feat(rootfs-to-image): support ROOTFS_EXPORT_DIR and configurable ROOTFS_COMPRESSION

Split the rootfs pipeline so it can produce either (or both) of:
  * a tarball — via ROOTFS_COMPRESSION=gzip (default, .tar.gz) / zstd
    (.tar.zst); matches existing behaviour, with zstd now available for
    faster creation and decompression on modern hardware.
  * an exported tree — via ROOTFS_EXPORT_DIR=<abs path>, which rsyncs
    the built rootfs to a host-side directory (e.g. an NFS export)
    without an intermediate archive.

Set ROOTFS_COMPRESSION=none when only the exported tree is wanted;
this skips archive creation entirely. If both ROOTFS_COMPRESSION=none
and -z ROOTFS_EXPORT_DIR are set the run is aborted — otherwise the
rootfs stage would produce nothing.

This is infrastructure; no existing callers need to change. The
netboot extension (added in a following commit) consumes
ROOTFS_EXPORT_DIR to populate the NFS export tree.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: iav

lib/functions/image/rootfs-to-image.sh

@@ -19,7 +19,7 @@ function calculate_image_version() {
 	calculated_image_version="${vendor_version_prelude}${BOARD^}_${RELEASE}_${BRANCH}_${kernel_version_for_image}${DESKTOP_ENVIRONMENT:+_$DESKTOP_ENVIRONMENT}${EXTRA_IMAGE_SUFFIX}"
 	[[ $BUILD_DESKTOP == yes ]] && calculated_image_version=${calculated_image_version}_desktop
 	[[ $BUILD_MINIMAL == yes ]] && calculated_image_version=${calculated_image_version}_minimal
-	[[ $ROOTFS_TYPE == nfs ]] && calculated_image_version=${calculated_image_version}_nfsboot
+	[[ $ROOTFS_TYPE == nfs || $ROOTFS_TYPE == nfs-root ]] && calculated_image_version=${calculated_image_version}_nfsboot
 	display_alert "Calculated image version" "${calculated_image_version}" "debug"
 }
 
@@ -40,7 +40,7 @@ function create_image_from_sdcard_rootfs() {
 	if [[ ${INCLUDE_HOME_DIR:-no} == yes ]]; then exclude_home=""; fi
 	# nilfs2 fs does not have extended attributes support, and have to be ignored on copy
 	if [[ $ROOTFS_TYPE == nilfs2 ]]; then rsync_ea=""; fi
-	if [[ $ROOTFS_TYPE != nfs ]]; then
+	if [[ $ROOTFS_TYPE != nfs && $ROOTFS_TYPE != nfs-root ]]; then
 		display_alert "Copying files via rsync to" "/ (MOUNT root)"
 		run_host_command_logged rsync -aHWh $rsync_ea \
 			--exclude="/boot" \
@@ -104,12 +104,54 @@ function create_image_from_sdcard_rootfs() {
 	PRE_UMOUNT_FINAL_IMAGE
 
 	if [[ $ROOTFS_TYPE == nfs ]]; then
-		display_alert "Creating rootfs archive" "rootfs.tgz" "info"
-		tar cp --xattrs --directory=$SDCARD/ --exclude='./boot/*' --exclude='./dev/*' --exclude='./proc/*' --exclude='./run/*' --exclude='./tmp/*' \
-			--exclude='./sys/*' $exclude_home . |
-			pv -p -b -r -s "$(du -sb "$SDCARD"/ | cut -f1)" \
-				-N "$(logging_echo_prefix_for_pv "create_rootfs_archive") rootfs.tgz" |
-			gzip -c > "$DEST/images/${version}-rootfs.tgz"
+		# ROOTFS_COMPRESSION: zstd (default, .tar.zst) | gzip (.tar.gz) | none (skip archive)
+		declare rootfs_compression="${ROOTFS_COMPRESSION:-zstd}"
+		declare archive_ext="" archive_filter=""
+		case "${rootfs_compression}" in
+			gzip) archive_ext="tar.gz"; archive_filter="gzip -c" ;;
+			zstd | zst) archive_ext="tar.zst"; archive_filter="zstd -T0 -c" ;;
+			none) ;;
+			*) exit_with_error "Unknown ROOTFS_COMPRESSION: '${rootfs_compression}' (expected: gzip|zstd|none)" ;;
+		esac
+		if [[ "${rootfs_compression}" == "none" && -z "${ROOTFS_EXPORT_DIR}" ]]; then
+			exit_with_error "ROOTFS_COMPRESSION=none requires ROOTFS_EXPORT_DIR (otherwise nothing is produced)"
+		fi
+
+		declare -g ROOTFS_ARCHIVE_PATH=""
+		if [[ "${rootfs_compression}" != "none" ]]; then
+			ROOTFS_ARCHIVE_PATH="${FINALDEST}/${version}-rootfs.${archive_ext}"
+			display_alert "Creating rootfs archive" "${version}-rootfs.${archive_ext}" "info"
+			# Subshell with pipefail so failures in tar/pv propagate (otherwise the
+			# exit code of the final compressor stage hides truncation mid-archive).
+			declare -a tar_excludes=(--exclude='./boot/*' --exclude='./dev/*' --exclude='./proc/*' --exclude='./run/*' --exclude='./tmp/*' --exclude='./sys/*')
+			[[ "${INCLUDE_HOME_DIR:-no}" != "yes" ]] && tar_excludes+=(--exclude='./home/*')
+			(
+				set -o pipefail
+				tar cp --xattrs --directory="$SDCARD/" "${tar_excludes[@]}" . |
+					pv -p -b -r -s "$(du -sb "$SDCARD"/ | cut -f1)" \
+						-N "$(logging_echo_prefix_for_pv "create_rootfs_archive") rootfs.${archive_ext}" |
+					${archive_filter} > "${ROOTFS_ARCHIVE_PATH}"
+			)
+		fi
+
+		# ROOTFS_EXPORT_DIR: when set, also rsync rootfs tree into this directory.
+		# Useful when the build host is the NFS server, or has the NFS export mounted,
+		# so netboot deployment is a single build step with no unpack/transport phase.
+		if [[ -n "${ROOTFS_EXPORT_DIR}" ]]; then
+			display_alert "Exporting rootfs tree" "${ROOTFS_EXPORT_DIR}" "info"
+			run_host_command_logged mkdir -pv "${ROOTFS_EXPORT_DIR}"
+			# --delete so files removed from the source rootfs don't survive in a
+			# reused export tree (otherwise the NFS root silently drifts from the image).
+			run_host_command_logged rsync -aHWh --delete $rsync_ea \
+				--exclude="/boot/*" \
+				--exclude="/dev/*" \
+				--exclude="/proc/*" \
+				--exclude="/run/*" \
+				--exclude="/tmp/*" \
+				--exclude="/sys/*" \
+				$exclude_home \
+				--info=progress0,stats1 "$SDCARD/" "${ROOTFS_EXPORT_DIR}/"
+		fi
 	fi
 
 	if [[ "${SHOW_DEBUG}" == "yes" ]]; then