uploader: reinit TeeReader on upload retry to reset pointer

On an upload fail and retry, teeReader would not be reset and would
point to the end of the buffer being uploaded. We need to reset
teeReader to begin reading from the beginning of fileContents again.

core/uploader.go

@@ -73,11 +73,12 @@ func Upload(input io.Reader, outputURI *url.URL, waitBetweenWrites, writeTimeout
 			return nil, fmt.Errorf("failed to read file")
 		}
 
-		// To count how many bytes we are trying to read then write (upload) to s3 storage
-		teeReader := io.TeeReader(bytes.NewReader(fileContents), byteCounter)
-
 		var out *drivers.SaveDataOutput
 		err = backoff.Retry(func() error {
+			// To count how many bytes we are trying to read then write (upload) to s3 storage
+			teeReader := io.TeeReader(bytes.NewReader(fileContents), byteCounter)
+			byteCounter.Count = 0
+
 			out, err = session.SaveData(context.Background(), "", teeReader, fields, segmentWriteTimeout)
 			if err != nil {
 				glog.Errorf("failed upload attempt for %s (%d bytes): %v", outputURI.Redacted(), byteCounter.Count, err)