makechrootpkg: sync_chroot: Make more general.

This is inspired by the thought that went in to the delete_chroot
is_subvolume commit.

sync_chroot($chrootdir, $copydir) copies `$chrootdir/root` to `$copydir`.
That seems a little silly; why do we care about "$chrootdir"?  Have it just
be sync_chroot(source, destination) like every other sync/copy command.

Where this becomes tricky is check to decide if we are going to use btrfs
subvolumes or not.  We don't care if "$source/.." is on btrfs; the root
could be a directly-mounted subvolume, but and the destination could be
another subvolume of the same btrfs mounted somewhere else.

The things we do care about are:

 - The source is a btrfs subvolume (so that we can snapshot it)
 - The source is on the same filesystem as the directory that the copy will
   be created in.
 - If the destination exists:
   * that it is not a mountpoint (so that we can delete and recreate it)
   * that it is a btrfs subvolume (so that we can quickly delete it)

On the last point, it isn't necessary for creating the new snapshot, just
for quick deletion.  That can be a separate check, where we use regular
`rm` for deleting the existing copy, but use subvolume snapshots for
creating the new one.

1 files changed, 8 insertions, 0 deletions

diff --git a/lib/archroot.sh b/lib/archroot.sh
index 87c28a2..6b1b52e 100644
--- a/lib/archroot.sh
+++ b/lib/archroot.sh
@@ -34,6 +34,14 @@ is_subvolume() {
 }
 
 ##
+#  usage : is_same_fs( $path_a, $path_b )
+# return : whether $path_a and $path_b are on the same filesystem
+##
+is_same_fs() {
+	[[ "$(stat -c %d "$1")" == "$(stat -c %d "$1")" ]]
+}
+
+##
 #  usage : subvolume_delete_recursive( $path )
 #
 #    Find all btrfs subvolumes under and including $path and delete them.