So you can guarantee that file_xxx_N is complete if file_xxx_N+1 (or later) exists? Fair enough.

But for reliability, the reader should check this condition. (And not process a file if it is the latest one). But that might have problems too (if there hasn't been any activity since, then you'll miss the last record).

Really, I'm nitpicking, because the race condition is probably unlikely to be hit. But systems like this often run unattended for a long time, on systems which sometimes bog down under load. Race conditions lead to unpredictable behaviour and lots of those time-consuming "oh...we sometimes get that problem, we don't know why" issues.

IMHO, the only safe way to do this is "create temp file, then rename".