limit up-front allocation when reading bytes

Fixes #73

util.go

@@ -5,6 +5,7 @@
 package tag
 
 import (
+	"bytes"
 	"encoding/binary"
 	"io"
 )
@@ -40,7 +41,18 @@ func readUint64LittleEndian(r io.Reader) (uint64, error) {
 	return binary.LittleEndian.Uint64(b), nil
 }
 
+// readBytesMaxUpfront is the max up-front allocation allowed
+const readBytesMaxUpfront = 10 << 20 // 10MB
+
 func readBytes(r io.Reader, n uint) ([]byte, error) {
+	if n > readBytesMaxUpfront {
+		b := &bytes.Buffer{}
+		if _, err := io.CopyN(b, r, int64(n)); err != nil {
+			return nil, err
+		}
+		return b.Bytes(), nil
+	}
+
 	b := make([]byte, n)
 	_, err := io.ReadFull(r, b)
 	if err != nil {