The FAT engine could create, read, write, and grow files, but never free clusters
or make directories — so overwriting a shorter file left a stale tail (a real bug:
corrupt boot-log re-flushes, and later corrupt compiler cache/.o files). This adds
the mutation half of the engine, with the corruption fix wired all the way through.
Engine (system/services/fat/engine.zig), all host-tested:
- freeChain: return a cluster chain to the pool (bounded against a corrupt cycle) —
the shared primitive under truncate and remove.
- truncate: free the chain and zero the entry's size/first-cluster (O_TRUNC).
- createDirectory (mkdir): allocate + initialise a cluster with "." and ".." and add
the directory entry to the parent.
- removeFile (unlink): free the chain and mark the 8.3 entry plus any preceding
long-name entries deleted, so a reused slot can't inherit an orphaned long name.
Refuses directories.
- createFile now shares a common addEntry helper with createDirectory.
O_TRUNC wired end to end: a truncate open-flag (vfs protocol) that the router already
forwards; runtime.fs.OpenOptions.truncate; and fat's handleOpen calls engine.truncate
on an existing file. The boot-log flush (log-flush + init) now opens with truncate, so
a shorter log on a later boot of the same stick leaves no stale tail — closing the
caveat from the boot-log work.
mkdir/unlink are engine-complete and host-tested but not yet exposed as VFS
operations / runtime.fs methods (they are new path-based ops needing router cases);
rename and the richer stat (mtime/mode, blocked on wall-clock) remain. See
docs/zig-self-hosting.md (Phase 2).
Verified: zig build, zig build test (8 engine host tests, incl. truncate, the
overwrite-no-stale-tail regression, remove, and mkdir), zig build check-fat-image, and
a sequential QEMU sweep — fat-mount, log-flush (DANOS.LOG read back at 11804 bytes,
clean, with the truncate-based final flush), vfs, vfs-client-death, usb-storage,
orderly-shutdown, initial-ramdisk, smoke — all green.
Add a FAT12/16/32 filesystem the VFS mounts at /mnt/usb, reading and writing a
USB stick through the block device. Verified end to end under QEMU: the fat
server mounts the volume, the VFS routes /mnt/usb to it, and a client lists the
root and reads a file (the ELF magic of /mnt/usb/system/kernel).
- engine.zig: the FAT engine over a BlockDevice interface — mount (a bare FAT or,
as QEMU's VVFAT and most real sticks present it, an MBR-partitioned disk), FAT
chain walk (12/16/32), cluster allocation, directory traversal with long-name
read, and file read / write / create. Host-tested against a RAM-backed FAT16
image (create, cluster-spanning write, mid-file overwrite, read-back, list).
- on-disk.zig: the align(1) boot-sector / directory / long-name / FSInfo structs
and the cluster-count FAT-type detection.
- fat.zig: the server — wraps the .block device (a DMA bounce buffer) in a
BlockDevice, mounts the FAT, serves the vfs-protocol as a backend, and mounts
itself into the VFS at /mnt/usb. Spawned by init as a boot service.
- runtime.block: the block-device client (geometry / read / write by physical
address, so whole sectors never cross IPC).
- Raise the kernel service-name registry (maximum_services) 8 -> 16: it is
indexed directly by ServiceId, and fat = 8 was being rejected, so the fat
server exited before registering.
- VFS: an absolute path with no matching mount is now not-found rather than
silently created in the flat ramfs — so /mnt/usb fails cleanly until mounted.
Tests: fat-mount (the full stack: block -> FAT -> VFS mount -> list + file read)
passes; host units cover the engine and on-disk structs; the vfs, shutdown, and
USB regression suite stays green (10/10).
Daniel Samson