Export backup loader interface (#901)

The logic in the load method will be copied to Dgraph so that it can
support converting keys and values from a different format. To
allow this, the loader interface has been exported as KVLoader.

Also add a note telling users to use the KVLoader if more complex logic
is needed to restore a backup.

backup.go

@@ -127,20 +127,23 @@ func writeTo(list *pb.KVList, w io.Writer) error {
 	return err
 }
 
-type loader struct {
+// KVLoader is used to write KVList objects in to badger. It can be used to restore a backup.
+type KVLoader struct {
 	db       *DB
 	throttle *y.Throttle
 	entries  []*Entry
 }
 
-func (db *DB) newLoader(maxPendingWrites int) *loader {
-	return &loader{
+// NewKVLoader returns a new instance of KVLoader.
+func (db *DB) NewKVLoader(maxPendingWrites int) *KVLoader {
+	return &KVLoader{
 		db:       db,
 		throttle: y.NewThrottle(maxPendingWrites),
 	}
 }
 
-func (l *loader) set(kv *pb.KV) error {
+// Set writes the key-value pair to the database.
+func (l *KVLoader) Set(kv *pb.KV) error {
 	var userMeta, meta byte
 	if len(kv.UserMeta) > 0 {
 		userMeta = kv.UserMeta[0]
@@ -162,7 +165,7 @@ func (l *loader) set(kv *pb.KV) error {
 	return nil
 }
 
-func (l *loader) send() error {
+func (l *KVLoader) send() error {
 	if err := l.throttle.Do(); err != nil {
 		return err
 	}
@@ -176,7 +179,8 @@ func (l *loader) send() error {
 	return nil
 }
 
-func (l *loader) finish() error {
+// Finish is meant to be called after all the key-value pairs have been loaded.
+func (l *KVLoader) Finish() error {
 	if len(l.entries) > 0 {
 		if err := l.send(); err != nil {
 			return err
@@ -187,15 +191,16 @@ func (l *loader) finish() error {
 
 // Load reads a protobuf-encoded list of all entries from a reader and writes
 // them to the database. This can be used to restore the database from a backup
-// made by calling DB.Backup().
+// made by calling DB.Backup(). If more complex logic is needed to restore a badger
+// backup, the KVLoader interface should be used instead.
 //
 // DB.Load() should be called on a database that is not running any other
 // concurrent transactions while it is running.
 func (db *DB) Load(r io.Reader, maxPendingWrites int) error {
 	br := bufio.NewReaderSize(r, 16<<10)
 	unmarshalBuf := make([]byte, 1<<10)
 
-	ldr := db.newLoader(maxPendingWrites)
+	ldr := db.NewKVLoader(maxPendingWrites)
 	for {
 		var sz uint64
 		err := binary.Read(br, binary.LittleEndian, &sz)
@@ -219,7 +224,7 @@ func (db *DB) Load(r io.Reader, maxPendingWrites int) error {
 		}
 
 		for _, kv := range list.Kv {
-			if err := ldr.set(kv); err != nil {
+			if err := ldr.Set(kv); err != nil {
 				return err
 			}
 
@@ -231,7 +236,7 @@ func (db *DB) Load(r io.Reader, maxPendingWrites int) error {
 		}
 	}
 
-	if err := ldr.finish(); err != nil {
+	if err := ldr.Finish(); err != nil {
 		return err
 	}
 	db.orc.txnMark.Done(db.orc.nextTxnTs - 1)