Otherwise if the file grows during scanning the block list will be out
of sync with the stated size and things get confused. We could fixup the
size afterwards based on the block list, but then we might see other
inconsistencies as the mtime should have changed to reflect the new size
etc. Better stick to the original state and let the next scan pick up
the change.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3442
By using copyBuffer we avoid a buffer allocation for each block we hash,
and by allocating space for the hashes up front we get one large backing
array instead of a small one for each block. For a 17 MiB file this
makes quite a difference in the amount of memory allocated:
benchmark old ns/op new ns/op delta
BenchmarkHashFile-8 102045110 100459158 -1.55%
benchmark old allocs new allocs delta
BenchmarkHashFile-8 415 144 -65.30%
benchmark old bytes new bytes delta
BenchmarkHashFile-8 4504296 48104 -98.93%
