Support for environment variables for data/config folders

[jintaxi]

Relevant components

• Standalone tray application (based on Qt Widgets)
• Plasmoid/applet for Plasma desktop
• Dolphin integration
• Command line tool (syncthingctl)
• Integrated Syncthing instance (libsyncthing)
• Backend libraries

Is your feature request specific to a certain platform/environment? Please specify.
Both for Windows and Linux.

Is your feature request related to a problem? Please describe.
In the Syncthing Tray -> Settings -> Startup -> Syncthing launcher -> Config directory/Data directory support for environment variables is needed. Such as %LocalAppData% in Windows.

Describe the solution you'd like
When writing something like %LocalAppData%/Syncthing, the program interprets it not as %LocalAppData%/Syncthing to a folder in the local directory, but actually follows the path specified in the environment variable.

Describe alternatives you've considered
I write the full path to the AppData/local folder, which makes it difficult to port the program (for example, for a USB stick), or leave it in the program folder (./data).

[Martchus]

Another alternative would be to use the environment variables mentioned by syncthing serve --help and not set anything in the settings dialog. At least I think the integrated Syncthing instance allows that as well.

However, I guess I can add support for those substituions (or would accept a PR adding this feature). I suppose under Windows it would go though all environment variables and substitues %NAME_OF_VARIABLE% with the value of that variable. Under Linux it would substitue ${NAME_OF_VARIABLE}. Implementing further syntax and escaping is probably not needed.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[Martchus]

Closing as this is now implemented. The implementation uses https://pkg.go.dev/os#ExpandEnv under the hood so the syntax is $VAR or ${VAR} and I suppose that also counts under Windows.

EDIT: I've just tested it under Windows and it works (but only with $, so e.g. $AppData/libsyncthing-test will put files under %AppData%/libsyncthing-test).