Treat it as a fatal error if we are asked to install a snap that would pull in the core snap. Ubuntu 20.04 and Ubuntu 22.04 official images should never require snaps that depend on an Ubuntu 16.04 runtime, this indicates a misconfiguration that will bloat the install.

debian/changelog

@@ -2,6 +2,10 @@ livecd-rootfs (2.750) UNRELEASED; urgency=medium
 
   * Do not look for a base snap on snaps of type base, because recursive
     dependencies are not allowed for snaps.  LP: #1957123.
+  * Treat it as a fatal error if we are asked to install a snap that would
+    pull in the core snap.  Ubuntu 20.04 and Ubuntu 22.04 official images
+    should never require snaps that depend on an Ubuntu 16.04 runtime, this
+    indicates a misconfiguration that will bloat the install.
 
  -- Steve Langasek <steve.langasek@ubuntu.com>  Tue, 11 Jan 2022 15:38:05 -0800
 

live-build/functions

@@ -597,8 +597,12 @@ _snap_preseed() {
             if [ "$snap_type" != base ]; then
                 local core_snap=$(echo "$snap_info" | awk '/^base:/ {print $2}')
 
-                # If snap info does not list a base use 'core'
+                # If snap info does not list a base the default is 'core'
-                core_snap=${core_snap:-core}
+                # which is now an error to use.
+                if [ -z "$core_snap" ]; then
+                    echo "Legacy snap with no base declaration found, refusing to install 'core' snap"
+                    exit 1
+                fi
 
                 _snap_preseed $CHROOT_ROOT $core_snap stable
             fi