For some files the Windows Volume Shadow Service (VSS) advertises the
file size as X in the directory listing but returns a different number
Y on stat-ing the file. If the file is opened and read there are Y
bytes available for reading.
Existing copy tools copy Y bytes rather than X so for consistency
rclone should do the same.
This fixes the problem by stat-ing the file immediately before opening
it. This will also reduce the unnecessary occurrence of "can't copy -
source file is being updated" errors; if the file has finished
changing by the time we come to copy it then we now can copy it
successfully.
See: https://forum.rclone.org/t/consistently-getting-corrupted-on-transfer-sizes-differ-syncing-to-an-smb-share/42218/
5b0f9dc4e3