syncing a directory

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

Is there a way (other than sync()) to make sure a rename() has made  
it to non-volatile storage?

Dovecot wants to be sure that having moved a file from (maildir) tmp/  
to new/ is going to survive a crash before reporting success to the  
client. The (possible Linuxish) way it attempts to do this is to open  
the directory (O_RDONLY, of course) and then fsync()ing the file  
handle. Set aside the question whether this does anything useful on  
Linux, NetBSD objects, presumably because fsync() on a read-only  
descriptor doesn't make sense.

I know well that metadata writes used to be synchronous anyway, but  
they are no more in the age of soft updates.

It looks reasonable what dovecot demands, and it seems reasonable  
NetBSD objects to the specific way it tries doing that. But what  
would be the correct way of doing it? What if the files in question  
reside on NFS?