Recover deleted file that is currently being written to

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http://unix.stackexchange.com – I started downloading a big file and accidently deleted it a while ago. I know how to get its current contents by cping /proc/<pid>/fd/<fd> but since the download is still in progress it'll be incomplete at the time I copy it someplace else. Can I somehow salvage the file right at the moment the download finishes but before the downloader closes the file and I lose it for good? (HowTos)