Permission errors accessing $SNAP_USER_COMMON location from remove hook

snapcraft
1

My remove hook is getting the following permission errors on Ubuntu (though it works great on Manjaro):

cannot open /home/myusername/snap/mysnap/common/mytextfile.txt: Permission denied

…and…

rm: cannot remove '/home/myusername/snap/mysnap/common/mytextfile.txt': Permission denied

mytextfile.txt is placed into that user folder via the snap app itself (using os.environ['SNAP_USER_COMMON'] in python3), after install, and while running as the user. The file even has 777 permissions. But then the remove hook, running as root, can’t access it. What do I need to do to enable the hook to access the file that the snap app itself placed there?

When I run:

snap connections mysnap

…I get this, so it appears as though the home interface is connected, even though I am running in strict mode and do not manually connect it anywhere:

Interface       Plug                            Slot             Notes
desktop         mysnap:desktop         :desktop         -
desktop-legacy  mysnap:desktop-legacy  :desktop-legacy  -
gsettings       mysnap:gsettings       :gsettings       -
home            mysnap:home            :home            -
network         mysnap:network         :network         -
network-bind    mysnap:network-bind    :network-bind    -
unity7          mysnap:unity7          :unity7          -
x11             mysnap:x11             :x11             -

Here are the relevant parts of my YAML:

plugs:
    network:
    network-bind:
    desktop:
    desktop-legacy:
    x11:
    unity7:
    home:
    gsettings:

hooks:
    remove:
        plugs: [network, network-bind, home]

parts:
    remove-hook:
        plugin: nil
        build-packages:
            - wget
        stage-packages:
            - wget
        override-build: |
            cat > myfile << 'EOF'
            
            #!/bin/sh
            
            filelocation="/home/myusername/snap/mysnap/common/mytextfile.txt"
            
            while IFS= read -r mystringvalue
            
            do
                
                echo "$mystringvalue"
                
                #wget stuff goes here...
                
            done < "$filelocation"
            
            rm "$filelocation"
            
            EOF
            
            install -D myfile $SNAPCRAFT_PART_INSTALL/snap/hooks/remove

Thank you in advance for any suggestions you can offer!

2

The remove hook runs as root, as you found. That means it cannot access user common folders by design. As you’re doing this in a remove hook, the file would be removed by snapd anyway, so you don’t need to remove it in the hook.

3

I need to read it in the hook though.

Can you suggest an alternative for passing variables from the user-run-app to the root-run-remove-hook? Surely there must be some way for them to talk to one another?