steam-runtime-sniper creates var/tmp-XXXXXX folder that confuses backup tools

Your system information

• Steam Runtime Version: sniper_platform_0.20240916.101795
• Distribution (e.g. Ubuntu 18.04): Gentoo ~amd64
• Link to your full system information (Help -> Steam Runtime Diagnostics) in a Gist: https://gist.github.com/Anonymous1157/f733a83c09bb2625ed08876005f4cf0d
• Have you checked for system updates?: Yes
• What compatibility tool are you using?: Proton 9.0 (temporarily switched to Proton-GE in an attempt to remove steam-runtime-sniper but it did not let me, but also Proton is not relevant to the issue)
• What versions are listed in steamapps/common/SteamLinuxRuntime/VERSIONS.txt? 0.20240806.0
• What versions are listed in steamapps/common/SteamLinuxRuntime_soldier/VERSIONS.txt? 0.20241118.108551
• What versions are listed in steamapps/common/SteamLinuxRuntime_sniper/VERSIONS.txt? 0.20241118.108552

Please describe your issue in as much detail as possible:

I'm running Steam from the Flatpak.

Steam Runtime Sniper creates a folder Steam/ubuntu12_64/steam-runtime-sniper/var/tmp-XXXXXX in its file structure where the XXXXXX changes every time Steam launches. I tried looking for other reported issues about this folder and it's allegedly being hardlinked from somewhere else rather than unpacking the runtime again every launch, but I'm also far from the only person that's had a weird issue with this folder.

I use rsync to backup my home directory. If I have launched Steam at any time since the last backup, the tmp-XXXXXX folder will change its name. At backup time approximately one gigabyte of data at the remote server will be deleted and a presumably identical gigabyte of data will be copied again as if it were completely new files.

Is there a way to make Steam stop recreating/renaming this folder? I tried to get my friends to reproduce this problem and apparently this folder was not getting renamed for them. I don't know any details about their setup other than it's a different distro but they were also using a Flatpak.

Of course I can work around the problem by excluding this folder in my rsync command. That's fine. But I think this is a problem that should be solved at the runtime so that less technical users don't have to worry about it. I think it's reasonable to expect someone could want a one-click backup solution with no strange gotchas to work around.

Steps for reproducing this issue:

1. Launch Steam
2. Backup your home directory
3. Close and relaunch Steam
4. Backup your home directory again
5. Watch your backup software delete Steam/ubuntu12_64/steam-runtime-sniper/var/tmp-XXXXXX and copy Steam/ubuntu12_64/steam-runtime-sniper/var/tmp-YYYYYY
6. Repeat ad nauseum until your NAS' storage wears out

Start by examining the Steam Runtime Sniper handling of the var/tmp-XXXXXX directory and reproduce the issue using the listed Steam launch and backup steps. Done means relaunching Steam no longer renames or recreates an equivalent directory, so rsync does not recopy the data.