Created attachment 145120 [details]
a minimal program for printing VNODE events on BSD
On OpenBSD /usr/include/sys/event.h defines a filer for watching responding file truncation:
#define·NOTE_TRUNCATE 0x0080 /* vnode was truncated */
On FreeBSD this is not defined, and does not appear to be covered by any other filers
$ echo "test" > file.txt # NOTE_WRITE
$ : > file.txt # NOTE_TRUNCATE
I'm filing this bug on behalf of the entr utility (http://entrproject.org) since this leaves an edge case where a file is altered without triggering an action.