Layouts with writable config file

I am creating a Python app that has XML configuration file. The app will write and update the configuration file when running. I would also like the file to persist after an update. After reading about Layouts I thought I would use $SNAP_DATA as it persists when refreshed.
My snapcraft.yaml had the following:

    bind-file: $SNAP_DATA/etc/config.xml

But when my app wrote to the config.xml file I would get permission error as it is owned by root. So I thinking maybe I should use $SNAP_DATA with the home directory but I am not sure of the layout syntax as I would not know the USER name in home folder? Any help would be greatly appreciated.

Environment variables

$SNAP_DATA is equivalent to /etc. In both cases non-privileged users cannot write.

$SNAP_USER_DATA is equivalent to the user’s home directory and can be read/written by the user. This directory is backed up and restored across snap refresh and snap revert operations.

SNAP_USER_COMMON is similar but will persist across snap refresh and snap revert operations.

Thanks for the reply.

Unfortunately when I try to use $SNAP_USER_COMMON I get the error:

error: cannot read snap file: layout “$SNAP_USER_COMMON/config.xml” uses invalid mount point:
reference to unknown variable “$SNAP_USER_COMMON”

The syntax I used is:

bind-file: $SNAP_USER_COMMON/config.xml

I must be declaring the layout incorrectly?

Layouts are global, not user-specific. So you cannot use the $SNAP_USER_* locations for them.

Thanks for the help.

So, I guess I shouldn’t use layouts for writable config files. What do you suggest is the procedure for declaring a writable config file in my snap?

If you are in control of the code of the app you’re snapping then make it write to $SNAP_USER_DATA or $SNAP_USER_COMMON. At a pinch you could use $HOME because that is also defaulted to pointing to the same location as $SNAP_USER_DATA.