Can't set partition location, probably due to constraint

[ukoda]

I'm trying to create a backup and restore utility for Raspberry Pi systems. I am using the version of pyparted from pip. When I try restore the partitions to a new disk the partition alignment is not as I specify which appears to be due to the constraint setting and leaves the last partition grossly undersized.

The simplest example of this issue can be seen when trying to create a single partition that would be the boot partition on a RPi system. Using a NVMe drive on a USB adaptor I can do this fine manually with fdisk:

root@san:/backup/san# fdisk -l /dev/sda
Disk /dev/sda: 119.24 GiB, 128035676160 bytes, 250069680 sectors
Disk model: Tech            
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x957e6c15

Device     Boot Start     End Sectors  Size Id Type
/dev/sda1        8192 1056767 1048576  512M  c W95 FAT32 (LBA)

This is the test code I wrote, based on the example code in this repo, to show the issue:

import parted

device = parted.getDevice('/dev/sda')
device.clobber()
disk = parted.freshDisk(device, "msdos")
geometry = parted.Geometry(
    device=device,
    start=8192,
    end=1056767
)
filesystem = parted.FileSystem('fat32', geometry=geometry)
partition = parted.Partition(
    disk=disk,
    type=parted.PARTITION_NORMAL,
    fs=filesystem,
    geometry=geometry
)
disk.addPartition(
    partition=partition, constraint=device.optimalAlignedConstraint
)
disk.commit()

However it results in:

Disk /dev/sda: 119.24 GiB, 128035676160 bytes, 250069680 sectors
Disk model: Tech            
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x5f47ce36

Device     Boot  Start     End Sectors  Size Id Type
/dev/sda1       522240 1044479  522240  255M  b W95 FAT32

I can get closer if I change the constraint to:

    partition=partition, constraint=device.minimalAlignedConstraint

to get:

Device     Boot Start     End Sectors  Size Id Type
/dev/sda1        8160 1056719 1048560  512M  b W95 FAT32

But this still adds up to a problem over 3 partitions.

I have tried several other options but can't get it to honor the actual values I want.

Also of note is the partition Id is being set to 'b', not 'c'. Not sure if this will cause problems with the restored system yet.

[ukoda]

For now I am working around the problem by using sfdisk from a subprocess call. Not ideal but it does give me the partitions I need to move forward with my utility.