transmission: 'Unable to save resume file. File exists' Error

Maintainer: @neheb

Environment:
OpenWrt SNAPSHOT Kernel v5.4.60
BT HomeHub 5A - xrx200
Transmission v3.00-4

Description:
The resume file that gets saved in the resume folder in transmission config is not being saved if it exists already. In v19.07.3 this does not seem to be an issue. I presume that transmission code was changed to implement something and may have not been tested in this regard. Anyways, the file can be saved successfully if you delete the old file manually and so an updated file gets saved. Now this issue seems pretty important because if a resume file gets saved at the start and does not save after that then in an event of restart, the transmission would think that it did not even download the file although a substantial portion may have been downloaded.

To reproduce this error, you just need to add a torrent file, start it and then pause it. If there is no error then the resume file has been saved just now so start it again and pause it and there you go. The error pops up instantly.
[Edit: Fix typos]

Reproduce on the stated OpenWrt SNAPSHOT environment with Transmission v3.00-4 by adding a torrent, starting it, pausing it, then repeating the start-and-pause cycle. Inspect the Transmission package and its resume-folder handling; done means an existing resume file can be updated on the second pause without the “File exists” error.