Wrong file given to -k option is potentially overwritten

[mikini]

Just got an Ubuntu 17.10 desktop iso squashed by trying to reuse its contents for retrieving the server iso using zsync.

I mixed up the -k and -i options giving the existing iso file instead as parameter for -k. Then almost immediately seeing the unexpected message I aborted using ctrl-c. Result was as could be expected; that the iso (given as -k parameter) was replaced with the .sync file because it was coincidentally the most recent.

Maybe a warning would be suitable if the file given as -k parameter exists and is not a valid .zsync file.
Maybe even also verify after download that the existing and updated .zsync files actually describe the same target file before overwriting the existing.

Command line dump:

user@machine:~/OS ISO/artful-desktop-amd64$ ll
total 1469080
drwxrwxr-x 2 user user       4096 Nov  9 18:41 ./
drwxrwxr-x 3 user user       4096 Oct  3 01:52 ../
-rw-rw-r-- 1 user user    3145949 Sep  8 10:02 artful-desktop-amd64.iso.zsync
-rw-rw-r-- 1 user user        479 Oct 19 11:15 SHA256SUMS
-rw-rw-r-- 1 user user        916 Oct 19 11:15 SHA256SUMS.gpg
-rw------- 1 user user 1501102080 Oct 18 18:53 ubuntu-17.10-desktop-amd64.iso
-rwxrw-r-- 1 user user         82 Jul 28 13:39 update.sh*
-rw-rw-r-- 1 user user       1024 Nov  9 18:41 .update.sh.swp
user@machine:~/OS ISO/artful-desktop-amd64$ zsync http://mirrors.dotsrc.org/ubuntu-cd/17.10/ubuntu-17.10-server-amd64.iso.zsync -k ubuntu-17.10-desktop-amd64.iso 
#################### 100.0% 202.8 kBps DONE     

No relevent local data found - I will be downloading the whole file. If that's not what you want, CTRL-C out. You should specify the local file is the old version of the file to download with -i (you might have to decompress it with gzip -d first). Or perhaps you just have no data that helps download the file
downloading from http://mirrors.dotsrc.org/ubuntu-cd/17.10/ubuntu-17.10-server-amd64.iso:
#------------------- 9.6% 13090.4 kBps 0:53 ETA  ^C
user@machine:~/OS ISO/artful-desktop-amd64$ ll
total 77988
drwxrwxr-x 2 user user     4096 Nov  9 18:43 ./
drwxrwxr-x 3 user user     4096 Oct  3 01:52 ../
-rw-rw-r-- 1 user user  3145949 Sep  8 10:02 artful-desktop-amd64.iso.zsync
-rw-rw-r-- 1 user user      479 Oct 19 11:15 SHA256SUMS
-rw-rw-r-- 1 user user      916 Oct 19 11:15 SHA256SUMS.gpg
-rw-rw-r-- 1 user user  1525990 Nov  9 18:43 ubuntu-17.10-desktop-amd64.iso
-rw------- 1 user user 75153408 Nov  9 18:43 ubuntu-17.10-server-amd64.iso.part
-rwxrw-r-- 1 user user      253 Nov  9 18:43 update.sh*
-rw-rw-r-- 1 user user     1024 Nov  9 18:42 .update.sh.swp

System information:

$ zsync -V
zsync v0.6.2 (compiled Dec  5 2012 12:53:16)
By Colin Phipps <cph@moria.org.uk>
Published under the Artistic License v2, see the COPYING file for details.
$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.3 LTS
Release:	16.04
Codename:	xenial