Will Thompson authored Nov 22, 2017 and Will Thompson committed Jun 15, 2018

This function only calls fsync() if @target exists and is non-empty. If
not, it doesn't provide the "old contents or new contents" guarantee
that one might expect. This has been the case since
d20a188b, and is justified either as a
performance optimization or by asserting that this function only
guarantees to not destroy existing data (implicitly defining
non-existence or emptiness as not data).

In addition, explicitly spell out that whether it's atomic in the
non-empty case is system-dependent. If the system administrator has
configured some funky filesystem options, they may be out of luck on the
atomicity front.

#1302