snap: Support canonical config paths, fixes #1403

This patch supports canonical config paths by the help of the
`personal-files` and the `system-files` security confinement interfaces.

Signed-off-by: 林博仁(Buo-ren Lin) <Buo.Ren.Lin@gmail.com>

snap/local/launchers/glances-launch

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+# This is the maintainence launcher for the snap, make necessary runtime
+# environment changes to make the snap work here.  You may also insert
+# security confinement/deprecation/obsoletion notice of the snap here.
+set \
+    -o errexit \
+    -o errtrace \
+    -o nounset \
+    -o pipefail
+
+# Use user's real home directory for canonical configuration path access
+HOME="$(
+    getent passwd "${USER}" \
+        | cut --delimiter=: --fields=6
+)"
+
+# Use snap's own folder for cache directory
+declare XDG_CACHE_HOME
+mkdir \
+    --parents \
+    "${SNAP_USER_DATA}"/.cache
+XDG_CACHE_HOME="${SNAP_USER_DATA}"/.cache
+export XDG_CACHE_HOME
+
+# Finally run the next part of the command chain
+exec "${@}"

snap/snapcraft.yaml

@@ -12,7 +12,9 @@ confinement: strict
 
 apps:
   glances:
-    command: bin/glances
+    command: >
+      bin/glances-launch
+      $SNAP/bin/glances
     plugs:
       - network
       - system-observe
@@ -27,6 +29,16 @@ apps:
       LANG: C.UTF-8
       LC_ALL: C.UTF-8
 
+plugs:
+  home-glances-config:
+    interface: personal-files
+    read:
+    - $HOME/.config/glances/glances.conf
+  etc-glances-config:
+    interface: system-files
+    read:
+    - /etc/glances/glances.conf
+
 parts:
   glances:
     plugin: python
@@ -50,3 +62,8 @@ parts:
     source: https://github.com/docker/docker-py.git
     source-tag: '3.7.0'
     source-depth: 1
+  launchers:
+    source: snap/local/launchers
+    plugin: dump
+    organize:
+      '*': bin/