filesystem: send minimal gaps to create_partition

Before calling create_partition, pre-split gaps to the desired size of
the partition.

subiquity/common/filesystem/boot.py

@@ -209,7 +209,7 @@ def get_add_part_plan(device, *, spec, args):
     create_part_plan = CreatePartPlan(gap=None, spec=spec, args=args)
 
     if gaps.largest_gap_size(device) >= size:
-        create_part_plan.gap = gaps.largest_gap(device)
+        create_part_plan.gap = gaps.largest_gap(device).split(size)[0]
         return create_part_plan
     else:
         new_parts = [p for p in partitions if not p.preserve]

subiquity/common/filesystem/manipulator.py

@@ -220,6 +220,7 @@ class FilesystemManipulator:
                     spec['size'], gap.size)
                 spec['size'] = gap.size
 
+        gap = gap.split(spec['size'])[0]
         self.create_partition(disk, gap, spec)
 
         log.debug("Successfully added partition")

subiquity/server/controllers/filesystem.py

@@ -148,18 +148,13 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
         if DeviceAction.TOGGLE_BOOT in DeviceAction.supported(disk):
             self.add_boot_disk(disk)
         gap = gaps.largest_gap(disk)
-        self.create_partition(
-            device=disk, gap=gap, spec=dict(
-                size=sizes.get_bootfs_size(disk),
-                fstype="ext4",
-                mount='/boot'
-                ))
-        gap = gaps.largest_gap(disk)
-        part = self.create_partition(
-            device=disk, gap=gap, spec=dict(
-                size=gaps.largest_gap_size(disk),
-                fstype=None,
-                ))
+        size = sizes.get_bootfs_size(disk)
+        gap_boot, gap_rest = gap.split(size)
+        spec = dict(size=size, fstype="ext4", mount='/boot')
+        self.create_partition(device=disk, gap=gap_boot, spec=spec)
+
+        spec = dict(size=gap_rest.size, fstype=None)
+        part = self.create_partition(device=disk, gap=gap_rest, spec=spec)
         vg_name = 'ubuntu-vg'
         i = 0
         while self.model._one(type='lvm_volgroup', name=vg_name) is not None:
@@ -376,7 +371,8 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
             'mount': data.partition.mount,
         }
 
-        self.create_partition(disk, data.gap, spec, wipe='superblock')
+        gap = gaps.at_offset(disk, data.gap.offset).split(requested_size)[0]
+        self.create_partition(disk, gap, spec, wipe='superblock')
         return await self.v2_GET()
 
     async def v2_delete_partition_POST(self, data: ModifyPartitionV2) \