feat(reader): added WaitAvailable() method

multireader.go

@@ -151,15 +151,9 @@ func (r *Reader) Read(data []byte) (int, error) {
 		err error
 	)
 
-	if r.closed {
-		return 0, ErrReaderClosed
-	}
-
-	for r.readPos >= r.multiReader.lastWritePos {
-		if r.multiReader.closed {
-			return 0, ErrMultiReaderClosed
-		}
-		r.cond.Wait()
+	_, err = r.waitAvailable()
+	if err != nil {
+		return 0, err
 	}
 
 	n, err = r.readAhead(data)
@@ -215,6 +209,26 @@ func (r *Reader) ReadAhead(data []byte) (int, error) {
 	return r.readAhead(data)
 }
 
+func (r *Reader) waitAvailable() (int, error) {
+	for r.readPos >= r.multiReader.lastWritePos {
+		if r.closed {
+			return 0, ErrReaderClosed
+		}
+		if r.multiReader.closed {
+			return 0, ErrMultiReaderClosed
+		}
+		r.cond.Wait()
+	}
+
+	return r.multiReader.lastWritePos - r.readPos, nil
+}
+
+func (r *Reader) WaitAvailable() (int, error) {
+	r.mtx.Lock()
+	defer r.mtx.Unlock()
+	return r.waitAvailable()
+}
+
 func (r *Reader) Close() error {
 	r.mtx.Lock()
 	defer r.mtx.Unlock()