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/meta.go

@@ -20,6 +20,7 @@ type metadataTracker struct {
 	mut     sync.RWMutex
 	counts  CountsSet
 	indexes map[metaKey]int // device ID + local flags -> index in counts
+	dirty   bool
 }
 
 type metaKey struct {
@@ -55,18 +56,31 @@ func (m *metadataTracker) Marshal() ([]byte, error) {
 
 // toDB saves the marshalled metadataTracker to the given db, under the key
 // corresponding to the given folder
-func (m *metadataTracker) toDB(db *Instance, folder []byte) error {
+func (m *metadataTracker) toDB(db *instance, folder []byte) error {
 	key := db.keyer.GenerateFolderMetaKey(nil, folder)
+
+	m.mut.RLock()
+	defer m.mut.RUnlock()
+
+	if !m.dirty {
+		return nil
+	}
+
 	bs, err := m.Marshal()
 	if err != nil {
 		return err
 	}
-	return db.Put(key, bs, nil)
+	err = db.Put(key, bs, nil)
+	if err == nil {
+		m.dirty = false
+	}
+
+	return err
 }
 
 // fromDB initializes the metadataTracker from the marshalled data found in
 // the database under the key corresponding to the given folder
-func (m *metadataTracker) fromDB(db *Instance, folder []byte) error {
+func (m *metadataTracker) fromDB(db *instance, folder []byte) error {
 	key := db.keyer.GenerateFolderMetaKey(nil, folder)
 	bs, err := db.Get(key, nil)
 	if err != nil {
@@ -99,6 +113,7 @@ func (m *metadataTracker) addFile(dev protocol.DeviceID, f FileIntf) {
 	}
 
 	m.mut.Lock()
+	m.dirty = true
 
 	if flags := f.FileLocalFlags(); flags == 0 {
 		// Account regular files in the zero-flags bucket.
@@ -141,6 +156,7 @@ func (m *metadataTracker) removeFile(dev protocol.DeviceID, f FileIntf) {
 	}
 
 	m.mut.Lock()
+	m.dirty = true
 
 	if flags := f.FileLocalFlags(); flags == 0 {
 		// Remove regular files from the zero-flags bucket
@@ -194,6 +210,7 @@ func (m *metadataTracker) removeFileLocked(dev protocol.DeviceID, flags uint32,
 // resetAll resets all metadata for the given device
 func (m *metadataTracker) resetAll(dev protocol.DeviceID) {
 	m.mut.Lock()
+	m.dirty = true
 	for i, c := range m.counts.Counts {
 		if bytes.Equal(c.DeviceID, dev[:]) {
 			m.counts.Counts[i] = Counts{
@@ -209,6 +226,7 @@ func (m *metadataTracker) resetAll(dev protocol.DeviceID) {
 // sequence number
 func (m *metadataTracker) resetCounts(dev protocol.DeviceID) {
 	m.mut.Lock()
+	m.dirty = true
 
 	for i, c := range m.counts.Counts {
 		if bytes.Equal(c.DeviceID, dev[:]) {
@@ -285,6 +303,7 @@ func (m *metadataTracker) Created() time.Time {
 func (m *metadataTracker) SetCreated() {
 	m.mut.Lock()
 	m.counts.Created = time.Now().UnixNano()
+	m.dirty = true
 	m.mut.Unlock()
 }