lib/db: Refactor: use a Lowlevel type underneath Instance (ref #5198) (#5212)

This adds a thin type that holds the state associated with the
leveldb.DB, leaving the huge Instance type more or less stateless. Also
moves some keying stuff into the DB package so that other packages need
not know the keying specifics.

(This does not, yet, fix the cmd/stindex program, in order to keep the
diff size down. Hence the keying constants are still exported.)

lib/db/leveldb_dbinstance_updateschema.go

@@ -38,8 +38,17 @@ func (e databaseDowngradeError) Error() string {
 	return fmt.Sprintf("Syncthing %s required", e.minSyncthingVersion)
 }
 
-func (db *Instance) updateSchema() error {
-	miscDB := NewNamespacedKV(db, string(KeyTypeMiscData))
+func UpdateSchema(ll *Lowlevel) error {
+	updater := &schemaUpdater{newInstance(ll)}
+	return updater.updateSchema()
+}
+
+type schemaUpdater struct {
+	*instance
+}
+
+func (db *schemaUpdater) updateSchema() error {
+	miscDB := NewMiscDataNamespace(db.Lowlevel)
 	prevVersion, _ := miscDB.Int64("dbVersion")
 
 	if prevVersion > dbVersion {
@@ -77,7 +86,7 @@ func (db *Instance) updateSchema() error {
 	return nil
 }
 
-func (db *Instance) updateSchema0to1() {
+func (db *schemaUpdater) updateSchema0to1() {
 	t := db.newReadWriteTransaction()
 	defer t.close()
 
@@ -159,7 +168,7 @@ func (db *Instance) updateSchema0to1() {
 
 // updateSchema1to2 introduces a sequenceKey->deviceKey bucket for local items
 // to allow iteration in sequence order (simplifies sending indexes).
-func (db *Instance) updateSchema1to2() {
+func (db *schemaUpdater) updateSchema1to2() {
 	t := db.newReadWriteTransaction()
 	defer t.close()
 
@@ -178,7 +187,7 @@ func (db *Instance) updateSchema1to2() {
 }
 
 // updateSchema2to3 introduces a needKey->nil bucket for locally needed files.
-func (db *Instance) updateSchema2to3() {
+func (db *schemaUpdater) updateSchema2to3() {
 	t := db.newReadWriteTransaction()
 	defer t.close()
 
@@ -209,7 +218,7 @@ func (db *Instance) updateSchema2to3() {
 // release candidates (dbVersion 3 and 4)
 // https://github.com/syncthing/syncthing/issues/5007
 // https://github.com/syncthing/syncthing/issues/5053
-func (db *Instance) updateSchemaTo5() {
+func (db *schemaUpdater) updateSchemaTo5() {
 	t := db.newReadWriteTransaction()
 	var nk []byte
 	for _, folderStr := range db.ListFolders() {
@@ -221,7 +230,7 @@ func (db *Instance) updateSchemaTo5() {
 	db.updateSchema2to3()
 }
 
-func (db *Instance) updateSchema5to6() {
+func (db *schemaUpdater) updateSchema5to6() {
 	// For every local file with the Invalid bit set, clear the Invalid bit and
 	// set LocalFlags = FlagLocalIgnored.