Add "rollback" command to "upgrade" script

This change builds on the previously added "set-bootfs" command of the
"rootfs-container" script, now providing an easy way to "rollback" after
booting to an upgraded rootfs container. This allows a user to easily
revert an upgrade, if the upgrade is deemed unsuccessful.

For example, one can run the following to perform an upgrade:

    $ sudo /var/dlpx-update/latest/upgrade -v not-in-place
    $ sudo reboot

Now, after the reboot, if it's determined that the upgrade was not
successful, one can run the following to rollback that prior upgrade:

    $ sudo /var/dlpx-update/latest/upgrade rollback
    $ sudo reboot

At this point, the system will boot to the original root filesystem that
was used prior ot the upgrade.

upgrade/upgrade-scripts/upgrade

@@ -17,6 +17,13 @@
 
 . "${BASH_SOURCE%/*}/common.sh"
 
+#
+# Any changes to this value needs to be careful to properly support
+# existing rootfs container datasets which may have this property
+# already set. Thus, changes here may require backwards compatibility.
+#
+ROLLBACK_PROPERTY="com.delphix:rollback-container"
+
 IMAGE_PATH=$(get_image_path)
 [[ -n "$IMAGE_PATH" ]] || die "failed to determine image path"
 
@@ -42,6 +49,7 @@ function usage() {
 
 	echo "$PREFIX_STRING [-v] in-place"
 	echo "$PREFIX_SPACES [-v] not-in-place"
+	echo "$PREFIX_SPACES rollback"
 
 	exit 2
 }
@@ -136,9 +144,23 @@ function cleanup_not_in_place_upgrade() {
 	return "$rc"
 }
 
+function get_mounted_rootfs_container() {
+	basename "$(dirname "$(zfs list -Hpo name /)")"
+}
+
 function upgrade_not_in_place() {
 	trap cleanup_not_in_place_upgrade EXIT
 
+	#
+	# We query the mounted rootfs container name here, so that if we
+	# can't get this information for whatever reason, we can easily
+	# handle the error and abort; rather than having to handle this
+	# error later, when it might require more work to handle it.
+	#
+	MOUNTED_CONTAINER="$(get_mounted_rootfs_container)"
+	[[ -n "$MOUNTED_CONTAINER" ]] ||
+		die "failed to determine mounted rootfs container"
+
 	CONTAINER=$("$IMAGE_PATH/upgrade-container" create not-in-place)
 	[[ -n "$CONTAINER" ]] || die "failed to create upgrade container"
 
@@ -178,10 +200,41 @@ function upgrade_not_in_place() {
 	"$IMAGE_PATH/upgrade-container" convert-to-rootfs "$CONTAINER" ||
 		die "failed to convert-to-rootfs '$CONTAINER'"
 
+	#
+	# In order for the "rollback" back command to work, we need to
+	# know which rootfs container to rollback to. Thus, we embed
+	# this information into the rootfs container using a ZFS
+	# property; rollback will read this information, to determine
+	# which rootfs container to rollback to.
+	#
+	zfs set \
+		"$ROLLBACK_PROPERTY=$MOUNTED_CONTAINER" \
+		"rpool/ROOT/$CONTAINER" ||
+		die "'zfs set com.delphix:rollback-container' failed"
+
 	"$IMAGE_PATH/rootfs-container" set-bootfs "$CONTAINER" ||
 		die "failed to set-bootfs '$CONTAINER'"
 }
 
+function rollback() {
+	MOUNTED_CONTAINER="$(get_mounted_rootfs_container)"
+	[[ -n "$MOUNTED_CONTAINER" ]] ||
+		die "failed to determine mounted rootfs container"
+
+	ROLLBACK_CONTAINER="$(zfs get -Hpo value \
+		"$ROLLBACK_PROPERTY" "rpool/ROOT/$MOUNTED_CONTAINER")"
+	[[ -n "$ROLLBACK_CONTAINER" && "$ROLLBACK_CONTAINER" != "-" ]] ||
+		die "failed to determine rollback rootfs container"
+
+	#
+	# The "rollback" operation is nothing more than "set-bootfs" of
+	# a specific rootfs container. Now that we have the specific
+	# rootfs container that we want to use as the next bootfs.
+	#
+	"$IMAGE_PATH/rootfs-container" set-bootfs "$ROLLBACK_CONTAINER" ||
+		die "failed to set-bootfs '$ROLLBACK_CONTAINER'"
+}
+
 [[ "$EUID" -ne 0 ]] && die "must be run as root"
 
 while getopts ':v' c; do
@@ -208,6 +261,10 @@ not-in-place)
 	verify_upgrade_is_allowed
 	upgrade_not_in_place "$@"
 	;;
+rollback)
+	shift 1
+	rollback "$@"
+	;;
 *)
 	usage "invalid option -- '$1'"
 	;;