lb_chroot_layered: use rsync to make more minimal overlay layers. (LP: #2028213)

debian/changelog

@@ -5,6 +5,10 @@ livecd-rootfs (23.10.8) UNRELEASED; urgency=medium
     for the StarFive VisionFive 2 board. As it is larger than U-Boot we need
     to increase the size of the loader 2 partition to accommodate it.
 
+  [ Michael Hudson-Doyle ]
+  * lb_chroot_layered: use rsync to make more minimal overlay layers.
+    (LP: #2028213)
+
  -- Michael Hudson-Doyle <michael.hudson@ubuntu.com>  Wed, 02 Aug 2023 13:12:09 +1200
 
 livecd-rootfs (23.10.7) mantic; urgency=medium

live-build/lb_chroot_layered

@@ -107,57 +107,6 @@ lb_chroot_includes () {
 	cd "${OLDPWD}"
 }
 
-reduce_pass_size () {
-	# Remove duplicated files between parent and current pass
-	# Note the empty directories created in a child pass are not removed
-	local pass=$1
-	local parent="$(get_parent_pass $pass)"
-
-	$(is_root_layer $pass) && return
-
-	pass_dir="overlay.${pass}"
-	parent_pass_dir="overlay.${parent}"
-
-	local list_pass=$(mktemp)
-	local list_parent=$(mktemp)
-
-	(cd $pass_dir && find . ! -type d  -printf "%h/%f|%s|%y|%U|%G|%m\n"|sort > $list_pass)
-	(cd $parent_pass_dir && find . ! -type d  -printf "%h/%f|%s|%y|%U|%G|%m\n"|sort > $list_parent)
-
-	# Only iterate on common files with same type, owner, permission and size
-	comm -12 $list_pass $list_parent|cut -d'|' -f1|while read f; do
-		# If file contents are different, keep it
-		if ! diff --brief --no-dereference "$pass_dir/$f" "$parent_pass_dir/$f" >/dev/null; then
-			continue
-		fi
-		p="$(dirname "$f")"
-		while [ "$p" != . ]; do
-			# As explained in the overlayfs documentation
-			# https://www.kernel.org/doc/html/latest/filesystems/overlayfs.html#whiteouts-and-opaque-directories
-			# an xattr of trusted.overlay.opaque indicates an 'opaque' directory
-			# that was deleted from the overlay. Removing files from within the
-			# directory, even if identical with one in the lower layer, will result
-			# in it going missing from the combined filesystem.
-			#
-			# It would be possible to be more clever, e.g. if the two directories
-			# are still similar, we could the delete the attribute and convert any
-			# still-needed deletions to whiteouts but it doesn't seem worth it in
-			# the cases seen so far.
-			if [ -n "$(getfattr -n trusted.overlay.opaque -h --only-values -- "$pass_dir/$p" 2>/dev/null)" ]; then
-				Echo_message "reduce_pass_size: '%s' would be removed from overlay but for trusted.overlay.opaque on '%s'" "$f" "$p"
-				continue 2
-			fi
-			p="$(dirname "$p")"
-		done
-		# Files are strictly identical between the 2 passes (only mod or access times differs). No need for unused delta.
-		Echo_message "reduce_pass_size: '%s' is strictly identical between %s and %s. Removing." "$f" "$pass" "$parent"
-		rm "$pass_dir/$f"
-	done
-
-	rm $list_pass
-	rm $list_parent
-}
-
 create_chroot_pass () {
 	local pass=$1
 	shift 1 # restore ${*}
@@ -182,8 +131,8 @@ create_chroot_pass () {
 	mkdir -p "$overlay_dir/"
 	lowerdirs=$(get_lowerdirs_for_pass $pass)
 	if [ -n "$lowerdirs" ]; then
-		mkdir -p chroot/
+		mkdir -p chroot "${overlay_dir}-initial"
-		mount_overlay "$lowerdirs" "$overlay_dir" chroot/
+		mount_overlay "$lowerdirs" "${overlay_dir}-initial" chroot/
 	else
 		ln -s "$overlay_dir/" chroot
 	fi
@@ -319,13 +268,22 @@ create_chroot_pass () {
 	lb chroot_devpts remove ${*}
 
 	if [ -n "$lowerdirs" ]; then
-		umount chroot
+		# Although the current chroot was created as an overlay over
-		rmdir chroot
+		# the previous layer, many operations can result in redundant
+		# files in the upperdir. Rather than trying to minimize the
+		# overlay by hand, we rsync the chroot into a fresh overlay,
+		# rely on rsyncs ability to avoid redundant file operations,
+		# and take _that_ overlay's upperdir as the content of the
+		# layer.
+		mkdir chroot-2
+		mount_overlay "$lowerdirs" "$overlay_dir" chroot-2/
+		rsync -aXHAS --del chroot/ chroot-2/
+		umount chroot chroot-2
+		rmdir chroot chroot-2
+		rm -rf ${overlay_dir}-initial
 	else
 		rm chroot
 	fi
-
-	reduce_pass_size $pass
 }
 
 if [ ! -d chroot.bootstrap/ ]; then