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Dir : ~/snap/core22/current/usr/lib/core/

Edit File: handle-writable-paths

#!/bin/sh
#
# Extracted code from:
# https://github.com/snapcore/core-build/blob/master/initramfs/scripts/ubuntu-core-rootfs
# to keep in the core22+ snap in sync with the writable-path file.
#
# Having it here means we don't need to rebuld all kernels if this file
# changes.

set -e

panic()
{
	echo "PANIC: $*"
	exit 1
}

# XXX: we really want the "synced" feature to go away 
sync_dirs()
{
	base="$1"
	source="$2"
	target="$3"

OLD_PWD="$PWD"
	cd "$base"

for file in "$source"/*
	do
		# Skip empty directories
		[ ! -e "$base/$file" -a ! -L "$base/$file" ] && continue

# If the target already exists as a file or link, there's nothing we can do
		[ -e "$target/$file" -o -L "$target/$file" ] && [ ! -d "$target/$file" ] && continue

# If the target doesn't exist, just copy it over
		if [ ! -e "$target/$file" -a ! -L "$target/$file" ]; then
			cp -Ra "$base/$file" "$target/$file"
			continue
		fi

# That leaves us with directories and a recursive call
		[ -d "$file" ] && sync_dirs "$base" "$file" "$target"
	done

cd "$OLD_PWD"
}

# Process the list of bind-mounts (but don't mount them - systemd will handle that)
# File format is documented in writable-paths(5).
handle_writable_paths()
{
	writable_paths="$1"
	fstab="$2"

[ -n "$writable_paths" ] || panic "need writeable paths"
	[ -e "$writable_paths" ] || panic "writeable paths does not exist"
	[ -n "$fstab" ] || panic "need fstab"

cat "$writable_paths" | while read line; do
		# tokenise
		set -- $line

# skip invalid/commented entries
		([ -z "$1" ] || \
		 [ -z "$2" ] || \
		 [ -z "$3" ] || \
		 [ -z "$4" ] || \
		 [ -z "$5" ]) && continue

# ignore anything that isn't an absolute path (including comments)
		case "$1" in
			/*) ;;
			*) continue ;;
		esac

# skip invalid mount points
		dstpath="${rootmnt}$1"
		[ ! -e "$dstpath" ] && continue

if [ "$3" = "temporary" ]; then
			# Temporary entries are simple, just mount a tmpfs
			echo "tmpfs $1 tmpfs $5 0 0" >> "$fstab"
		elif [ "$3" = "persistent" ] || \
		     [ "$3" = "synced" ]; then
			# Figure out the source path
			if [ "$2" = "auto" ]; then
				srcpath="${rootmnt}/writable/system-data${1}"
				path="/writable/system-data${1}"
			else
				srcpath="${rootmnt}/writable/$2"
				path="/writable/$2"
			fi

if [ ! -e "$srcpath" ]; then
				# Process new persistent or synced paths
				dstown=$(stat -c "%u:%g" "$dstpath")
				dstmode=$(stat -c "%a" "$dstpath")
				mkdir -p ${srcpath%/*}
				if [ ! -d "$dstpath" ]; then
					# Deal with redirected files
					if [ "$4" = "transition" ]; then
						cp -a "$dstpath" "$srcpath"
					else
						touch "$srcpath"
						chown "$dstown" "$srcpath"
						chmod "$dstmode" "$srcpath"
					fi
				else
					# Deal with redirected directories
					if [ "$4" = "transition" ] || [ "$3" = "synced" ]; then
						cp -aR "$dstpath" "$srcpath"
					else
						mkdir "$srcpath"
						chown "$dstown" "$srcpath"
						chmod "$dstmode" "$srcpath"
					fi
				fi
			elif [ "$3" = "synced" ]; then
				# Process existing synced paths
				sync_dirs "$dstpath" . "$srcpath"
			fi

# mount all /etc dirs right now, not later when fstab is
                        # processed, as it will cause races.
                        case $1 in
                            /etc*)
                                [ -e "${rootmnt}/writable/system-data/$1" ] || mkdir -p "${rootmnt}/writable/system-data/$1"
                                mount -o bind "${rootmnt}/writable/system-data/$1" "${rootmnt}/$1"
                                ;;
                            *)
                                # Write the fstab entry
                                if [ "$5" = "none" ]; then
                                        echo "$path $1 none bind 0 0" >> "$fstab"
                                else
                                        echo "$path $1 none bind,$5 0 0" >> "$fstab"
                                fi
                                ;;
                        esac
		else
			continue
		fi
	done

# now apply the defaults on top
	handle_writable_defaults
}

# handle_writable_default will apply system defaults *once* after
# the handle_writable_path got populated
handle_writable_defaults()
{
        srcpath="${rootmnt}/writable/system-data/_writable_defaults"
        if [ ! -d "$srcpath" ] || [ -e "${srcpath}/.done" ]; then
                return
        fi

dstpath="${rootmnt}/writable/system-data"
        for fileordir in "$srcpath"/* ; do
            # skip empty $srcpath/
            [ ! -e "$fileordir" ] && [ ! -L "$fileordir" ] &&  continue

# copy
            cp -a "$fileordir" "$dstpath"
        done
        touch "$srcpath/.done"
}

main()
{
	writable_paths="${1:-${rootmnt}/etc/system-image/writable-paths}"
	fstab="${2:-${rootmnt}/etc/fstab}"

# Add writable overlays
	if [ -e "$writable_paths" ]; then
		touch "${rootmnt}/run/image.fstab"
		mount -o bind "${rootmnt}/run/image.fstab" "$fstab" || panic "Cannot bind mount fstab"
		echo "# Auto-generated by $0" >> "$fstab"
		echo "# DO NOT EDIT THIS FILE BY HAND - YOUR CHANGES WILL BE OVERWRITTEN" >> "$fstab"
		echo "# (See writable-paths(5) for details)" >> "$fstab"
		echo "/dev/root / rootfs defaults,ro 0 0" >> "$fstab"
		handle_writable_paths "$writable_paths" "$fstab"
		echo "/run/mnt/ubuntu-seed /var/lib/snapd/seed none bind,ro 0 0" >> "$fstab"
	fi

# IMPORTANT: ensure we synced everything back to disk
	sync
}

# for unit test
if [ "$TEST_IMPORT" = "1" ]; then
    return
fi

rootmnt="$1"
if [ -z "$rootmnt" ]; then
	echo "need rootmnt as the first argument"
	exit 1
fi
# "$2" is the (optional) writable-paths file
# "$3" is the (optional) /etc/fstab file
main "$2" "$3"