OP is not asking for something that will guarantee a complete, safe write on all systems under all conditions in all logically-possible universes. OP is asking

OP is not asking for something that will guarantee a complete, safe write on all systems under all conditions in all logically-possible universes. OP is asking for something that, if it loses data due to the way the filesystem or operating system behaves, would be treated as a bug in the filesystem or OS, rather than a "lol, this idiot didn't even do (set of syscalls unique to the particular FS/OS combination), not our fault it didn't actually write after it said it would".