What version of Go?

1.10 or tip

What operating system and processor architecture?

darwin, *

What is the matter?

According to this and other sites, fcntl(fd, F_FULLFSYNC) is necessary for proper fsync on MacOS (OS X): https://danluu.com/file-consistency/

Github search finds no uses of "F_FULLFSYNC" in the src tree.

If there is a rationale for the present implementation, it should be documented in File.Sync() with directions on how to invoke the fcntl variant.