Fix create archive to a continuous writing source file failed (#388)

* Fix create archive to a continuous writing source file failed

#387
Signed-off-by: Yan Zhu <hackzhuyan@gmail.com>

* Update archiver.go

---------

Signed-off-by: Yan Zhu <hackzhuyan@gmail.com>
Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

archiver.go

@@ -221,8 +221,14 @@ func openAndCopyFile(file File, w io.Writer) error {
 		return err
 	}
 	defer fileReader.Close()
-	_, err = io.Copy(w, fileReader)
-	return err
+	// When file is in use and size is being written to, creating the compressed
+	// file will fail with "archive/tar: write too long." Using CopyN gracefully
+	// handles this.
+	_, err = io.CopyN(w, fileReader, file.Size())
+	if err != nil && err != io.EOF {
+		return err
+	}
+	return nil
 }
 
 // fileIsIncluded returns true if filename is included according to