Since i/o operations are not synchronized, it is possible that one thread closes file handle, opened in another thread. With addition of `hidden` file attribute, an old bug in `part_file::open_file()` revealed both problems, causing a lot of random i/o errors in parts file. Fixing `open_file` bug should reduce the number of such errors back to 'normal'.
handle short reads when exporting a file. Pieces in the part file are not guaranteed to be fully written so short reads need to be handled rather than asserting.