A few simple tweaks to reduce size of live servers installer.squashfs

- Do not run apt-get update (which can bring in package lists if we are
    unlikely wrt publisher schedules).
  - Run apt-get clean to clear out downloaded debs of curtin/casper and
    dependencies.
  - Do not install user-setup.
  - Use the core snap from the base filesystem if present.

debian/changelog

@@ -4,6 +4,13 @@ livecd-rootfs (2.551) UNRELEASED; urgency=medium
     installer.
   * Use right lsb_release argument for HWE kernel name (-sc -> bionic, need
     -sr -> 18.04).
+  * A few simple tweaks to reduce size of live servers installer.squashfs:
+    - Do not run apt-get update (which can bring in package lists if we are
+      unlikely wrt publisher schedules).
+    - Run apt-get clean to clear out downloaded debs of curtin/casper and
+      dependencies.
+    - Do not install user-setup.
+    - Use the core snap from the base filesystem if present.
 
  -- Michael Hudson-Doyle <michael.hudson@ubuntu.com>  Mon, 17 Dec 2018 13:40:21 +1300
 

live-build/ubuntu-server/hooks/032-installer-squashfs.binary

@@ -51,8 +51,9 @@ EOF
 
 # Install any requirements for the installer, for things we don't want
 # to see on the installed system
-chroot $INSTALLER_ROOT apt-get update
-chroot $INSTALLER_ROOT apt-get -y install user-setup curtin lupin-casper
+chroot $INSTALLER_ROOT apt-get -y install curtin lupin-casper
+chroot $INSTALLER_ROOT apt-get clean
+
 
 # For bug #1743643 "Install to dirty disk with swap fails" remove the
 # "helpful" casper script that mounts any swap partitions it finds.
@@ -65,15 +66,17 @@ touch $INSTALLER_ROOT/etc/cloud/cloud-init.disabled
 chroot $INSTALLER_ROOT mkdir -p /var/lib/snapd/seed/snaps /var/lib/snapd/seed/assertions
 chroot $INSTALLER_ROOT sh -c '
 set -x;
-cd /var/lib/snapd/seed;
-sudo SNAPPY_STORE_NO_CDN=1 snap download core;
-sudo SNAPPY_STORE_NO_CDN=1 snap download subiquity;
+mkdir -p /var/lib/snapd/seed/snaps/
+cd /var/lib/snapd/seed/snaps/;
+if [ ! -e core_*.snap ]; then
+	SNAPPY_STORE_NO_CDN=1 snap download core
+fi
+SNAPPY_STORE_NO_CDN=1 snap download subiquity;
 
-CORE_SNAP=$(ls -1 core*.snap);
+CORE_SNAP=$(ls -1 core_*.snap);
 SUBIQUITY_SNAP=$(ls -1 subiquity*.snap);
 
 mv *.assert /var/lib/snapd/seed/assertions/;
-mv *.snap /var/lib/snapd/seed/snaps/;
 
 cat <<EOF > /var/lib/snapd/seed/seed.yaml
 snaps: