduce more variations as
+	// required.
+	AllGlobalFiles(folder string) (iter.Seq[FileMetadata], func() error)
+	AllGlobalFilesPrefix(folder string, prefix string) (iter.Seq[FileMetadata], func() error)
+	AllLocalBlocksWithHash(hash []byte) (iter.Seq[BlockMapEntry], func() error)
+	AllLocalFiles(folder string, device protocol.DeviceID) (iter.Seq[protocol.FileInfo], func() error)
+	AllLocalFilesBySequence(folder string, device protocol.DeviceID, startSeq int64, limit int) (iter.Seq[protocol.FileInfo], func() error)
+	AllLocalFilesWithPrefix(folder string, device protocol.DeviceID, prefix string) (iter.Seq[protocol.FileInfo], func() error)
+	AllLocalFilesWithBlocksHash(folder string, h []byte) (iter.Seq[FileMetadata], func() error)
+	AllLocalFilesWithBlocksHashAnyFolder(h []byte) (iter.Seq2[string, FileMetadata], func() error)
+	AllNeededGlobalFiles(folder string, device protocol.DeviceID, order config.PullOrder, limit, offset int) (iter.Seq[protocol.FileInfo], func() error)
+
+	// Cleanup
+	DropAllFiles(folder string, device protocol.DeviceID) error
+	DropDevice(device protocol.DeviceID) error
+	DropFilesNamed(folder string, device protocol.DeviceID, names []string) error
+	DropFolder(folder string) error
+
+	// Various metadata
+	GetDeviceSequence(folder string, device protocol.DeviceID) (int64, error)
+	ListFolders() ([]string, error)
+	ListDevicesForFolder(folder string) ([]protocol.DeviceID, error)
+	RemoteSequences(folder string) (map[protocol.DeviceID]int64, error)
+
+	// Counts
+	CountGlobal(folder string) (Counts, error)
+	CountLocal(folder string, device protocol.DeviceID) (Counts, error)
+	CountNeed(folder string, device protocol.DeviceID) (Counts, error)
+	CountReceiveOnlyChanged(folder string) (Counts, error)
+
+	// Index IDs
+	DropAllIndexIDs() error
+	GetIndexID(folder string, device protocol.DeviceID) (protocol.IndexID, error)
+	SetIndexID(folder string, device protocol.DeviceID, id protocol.IndexID) error
+
+	// MtimeFS
+	DeleteMtime(folder, name string) error
+	GetMtime(folder, name string) (ondisk, virtual time.Time)
+	PutMtime(folder, name string, ondisk, virtual time.Time) error
+
+	KV
+}
+
+// Generic KV store
+type KV interface {
+	GetKV(key string) ([]byte, error)
+	PutKV(key string, val []byte) error
+	DeleteKV(key string) error
+	PrefixKV(prefix string) (iter.Seq[KeyValue], func() error)
+}
+
+type BlockMapEntry struct {
+	BlocklistHash []byte
+	Offset        int64
+	BlockIndex    int
+	Size          int
+}
+
+type KeyValue struct {
+	Key   string
+	Value []byte
+}
+
+type FileMetadata struct {
+	Name       string
+	Sequence   int64
+	ModNanos   int64
+	Size       int64
+	LocalFlags int64
+	Type       protocol.FileInfoType
+	Deleted    bool
+	Invalid    bool
+}
+
+func (f *FileMetadata) ModTime() time.Time {
+	return time.Unix(0, f.ModNanos)
+}
+
+func (f *FileMetadata) IsReceiveOnlyChanged() bool {
+	return f.LocalFlags&protocol.FlagLocalReceiveOnly != 0
+}
+
+func (f *FileMetadata) IsDirectory() bool {
+	return f.Type == protocol.FileInfoTypeDirectory
+}
+
+func (f *FileMetadata) ShouldConflict() bool {
+	return f.LocalFlags&protocol.LocalConflictFlags != 0
+}