Archive files corrupted when they are in private file system

Setting to RTBC thanks to #4

I'm no longer employed at the company which worked on that particular project. So I can just try to recall what happened:

From the documentation of clearstatcache (https://secure.php.net/manual/en/function.clearstatcache.php):

Affected functions include stat(), lstat(), file_exists(), is_writable(), is_readable(), is_executable(), is_file(), is_dir(), is_link(), filectime(), fileatime(), filemtime(), fileinode(), filegroup(), fileowner(), filesize(), filetype(), and fileperms().

I guess this happens if you archive enough files at once so that more than one batch iteration is necessary. Then we may get the too small size cached by the following lines:

if (file_exists($destination)) { // <-- file_exists() is affected
  $opened = $zip->open(drupal_realpath($destination));
}

@joelpitt, no there are no problems when you use a public file system as the file will be downloaded directly, ignoring the contents of the file_managed table. The issue just appears on private file systems because Drupal reads the file length from the database and delivers only that amount of bytes to the client.