[PATCH 1/2] Check if last partition ends at GPT backup header

["T. Schaffner" <tobias.schaffner@siemens.com>]

From: Tobias Schaffner <tobias.schaffner@siemens.com>

The GPT backup header has a fixed size of 33 512 byte blocks. Check if the last
partition ends at the block before the start of the GUID partition table backup
header. If so the partition is fully expanded.

Signed-off-by: Tobias Schaffner <tobias.schaffner@siemens.com>
---
 .../files/expand-last-partition.sh            | 21 +++++++++----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh b/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
index 57055cc..0d662cc 100755
--- a/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
+++ b/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
@@ -22,24 +22,23 @@ if [ "${ROOT_DEV}" = "${BOOT_DEV}" ]; then
 	exit 1
 fi
 
-# this value is in blocks. Normally a block has 512 bytes.
-BUFFER_SIZE=32768
 BOOT_DEV_NAME=${BOOT_DEV##*/}
+LAST_PART_NAME="$(lsblk -l -o NAME "${BOOT_DEV}" | tail -1)"
+LAST_PART="/dev/${LAST_PART_NAME}"
+
 DISK_SIZE="$(cat /sys/class/block/"${BOOT_DEV_NAME}"/size)"
-ALL_PARTS_SIZE=0
-for PARTITION in /sys/class/block/"${BOOT_DEV_NAME}"/"${BOOT_DEV_NAME}"*; do
-	PART_SIZE=$(cat "${PARTITION}"/size)
-	ALL_PARTS_SIZE=$((ALL_PARTS_SIZE + PART_SIZE))
-done
+LAST_PART_SIZE="$(cat /sys/class/block/"${LAST_PART_NAME}"/size)"
+LAST_PART_START="$(cat /sys/class/block/"${LAST_PART_NAME}"/start)"
+
+# The GUID partition table stores its backup in the last 33 blocks of the table.
+# Therefore the last partition ends 33 before the end of the disk if expanded.
+GPT_BACKUP_SIZE=33
 
-MINIMAL_SIZE=$((ALL_PARTS_SIZE + BUFFER_SIZE))
-if [ "$DISK_SIZE" -lt "$MINIMAL_SIZE" ]; then
+if [ $((LAST_PART_START + LAST_PART_SIZE + GPT_BACKUP_SIZE)) -lt "${DISK_SIZE}" ]; then
 	echo "Disk is practically already full, doing nothing." >&2
 	exit 0
 fi
 
-LAST_PART="$(sfdisk -d "${BOOT_DEV}" 2>/dev/null | tail -1 | cut -d ' ' -f 1)"
-
 # Transform the partition table as follows:
 #
 # - Remove any 'last-lba' header so sfdisk uses the entire available space.
-- 
2.34.1