### Bug Description Following https://github.com/canonical/craft-parts/pull/1109, a `FilesystemMounts` object can be declared to distribute content from the overlay to the partitions. For example with: ``` filesystems: default: - mount: / device: default - mount: /foo device: bar ``` Content from the `/foo` directory of the overlay will be staged to the stage directory of the `bar` partition. After the STAGE step the content will be: ``` parts/ part1/ layer/ baz/ a foo/ qux partitions/ bar/ stage/ qux stage/ baz/ a foo/ ``` However, the current implementation of `_migrate_overlay_files_to_prime`, called by `_run_prime` will determine that the `foo` directory is visible when migrating content for the `bar` partition and will thus fail because this directory was *not* staged in this partition. This is due to the fact that the part layer directory is used as a `lowerdir` and the **partition's stage** directory as a `upperdir` to determine file visible in the layer. ### To Reproduce With `filesystems.yaml`: ``` filesystems: default: - mount: "/" device: "default" - mount: "/boot/efi" device: "(volume/pc/efi)" ``` Run: ``` mkdir /tmp/overlay python3 -mcraft_parts --filesystem-mounts filesystems.yaml --partitions default,volume/pc/efi,volume/pc/rootfs --overlay-base /tmp/overlay --trace -v prime ``` ### part yaml ```shell parts: part1: plugin: nil override-build: | echo "test-part1" > $CRAFT_PART_INSTALL/build-a overlay-script: | mkdir -p $CRAFT_OVERLAY/boot/efi/foo part2: after: [part1] plugin: nil overlay-script: | echo "a2" > $CRAFT_OVERLAY/a ``` ### Relevant log output ```shell [...] Execute: Prime part1 DEBUG:craft_parts.executor.executor:execute action part1:Action(part_name='part1', step=Step.PRIME, action_type=ActionType.RUN, reason=None, project_vars=None, properties=ActionProperties(changed_files=None, changed_dirs=None)) DEBUG:craft_parts.executor.part_handler:priming overlay files DEBUG:craft_parts.executor.part_handler:migrate default partition part 'part2' layer to prime DEBUG:craft_parts.overlays.overlays:check layer visibility in /workdir/parts/part2/layer/boot/efi DEBUG:craft_parts.overlays.overlays:layer visibility files=set(), dirs=set() DEBUG:craft_parts.executor.part_handler:excluding already migrated files: set() DEBUG:craft_parts.executor.part_handler:excluding already migrated dirs: set() DEBUG:craft_parts.overlays.overlays:check layer visibility in /workdir/parts/part2/layer DEBUG:craft_parts.overlays.overlays:check if path is visible: root=/workdir/prime, relpath=a DEBUG:craft_parts.overlays.overlays:layer visibility files=set(), dirs=set() DEBUG:craft_parts.executor.part_handler:excluding already migrated files: set() DEBUG:craft_parts.executor.part_handler:excluding already migrated dirs: set() DEBUG:craft_parts.executor.part_handler:migrate default partition part 'part1' layer to prime DEBUG:craft_parts.overlays.overlays:check layer visibility in /workdir/parts/part1/layer/boot/efi DEBUG:craft_parts.overlays.overlays:check if path is visible: root=/workdir/partitions/volume/pc/efi/prime, relpath=.wh..wh..opq DEBUG:craft_parts.overlays.overlays:check if path is visible: root=/workdir/partitions/volume/pc/efi/prime, relpath=foo DEBUG:craft_parts.overlays.overlays:check if path is visible: root=/workdir/partitions/volume/pc/efi/prime, relpath=foo/.wh..wh..opq DEBUG:craft_parts.overlays.overlays:layer visibility files={'.wh..wh..opq', 'foo/.wh..wh..opq'}, dirs={'foo'} DEBUG:craft_parts.executor.part_handler:excluding already migrated files: set() DEBUG:craft_parts.executor.part_handler:excluding already migrated dirs: set() Error: /workdir/stage/boot/efi/foo: No such file or directory. ```