On Ubuntu 26.04 (Resolute) hosts and GitHub Actions VM runners, `imagecraft pack` fails when formatting partitions: ```text mkfs.vfat: failed whilst writing root directory CalledProcessError: Command '['mkfs.vfat', '-n', 'system-boot', '.../.devices/pc_efi.img']' returned non-zero exit status 1. ``` This causes `tests/integration/test_application.py::test_imagecraft_pack` to fail when run with root privileges. ### Root Cause - Ubuntu 26.04 upgraded `fusefile` from 1.2 (`libfuse2`) to 2.0 (`libfuse3`). - `fusefile 2.0` defines `.write_buf = fusefile_write_buf`, causing `libfuse3` to enable splice reads from `/dev/fuse`. - Writes ≥ 4096 bytes (e.g. `mkfs.vfat` root directory, `mke2fs` journals) arrive with `FUSE_BUF_IS_FD` set. - `fusefile.c:816` explicitly returns `-EIO` when `FUSE_BUF_IS_FD` is present instead of reading from the file descriptor. - Unprivileged LXD containers mask the bug because user namespaces block `splice(2)`, forcing `libfuse3` to fall back to in-memory buffers. ### Reproduction On an Ubuntu 26.04 host/VM: 1. Running the integration test: ```bash sudo env "PATH=$PATH" uv run pytest tests/integration/test_application.py::test_imagecraft_pack -vv -s ``` 2. Minimal `fusefile` CLI reproducer: ```bash truncate -s 100M /tmp/disk.img touch /tmp/part.img fusefile /tmp/part.img /tmp/disk.img/1048576+67108864 mkfs.vfat -n system-boot /tmp/part.img # Fails: mkfs.vfat: failed whilst writing root directory ``` ### Proposed Workarounds / Fixes - Work around the `fusefile 2.0` splice issue in Imagecraft (e.g., wrapper, preload shim, or alternative virtual device approach). - Or run root integration tests in an unprivileged LXD container where FUSE splice is disallowed. --- *Note: This issue was written by a toaster (`gemini-3.8-flash` via `opencode` harness).*