files that change during transfer

[Joshua Haberman]

What does rsync do if a source file changes while rsync is running?

I would like to be able to tell rsync "sync from this file, but  
ignore any data that is appended to the file while you're running."   
In other words, I want rsync to at some point read the file's size,  
and from that point on pretend the file is always that size, even if  
it grows in reality.

Basically I want to be able to sync from a file that's growing,  
without rsync either retrying over and over (since the source file  
keeps changing), or leaving the destination file in a corrupted state  
(because it noticed some changes in the source file but not others).

Is there a way to do this with the existing rsync?  If not, could  
this behavior be added (perhaps as a command-line option?)  I can  
simulate this behavior by copying the growing file to a temporary  
file and syncing from that, but that can be expensive if the source  
file is large.

Thanks,
Josh