Use parted for partitioning (#285)

Previously, fdisk was used by sending commands into its stdin, which is
not very robust (since it heavily relies on the interactive prompts
offered by fdisk as well as the default values it offers, which seem
prone to changing in future version).

It seems likely that in the past, fdisk was easier than parted since it
provides default values that make it easier to create adjacent
partitions, without precalculating all positions in the script. However
now that partitions are manually being aligned, all data must be
calculated anyway.

This commit changes the partition generation to use parted rather than
fdisk. For this, it rewrites various calculations and renames variables
to be easier to read as well. All values are now in number of bytes,
rather than mixing bytes and sectors.

This commit also makes makes sure that the boot partition and root
partition are always adjacent (previously the root partition was aligned
without also rounding the boot partition size, leaving some empty space
in between).

As a side effect of using parted, this also causes the "bootcode" part
of the MBR to be filled with some default x86 bootcode. This is totally
irrelevant for booting the Raspberry Pi, but it does prevent triggering
a bug in parted. When using parted to change the partition table (e.g.
when resizing the root partition on first boot by raspi-config's
init_resize.sh), the disk identifier would be changed due to this bug,
which would change the PARTUUID of all partitions. The init_resize.sh
script would work around this by updating the PARTUUID in e.g. fstab,
but that's fragile at best.  This commit prevents the bug from
triggering and keeps the disk identifier the same.

See https://debbugs.gnu.org/35714 for details about this parted bug.

This commit fixes #284.

export-image/prerun.sh

@@ -10,32 +10,27 @@ rm -rf "${ROOTFS_DIR}"
 mkdir -p "${ROOTFS_DIR}"
 
 BOOT_SIZE="$((256 * 1024 * 1024))"
-TOTAL_SIZE=$(du --apparent-size -s "${EXPORT_ROOTFS_DIR}" --exclude var/cache/apt/archives --block-size=1 | cut -f 1)
+ROOT_SIZE=$(du --apparent-size -s "${EXPORT_ROOTFS_DIR}" --exclude var/cache/apt/archives --exclude boot --block-size=1 | cut -f 1)
 
-ROUND_SIZE="$((4 * 1024 * 1024))"
+# All partition sizes and starts will be aligned to this size
-ROUNDED_ROOT_SECTOR=$(((BOOT_SIZE + ROUND_SIZE) / ROUND_SIZE * ROUND_SIZE / 512 + 8192))
+ALIGN="$((4 * 1024 * 1024))"
-IMG_SIZE=$(((BOOT_SIZE + TOTAL_SIZE + (800 * 1024 * 1024) + ROUND_SIZE - 1) / ROUND_SIZE * ROUND_SIZE))
+# Add this much space to the calculated file size. This allows for
+# some overhead (since actual space usage is usually rounded up to the
+# filesystem block size) and gives some free space on the resulting
+# image.
+ROOT_MARGIN=$((800*1024*1024))
+
+BOOT_PART_START=$((ALIGN))
+BOOT_PART_SIZE=$(((BOOT_SIZE + ALIGN - 1) / ALIGN * ALIGN))
+ROOT_PART_START=$((BOOT_PART_START + BOOT_PART_SIZE))
+ROOT_PART_SIZE=$(((ROOT_SIZE + ROOT_MARGIN + ALIGN  - 1) / ALIGN * ALIGN))
+IMG_SIZE=$((BOOT_PART_START + BOOT_PART_SIZE + ROOT_PART_SIZE))
 
 truncate -s "${IMG_SIZE}" "${IMG_FILE}"
-fdisk -H 255 -S 63 "${IMG_FILE}" <<EOF
-o
-n
 
-
+parted --script "${IMG_FILE}" mklabel msdos
-8192
+parted --script "${IMG_FILE}" unit B mkpart primary fat32 "${BOOT_PART_START}" "$((BOOT_PART_START + BOOT_PART_SIZE - 1))"
-+$((BOOT_SIZE / 512))
+parted --script "${IMG_FILE}" unit B mkpart primary ext4 "${ROOT_PART_START}" "$((ROOT_PART_START + ROOT_PART_SIZE - 1))"
-p
-t
-c
-n
-
-
-${ROUNDED_ROOT_SECTOR}
-
-
-p
-w
-EOF
 
 PARTED_OUT=$(parted -sm "${IMG_FILE}" unit b print)
 BOOT_OFFSET=$(echo "$PARTED_OUT" | grep -e '^1:' | cut -d':' -f 2 | tr -d B)