add skipreader , fix GetFile can use offset

Author: codeskyblue

driver.go

@@ -175,7 +175,7 @@ func (driver *QiniuDriver) MakeDir(path string) error {
 	return err
 }
 
-func (driver *QiniuDriver) GetFile(key string, start int64) (int64, io.ReadCloser, error) {
+func (driver *QiniuDriver) GetFile(key string, offset int64) (int64, io.ReadCloser, error) {
 	stat, err := driver.Stat(key)
 	if err != nil {
 		return 0, nil, err
@@ -193,7 +193,7 @@ func (driver *QiniuDriver) GetFile(key string, start int64) (int64, io.ReadClose
 		return 0, nil, err
 	}
 
-	return stat.Size(), resp.Body, nil
+	return stat.Size(), NewSkipReadCloser(resp.Body, offset), nil
 }
 
 func (driver *QiniuDriver) PutFile(key string, data io.Reader, appendData bool) (int64, error) {

skipreader.go

@@ -0,0 +1,32 @@
+package qiniudriver
+
+import (
+	"io"
+	"io/ioutil"
+)
+
+func NewSkipReadCloser(rd io.ReadCloser, count int64) io.ReadCloser {
+	return &SkipReadCloser{
+		ReadCloser: rd,
+		count:      count,
+	}
+}
+
+type SkipReadCloser struct {
+	io.ReadCloser
+	count   int64
+	skipped bool
+}
+
+func (s *SkipReadCloser) Read(data []byte) (int, error) {
+	if !s.skipped {
+		if s.count > 0 {
+			_, err := io.CopyN(ioutil.Discard, s.ReadCloser, s.count)
+			if err != nil {
+				return 0, err
+			}
+		}
+		s.skipped = true
+	}
+	return s.ReadCloser.Read(data)
+}