I just discovered the kFSEventStreamEventFlagHistoryDone flag.
This solves my problem much more elegantly.
I'll file a documentation bug. Neither the File System Events
Programming guide nor the Xcode documentation mention this flag.
I'll be happy to file a bug report on FSEventsFlushAsync()
returning 0 if someone could confirm that it is, in fact, a bug
and not something stupid that I'm doing.