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/db_service.go

@@ -0,0 +1,141 @@
+// 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 (
+	"context"
+	"time"
+
+	"github.com/syncthing/syncthing/internal/db"
+)
+
+const (
+	internalMetaPrefix = "dbsvc"
+	lastMaintKey       = "lastMaint"
+)
+
+type Service struct {
+	sdb                 *DB
+	maintenanceInterval time.Duration
+	internalMeta        *db.Typed
+}
+
+func newService(sdb *DB, maintenanceInterval time.Duration) *Service {
+	return &Service{
+		sdb:                 sdb,
+		maintenanceInterval: maintenanceInterval,
+		internalMeta:        db.NewTyped(sdb, internalMetaPrefix),
+	}
+}
+
+func (s *Service) Serve(ctx context.Context) error {
+	// Run periodic maintenance
+
+	// Figure out when we last ran maintenance and schedule accordingly. If
+	// it was never, do it now.
+	lastMaint, _, _ := s.internalMeta.Time(lastMaintKey)
+	nextMaint := lastMaint.Add(s.maintenanceInterval)
+	wait := time.Until(nextMaint)
+	if wait < 0 {
+		wait = time.Minute
+	}
+	l.Debugln("Next periodic run in", wait)
+
+	timer := time.NewTimer(wait)
+	for {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case <-timer.C:
+		}
+
+		if err := s.periodic(ctx); err != nil {
+			return wrap(err)
+		}
+
+		timer.Reset(s.maintenanceInterval)
+		l.Debugln("Next periodic run in", s.maintenanceInterval)
+		_ = s.internalMeta.PutTime(lastMaintKey, time.Now())
+	}
+}
+
+func (s *Service) periodic(ctx context.Context) error {
+	t0 := time.Now()
+	l.Debugln("Periodic start")
+
+	s.sdb.updateLock.Lock()
+	defer s.sdb.updateLock.Unlock()
+
+	t1 := time.Now()
+	defer func() { l.Debugln("Periodic done in", time.Since(t1), "+", t1.Sub(t0)) }()
+
+	if err := s.garbageCollectBlocklistsAndBlocksLocked(ctx); err != nil {
+		return wrap(err)
+	}
+
+	_, _ = s.sdb.sql.ExecContext(ctx, `ANALYZE`)
+	_, _ = s.sdb.sql.ExecContext(ctx, `PRAGMA optimize`)
+	_, _ = s.sdb.sql.ExecContext(ctx, `PRAGMA incremental_vacuum`)
+	_, _ = s.sdb.sql.ExecContext(ctx, `PRAGMA wal_checkpoint(TRUNCATE)`)
+
+	return nil
+}
+
+func (s *Service) garbageCollectBlocklistsAndBlocksLocked(ctx context.Context) error {
+	// Remove all blocklists not referred to by any files and, by extension,
+	// any blocks not referred to by a blocklist. This is an expensive
+	// operation when run normally, especially if there are a lot of blocks
+	// to collect.
+	//
+	// We make this orders of magnitude faster by disabling foreign keys for
+	// the transaction and doing the cleanup manually. This requires using
+	// an explicit connection and disabling foreign keys before starting the
+	// transaction. We make sure to clean up on the way out.
+
+	conn, err := s.sdb.sql.Connx(ctx)
+	if err != nil {
+		return wrap(err)
+	}
+	defer conn.Close()
+
+	if _, err := conn.ExecContext(ctx, `PRAGMA foreign_keys = 0`); err != nil {
+		return wrap(err)
+	}
+	defer func() { //nolint:contextcheck
+		_, _ = conn.ExecContext(context.Background(), `PRAGMA foreign_keys = 1`)
+	}()
+
+	tx, err := conn.BeginTxx(ctx, nil)
+	if err != nil {
+		return wrap(err)
+	}
+	defer tx.Rollback() //nolint:errcheck
+
+	if res, err := tx.ExecContext(ctx, `
+		DELETE FROM blocklists
+		WHERE NOT EXISTS (
+			SELECT 1 FROM files WHERE files.blocklist_hash = blocklists.blocklist_hash
+		)`); err != nil {
+		return wrap(err, "delete blocklists")
+	} else if shouldDebug() {
+		rows, err := res.RowsAffected()
+		l.Debugln("Blocklist GC:", rows, err)
+	}
+
+	if res, err := tx.ExecContext(ctx, `
+		DELETE FROM blocks
+		WHERE NOT EXISTS (
+			SELECT 1 FROM blocklists WHERE blocklists.blocklist_hash = blocks.blocklist_hash
+		)`); err != nil {
+		return wrap(err, "delete blocks")
+	} else if shouldDebug() {
+		rows, err := res.RowsAffected()
+		l.Debugln("Blocks GC:", rows, err)
+	}
+
+	return wrap(tx.Commit())
+}