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.
The first step of the Zig self-hosting roadmap (docs/zig-self-hosting.md): give
danos programs a danos-native file API and remove the premature POSIX compatibility
shim. This also resolves the earlier misplacement of a full-write helper into the
compat layer — that behaviour now lives natively in runtime.fs.File.writeAll.
- library/runtime/fs.zig: the danos-native file client over the VFS (open/read/
write/writeAll/seekTo/attributes/close, directory listing, mount). Handles are
*values* — a File/Directory owns its VFS node id and byte offset — so there is no
per-process fd table or descriptor limit, unlike the POSIX fd model the shim
emulated. This is where the operations that later become std.os.danos are staged.
- Retire library/posix/ (unistd, stdio): only five call sites used it, all file
operations, all migrated to runtime.fs — fat (mount), the vfs-test and fat-test
clients, and init/log-flush (the boot-log flush). stdio was already dead.
- build.zig: drop the posix module, its addUserBinary parameter, the per-binary
import, and the ~26 call-site arguments.
- Docs: the VFS protocol's client is now runtime.fs; the docs index and
coding-standards note posix is retired and the foreign-ABI naming exception now
applies to the future std.os.danos seam; the process-lifecycle note points the
future musl layer at that same seam rather than the deleted directory.
Deferred by design (see the roadmap): the C-ABI runtime.os errno seam is built at
fork time (its shape must match std/os/danos.zig); truncate/mkdir/rename are
Phase 2; stdio-byte fds and cwd are later slices.
Verified: zig build, zig build test, zig build check-fat-image, and a sequential
QEMU sweep — vfs, vfs-client-death (the park/hold-handle path), fat-mount, log-flush,
orderly-shutdown, initial-ramdisk (log-flush silent in the bare sweep), smoke, init,
usb-storage, device-manager — all green.
On a headless or real board nothing captures serial, so the boot log — the whole
diagnostic stream — is lost at power-off. This retains it in the kernel and copies
it to the boot USB volume as /mnt/usb/DANOS.LOG, the on-disk equivalent of QEMU's
`-serial file:`. Pull the stick, read DANOS.LOG on another machine.
How it fits together:
- Kernel RAM sink (log.zig): a fixed 256 KiB in-image buffer registered as a log
sink in kmain, right after serial. Because userspace debug_write funnels through
log.write, it captures the entire stream — kernel lines and every service's
output — from the first line. Fills linearly and stops when full (earliest boot
output, the most valuable, is kept); no allocation, so it is panic-safe.
- klog_read syscall (32): copies that buffer out to a user buffer, the mirror of
debug_write — same overflow-safe user-half bounds check, kernel -> user copy,
under the kernel lock so the snapshot can't grow mid-copy. Wrapped by
runtime.system.klogRead.
- log-flush (new one-shot, in the initial-ramdisk): waits for the fat server to
mount /mnt/usb, then copies the whole log to /mnt/usb/DANOS.LOG. init spawns it
once the boot services are up (fire-and-forget; it polls the mount itself). If
no volume is mounted — no stick, or the no-VFS ramdisk sweep — it exits silently.
- init shutdown flush: init repeats the copy inline at the top of shutDown(),
BEFORE it tears down the storage services (the fat server is stopped first), so
a clean poweroff captures the fullest log while /mnt/usb is still writable.
- unistd.writeAll: loops write() past the 224-byte VFS payload cap; both flush
paths use it.
The filename is 8.3 (DANOS.LOG) at the mount root — the FAT short-name rule, and
there is no mkdir on the FAT path yet. Extend-only writes mean the two same-session
flushes never leave stale bytes (the shutdown log is a superset of the boot log);
a shorter log on a later boot of the same stick can leave a stale tail — a noted,
cosmetic limitation, not worth pulling O_TRUNC into the FAT write path for now.
Verified end to end under QEMU: a new `log-flush` case (reusing the orderly-shutdown
build) asserts both markers then S5, and DANOS.LOG is read back out of the image
afterwards (11804 bytes, containing the kernel init line, the FAT mount line, and
the boot flush marker). Regression stays green: zig build, zig build test, zig
build check-fat-image, and a sequential QEMU sweep — smoke, init, initial-ramdisk
(log-flush silent in the bare sweep), orderly-shutdown, fat-mount, usb-storage,
usb-hid, vfs, input, device-manager, process, signals, dma, fault-pf.
Daniel Samson