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.
Turn the flat-ramfs VFS into a router: a mount table maps an absolute path prefix
(e.g. /mnt/usb) to a backend server's endpoint, and open/read/write/status/
readdir/close on a path under a mount are forwarded to that backend, which speaks
the same vfs-protocol. This is what a FAT filesystem mounts into.
- protocol: append readdir / mount / unmount operations, a NodeKind enum (the FSH
file types) that now fills FileStatus.kind, a DirectoryEntry record, and a
directory open flag. Appended values keep existing clients and tests unchanged.
- vfs.zig: a mount table, longest-prefix routing, forwarding of every op on a
backend handle, mount/unmount handlers (the backend arrives as the call's
capability), and release-on-death that also closes the backend's handles.
- path.zig: pure, host-tested mount-prefix matching that never captures a
non-boundary like /mnt/usbextra.
- unistd: mount(), opendir / readdir / closedir clients.
Bare names still resolve in the flat ramfs — the backward-compat contract; the
vfs and vfs-client-death tests pass unchanged. End-to-end mount+read is exercised
by the FAT server (M6). Host units cover path matching and protocol sizes.
Follow-up to the monorepo re-org. Suite 35/35 plus host tests green.
POSIX compatibility is now its own library, library/posix/ (unistd, stdio),
layered strictly over the runtime — it calls the runtime's IPC/heap, never
system calls directly. The runtime is now POSIX-free (the danos-native
application ABI). The VFS wire protocol is danos-native throughout
(Stat -> FileStatus, .stat -> .status, O_CREAT -> create); the POSIX layer
maps the POSIX spellings at the boundary. The coding standard's ABI-name
exception is scoped to one place: a file is allowed POSIX spellings only if it
lives under library/posix/ — everywhere else, danos naming with no exception.
Naming fixes, all mechanical:
- initrd -> initial-ramdisk: the source file, the module, the tool
(make-initial-ramdisk.py), the artifact (initial-ramdisk.img, including the
bootloader's load path), and the identifiers.
- system/kernel/device-service.zig -> devices-broker.zig: it is ring-0 kernel
code (the trusted device table + claim capability), not a ring-3 service. The
future user-space device *manager* (policy) will live in system/services/.
- Dropped the daemon `d` suffix: hpetd -> hpet, busd -> bus. A driver lives in
system/drivers/, so the folder already says what it is; encoding the role in
the name too is redundant. The coding standard drops that exception.
- system/devices/aml/interp.zig -> interpreter.zig (the type was already
Interpreter).
The source layout now mirrors the runtime filesystem hierarchy
(docs/danos-file-system-hierarchy-FSH.md): what lives under system/ in the
source is what a running danos represents under /system. Each service and
driver is a sub-project directory that is its own Zig module — cross-project
references go by module name, never by a path into another project's files.
Moves (all git mv, history preserved):
- src/ -> system/ (danos internals; the self-representation)
root.zig -> danos.zig (the kernel<->user contract module)
kernel/arch/ -> kernel/architecture/ (arch -> architecture)
device/ -> devices/ (what /system/devices reflects)
boot/ -> /boot (the loaders, top level)
- sbin/ -> split by role:
init, vfs -> system/services/<name>/<name>.zig
hpetd, busd -> system/drivers/<name>/<name>.zig
vfs-test -> system/services/vfs/vfs-test.zig (inside the vfs project)
- lib/ -> library/runtime/ (room for other libraries beside runtime)
The VFS wire protocol becomes its own module, system/services/vfs/protocol.zig
("vfs-protocol"): the vfs sub-project exposes its interface, and the runtime's
file layer imports it by name. First instance of the "protocol module" pattern
(docs/driver-model.md); usb/block will expose theirs the same way.
Also: fix a naming-standard violation in the protocol — Op -> Operation (and
req -> request, _pad -> _padding). Docs updated: /system/services added to the
FHS doc, a repository-layout section added to the docs index, and stale source
paths swept across comments and docs.
Runtime boot paths are unchanged (the bootloader still loads /sbin/init);
aligning the runtime filesystem to the FHS is a separate follow-up. Suite 35/35
plus host tests green.
Daniel Samson