subiquity: LazyUnmount=yes /usr/lib/modules mountpoint.

Subiquity images mount /usr/lib/modules from a squashfs, which systemd tries to unmount on shutdown, whilst they are still being in use. This produces an error on shutdown, flooding the screen with messages as systemd switches to a more verbose shutdown. Specify LazyUnmount=yes on that mount, such that unmount call succeeds without flooding the shutdown log.

debian/changelog

@@ -1,3 +1,14 @@
+livecd-rootfs (2.616) UNRELEASED; urgency=medium
+
+  * Subiquity images mount /usr/lib/modules from a squashfs, which systemd
+    tries to unmount on shutdown, whilst they are still being in use. This
+    produces an error on shutdown, flooding the screen with messages as
+    systemd switches to a more verbose shutdown. Specify LazyUnmount=yes
+    on that mount, such that unmount call succeeds without flooding the
+    shutdown log.
+
+ -- Dimitri John Ledkov <xnox@ubuntu.com>  Fri, 04 Oct 2019 19:35:56 +0100
+
 livecd-rootfs (2.615) eoan; urgency=medium
 
   * magic-proxy: dump proxy log to stdout on failure

live-build/ubuntu-server/hooks/033-kernel-bits.binary

@@ -67,8 +67,11 @@ prereqs) exit 0;;
 esac
 
 echo ${kernel_metapkg} > /run/kernel-meta-package
-mkdir -p \$rootmnt/lib/modules
-mount \$rootmnt/cdrom/casper/extras/modules.squashfs-$flavor \$rootmnt/lib/modules
+mkdir -p \$rootmnt/usr/lib/modules
+mount \$rootmnt/cdrom/casper/extras/modules.squashfs-$flavor \$rootmnt/usr/lib/modules
+mkdir -p /run/systemd/system/usr-lib-modules.mount.d
+echo '[Mount]' >> /run/systemd/system/usr-lib-modules.mount.d/lazy.conf
+echo 'LazyUnmount=yes' >> /run/systemd/system/usr-lib-modules.mount.d/lazy.conf
 EOF
 	chmod +x "$KERNEL_BITS_ROOT"/etc/initramfs-tools/scripts/init-bottom/live-server