[acuozzo]

Is this guaranteed to be atomic on all filesystems?

[chasil]

For POSIX, yes.

https://rcrowley.org/2010/01/06/things-unix-can-do-atomicall...

Windows has a deep well of POSIX in the kernel (plus hard file locks), and it appears to hold there.

https://en.wikipedia.org/wiki/Microsoft_POSIX_subsystem

[elteto]

It’s even atomic on NFS. In fact, it’s probably the only reliable locking mechanism on NFS.