fsync write barrier for all client or server side writing of state to disk

[david415]

I'm looking at you authority client, catshadow and katzenpost/client.

[david415]

a wise man once said """ideally it would call fsync before closing the file, but instead the best we can do is resort to either fsync FOR EACH WRITE or pull in some library that does proper file i/o

but seeing as this seems systemic in the code base we might benefit from doing the latter eventually, or writing our own set of abstractions for it

anyway - if you're writing files to disk for key rotations, you definitely want them to actually get written to the disk, so os.file.O_SYNC in the flags arg to WriteFile is the 5-second fix for that problem, and we can tackle it properly later

python's stdlib has the same stupid data-eating stdlib design, see Durability here https://blog.gocept.com/2013/07/15/reliable-file-updates-with-python/"""