posix, windows: introduce cachestat This class contains a stat result, and possibly other file info to reliably determine between two points in time whether a file has changed. Uniquely identifying a file gives us that reliability because we either atomic rename or append. So one of two will happen: the file 'id' will change, or the size of the file will change. posix implements it simply by calling os.stat() and checking if the result has st_ino. For now on Windows we always assume the path is uncacheable. This can be improved on NTFS due to file IDs: http://msdn.microsoft.com/en-us/library/aa363788(v=vs.85).aspx So we need to find out if a file path is on an NTFS drive, for that we have: - GetVolumeInformation, which unfortunately only works with a root path (but is available on XP) - GetVolumeInformationByHandleW, works on a full file path but requires Vista or higher