fix: a partition that is being created needs a length

archinstall/lib/models/device.py

@@ -930,6 +930,15 @@ def __post_init__(self) -> None:
 		if self.fs_type is None and self.status == ModificationStatus.MODIFY:
 			raise ValueError('FS type must not be empty on modifications with status type modify')
 
+		# A partition that is being created needs a real length. The interactive
+		# prompt has always enforced this, but a configuration file or a script
+		# could put a zero-length partition into the layout, where it survived until
+		# libparted refused it while writing - long after the layout had been
+		# summarised and confirmed. EXIST and MODIFY describe what is already on the
+		# disk, so their length is reported rather than chosen and is left alone.
+		if self.status == ModificationStatus.CREATE and self.length <= Size(0, Unit.B, self.start.sector_size):
+			raise ValueError(f'A created partition needs a length, got {self.length.format_size(Unit.B)}')
+
 	@override
 	def __hash__(self) -> int:
 		return hash(self._obj_id)

tests/test_partition_length.py

@@ -0,0 +1,61 @@
+from pathlib import Path
+
+import pytest
+
+from archinstall.lib.models.device import (
+	FilesystemType,
+	ModificationStatus,
+	PartitionModification,
+	PartitionType,
+	SectorSize,
+	Size,
+	Unit,
+)
+
+
+def _make(
+	sector_size: SectorSize,
+	status: ModificationStatus,
+	length: Size,
+	dev_path: Path | None = None,
+) -> PartitionModification:
+	return PartitionModification(
+		status=status,
+		type=PartitionType.PRIMARY,
+		start=Size(1, Unit.MiB, sector_size),
+		length=length,
+		fs_type=FilesystemType.EXT4,
+		mountpoint=Path('/'),
+		dev_path=dev_path,
+	)
+
+
+@pytest.mark.parametrize(
+	'length',
+	[
+		Size(0, Unit.B, SectorSize.default()),
+		Size(0, Unit.sectors, SectorSize.default()),
+	],
+)
+def test_a_created_partition_needs_a_length(length: Size) -> None:
+	"""A zero-length partition cannot be written to a disk."""
+	with pytest.raises(ValueError, match='needs a length'):
+		_make(SectorSize.default(), ModificationStatus.CREATE, length)
+
+
+def test_a_created_partition_with_a_length_is_accepted() -> None:
+	sector_size = SectorSize.default()
+
+	part = _make(sector_size, ModificationStatus.CREATE, Size(1, Unit.MiB, sector_size))
+
+	assert part.end > part.start
+
+
+def test_existing_partitions_report_their_length() -> None:
+	"""EXIST and MODIFY describe what is on the disk, so their length is not up for
+	validation - a partition found on the disk is accepted as it is."""
+	sector_size = SectorSize.default()
+
+	for status in (ModificationStatus.EXIST, ModificationStatus.MODIFY):
+		part = _make(sector_size, status, Size(0, Unit.B, sector_size), Path('/dev/sda1'))
+		assert part.length == Size(0, Unit.B, sector_size)