makechrootpkg: delete_chroot: Fix the is-btrfs-subvolume check.

First of all, it ran `is_btrfs "$chrootdir"` to decide if it was on
btrfs, but $chrootdir wasn't defined locally; it just happens to work
because $chrootdir was defined in main().  (I noticed this because in
Parabola, it is called differently, so $chrootdir was empty).

So I was tempted to just change it to `is_btrfs "$copydir"`, but if
$copydir is just a regular directory on a btrfs filesystem, then it
It would leave much of $copydir intact.  What we really care about is
if $copydir is a btrfs subvolume; which we can check by combining the
is_btrfs check with inspecting the inum of the directory.

I put this combined check in lib/archroot.sh:is_subvolume.

https://lists.archlinux.org/pipermail/arch-projects/2013-September/003901.html

lib/archroot.sh

@@ -24,6 +24,15 @@ is_btrfs() {
 	[[ -e "$1" && "$(stat -f -c %T "$1")" == btrfs ]]
 }
 
+##
+#  usage : is_subvolume( $path )
+# return : whether $path is a the root of a btrfs subvolume (including
+#          the top-level subvolume).
+##
+is_subvolume() {
+	[[ -e "$1" && "$(stat -f -c %T "$1")" == btrfs && "$(stat -c %i "$1")" == 256 ]]
+}
+
 ##
 #  usage : subvolume_delete_recursive( $path )
 #
@@ -32,7 +41,7 @@ is_btrfs() {
 subvolume_delete_recursive() {
 	local subvol
 
-	is_btrfs "$1" || return 0
+	is_subvolume "$1" || return 0
 
 	while IFS= read -d $'\0' -r subvol; do
 		if ! btrfs subvolume delete "$subvol" &>/dev/null; then

makechrootpkg.in

@@ -144,7 +144,7 @@ delete_chroot() {
 	local copy=${1:-$2}
 
 	stat_busy "Removing chroot copy [%s]" "$copy"
-	if is_btrfs "$chrootdir" && ! mountpoint -q "$copydir"; then
+	if is_subvolume "$copydir" && ! mountpoint -q "$copydir"; then
 		subvolume_delete_recursive "$copydir" ||
 			die "Unable to delete subvolume %s" "$copydir"
 	else