snap seeding: Defer validation for regular image builds

When getting the list of snaps to include on an image via germinate, we
process the snaps in alphabetical order. Currently we seed several
packages `gnome-foo` and also `gtk-common-themes`, the default provider
of `gtk-3-themes`. Since `gtk-common-themes` is alphabetically after
`gnome-foo`, the `seed.yaml` we generate is invalid when we are part way
through generating.

What we really care about, though, is not that the `seed.yaml` is always
valid at every step, but it is that it is valid at the *end* of seeding.
So for the germinate case, let's defer validation to happen once at the
end. Other callsites of `snap_preseed`, where callers are careful to
seed snaps in the right order, are unaffected by this.

live-build/auto/build

@@ -85,8 +85,9 @@ preinstall_snaps() {
 	snap_prepare chroot
 
 	for snap in "$@"; do
-		snap_preseed chroot "${snap}"
+		SNAP_NO_VALIDATE_SEED=1 snap_preseed chroot "${snap}"
 	done
+	snap_validate_seed chroot
 	lb chroot_resolv remove
 }
 

live-build/functions

@@ -650,13 +650,22 @@ snap_preseed() {
             ;;
     esac
 
-
     # Do basic validation of generated snapd seed.yaml, doing it here
     # means we catch all the places(tm) that snaps are added but the
     # downside is that each time a snap is added the seed must be valid,
-    # i.e. snaps with bases need to add bases first etc.
-    if [ -e chroot/var/lib/snapd/seed/seed.yaml ]; then
-        snap debug validate-seed "$CHROOT_ROOT/var/lib/snapd/seed/seed.yaml"
+    # i.e. snaps with bases need to add bases first etc
+    #
+    # Skip validation by setting SNAP_NO_VALIDATE_SEED=1.
+    if [ -z "${SNAP_NO_VALIDATE_SEED}" ]; then
+        snap_validate_seed "${CHROOT_ROOT}"
+    fi
+}
+
+snap_validate_seed() {
+    local CHROOT_ROOT=$1
+
+    if [ -e "${CHROOT_ROOT}/var/lib/snapd/seed/seed.yaml" ]; then
+        snap debug validate-seed "${CHROOT_ROOT}/var/lib/snapd/seed/seed.yaml"
     fi
 }