Upload if hash matches about dufs

[sigoden]

This strategy does not solve the problem. This policy has a race condition that cannot be resolved.

Let me explain:

Assume ClientA and ClientB edit file1 at the same time

ClientA gets hash1 of file1
ClientB gets hash1 of file1
ClientB starts writing file1, hash of file1 still be hash1, read to write
ClientA starts writing file2, hash of file1 still be hash1, read to write
Ops, Two writing at the same time.

Using locks can solve such problems. However, dufs is stateless and doesn't support locks. If you need to support concurrent writes, consider software that provides locking mechanisms.