Preserve apt preferences created by any package we install (i.e. ubuntu-advantage-tools) against live-build's attempt to delete them. (LP: #1855354)

debian/changelog

@@ -1,3 +1,11 @@
+livecd-rootfs (2.634) UNRELEASED; urgency=medium
+
+  * Preserve apt preferences created by any package we install (i.e.
+    ubuntu-advantage-tools) against live-build's attempt to delete them.
+    (LP: #1855354)
+
+ -- Michael Hudson-Doyle <michael.hudson@ubuntu.com>  Thu, 12 Dec 2019 12:55:17 +1300
+
 livecd-rootfs (2.633) focal; urgency=medium
 
   * Snapd for uc20 model assertions does not support global channel overrides.

live-build/auto/build

@@ -314,6 +314,12 @@ EOF
 		undivert_grub chroot
 	fi
 
+	if [ -d chroot/etc/apt/preferences.d.save ]; then
+		# https://twitter.com/infinite_scream
+		mv chroot/etc/apt/preferences.d.save/* chroot/etc/apt/preferences.d/
+		rmdir chroot/etc/apt/preferences.d.save
+	fi
+
 	# Let all configuration non multi-layered project here.
 	# If those are moving to a multi-layer layout, this needs to be
 	# done in chroot hooks.

live-build/auto/config

@@ -1096,6 +1096,17 @@ rm -f /etc/fstab
 EOF
 fi
 
+if [ $PROJECT = ubuntu-base ]; then
+	cat > config/hooks/100-preserve-ua-tools-prefs.chroot <<EOF
+#! /bin/sh -ex
+
+# live-build "helpfully" removes /etc/apt/preferences.d/* so we put a
+# copy somewhere it won't touch it.
+
+cp -a /etc/apt/preferences.d /etc/apt/preferences.d.save
+EOF
+fi
+
 if [ $PROJECT = ubuntukylin ]; then
 	cat > config/hooks/100-ubuntukylin.chroot <<EOF
 #! /bin/sh