This uses the length of the file known at the start of the copy to determine the amount of data to copy.
* If a file is appended to during the copy, the original bytes are copied.
* If a file is truncated during a copy, or the attempt to read the data fails with a truncated stream,
distcp will now fail. Until now these failures were not detected.
Contributed by Mukund Thakur.
Change-Id: I576a49d951fa48d37a45a7e4c82c47488aa8e884
51c64b357d