SPEC-INDEX.md
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Status: Stable, normative for durable local staging state. Local-only;
not exchanged between peers. See SPEC-CONVENTIONS §2.
Scope: the on-disk layout of .mkit/index, used by the staging area.
.mkit/index (the INDEX_FILE constant) is the staging area — a list
of paths that will be included in the next commit, each paired with an
object hash, a status byte, and a stat cache that lets add/status
prove a worktree file unchanged in O(stat) instead of O(content).
It is repo-local; it is never serialized to a transport and never
signed.
Paths, status/mode selections, staged object hashes and deletion tombstones are authoritative user state. They MUST survive supported format upgrades; a working file may already contain different bytes from its staged version. Only the stat cache is disposable. Local-only does not imply reconstructible.
offset size field
0 4 magic "MKIX" = 0x4D 0x4B 0x49 0x58
4 1 version 0x03
5 4 entry_count u32 LE
9 … entries entry_count × entry
… 32 checksum BLAKE3 of all preceding file bytesEach entry:
[u8 status] see §3
[32 bytes object_hash] BLAKE3 of the staged object; [0;32] for status=removed
[u64 LE mtime_ns] stat cache: worktree mtime (ns since Unix epoch,
saturating) observed when object_hash was computed;
0 = no cache (always re-hash)
[u64 LE size] stat cache: file size observed at the same time;
meaningful only when mtime_ns != 0
[u64 LE ino] stat cache: inode number; 0 = don't check.
Catches replace-by-rename swaps.
[u64 LE ctime_ns] stat cache: status-change time (saturating ns);
0 = don't check. ctime cannot be set from
userspace, catching timestamp restoration.
[u16 LE path_len] 0 .. 4096
[path_len bytes path] UTF-8 relative path, forward slashes onlyPath validation rules (enforced by writers; readers reject violations
with IndexError::Corrupt for length bounds and IndexError::InvalidPath
for content):
/.MAX_PATH_LEN)."." or "..".//, leading/trailing /).0x00 (NUL) or \\ (backslash)..mkit or .git, and does not start with .mkit/
or .git/. (Repo metadata cannot be staged.)These rules are looser than SPEC-OBJECTS §4.1: per-segment Windows
reserved-name, trailing dot/space, and case-insensitive .mkit/.git
checks are NOT enforced at the index layer. Staging code that needs to
guarantee a clean commit MUST additionally validate via the
SPEC-OBJECTS §4.1 grammar before writing the tree object — the index
itself is local-only and a more permissive checkpoint.
0x00 removed entry marks a path scheduled for deletion; object_hash MUST be [0;32]
0x01 blob regular file, object_hash is a blob
0x02 tree reserved; not currently emitted (subtree staging is flattened to blobs)
0x03 symlink object_hash is a blob whose data is the link target
0x04 executable regular file with the POSIX executable bit set
(mode 0x04 in SPEC-OBJECTS §4.2)Writers MUST emit 0x04 for staged executables. Readers MUST accept
all five defined codes. Any other value → IndexError::BadStatus(byte).
The stat cache is an optimization, never a source of truth: an
implementation MAY ignore it entirely (treat every entry as
mtime_ns = 0) without changing any observable result, only cost.
A consumer MAY skip re-reading and re-hashing a worktree file and reuse
object_hash only when ALL hold:
mtime_ns != 0 (zero is the no-cache sentinel);mtime_ns;size;ino and the ctime equals ctime_ns;status (a plain blob has the
exec bits clear; an executable has any set; on platforms where the
exec bit is not observable, both classes match). Symlink entries
never stat-match — targets are re-read.Racy-clean (normative): a file modified within the filesystem's
timestamp granularity of when it was hashed can carry the same
mtime+size with different bytes. Readers MUST therefore treat as
uncached any entry whose mtime_ns is not safely older than the
index file's own mtime. The window is judged PER ENTRY: the tight
window (10ms) applies only when both the index file's mtime and the
entry's recorded mtime show sub-second precision — this follows the
common coarse-clock granularity on the platforms mkit targets; an
entry with a whole-second mtime (vfat/SMB mounts, tar/touch-truncated
timestamps) keeps the conservative 1-second window. Racy entries are
re-hashed on use; a later index write (whose file mtime is then newer)
heals the cache. This is git's racy-git rule applied at read time,
which preserves the on-disk cache instead of destroying it.
Producers record all four cache fields from the metadata of the
opened file descriptor used for hashing (not a separate pre-open
stat), closing the window where the path is swapped between stat and
read. Consumers that heal the cache after verifying an entry clean
(for example, status) MUST likewise record the hash-time observation, never
a stat taken after verification — verify-then-stat can pair a fresh
stat with a stale hash.
Commands that rebuild the index from a tree (commit's post-commit sync,
checkout) SHOULD carry the cache over from the outgoing index for
entries whose path, status, and object_hash are unchanged.
Writes use atomic-rename: write to a sibling tempfile (named
.<file>.tmp.<pid>.<seq>), fsync the tempfile, rename(2) into
place, then fsync the parent directory so the rename itself is
durable across power loss. The .mkit/ directory is created if absent.
(The index write is deliberately NOT part of the object store's batched
durability schedule — it is one of the durable pointers that schedule
orders itself against; see SPEC-OBJECTS §10.1.)
Readers MUST enforce:
MAX_INDEX_BYTES) → IndexError::TooLarge.
Bounded reads MUST enforce this even if the file grows during reading.IndexError::Corrupt,
before interpreting entries. The checksum detects corruption, not malicious
replacement by an actor who can rewrite the entire local file.Only version 3 is supported. Readers MUST reject every other version without rewriting the index. This is a deliberate pre-release format break: there is no legacy reader or migration API. An unsupported index MUST NOT be rebuilt from HEAD or working files, because those cannot recover its staged selections.
GC MUST abort before sweeping if any registered worktree index is corrupt or unsupported, since its staged hashes might be the only retention roots.
Additionally, readers MUST reject a count header that cannot possibly
fit in the remaining buffer (each entry is at minimum 67 bytes —
1 status + 32 hash + 32 stat cache + 2 path_len + 0 path). A 9-byte
buffer declaring count = u32::MAX is rejected as IndexError::Corrupt
before the entry-allocation loop runs.
Readers MUST reject any trailing bytes after the declared entry list and its checksum.
Readers MUST also reject duplicate exact paths as IndexError::DuplicatePath;
an index cannot contain two live interpretations for the same repo-relative
path.
The golden vectors under rust/tests/golden/refs-index/ pin the
current format explicitly: index_empty.bin / index_3entries.bin
carry the entries above and must round-trip byte-for-byte
(deserialize → serialize is the identity). Entries within a serialized
index MUST appear in the order they were added to the in-memory
Index — the format does not require path-sorted output, so
byte-identity is a property of a specific fixture's construction, not
a general "any two logically-equal indexes serialize identically"
guarantee. The generator examples/generate_refs_index_goldens.rs
re-emits current filenames byte-identically and records every fixture BLAKE3 in
MANIFEST.txt.
| Invariant | Enforced by |
|---|---|
| A file is parsed only under a known layout | "MKIX" magic → IndexError::BadMagic; explicit supported version matrix → IndexError::UnsupportedVersion (§2, §5) |
| A header cannot force pathological allocation | entry_count checked against the minimum entry size and remaining buffer → IndexError::Corrupt; 64 MiB cap → IndexError::TooLarge (§5) |
| The file encodes exactly its declared entries | trailing bytes rejected (§5) |
| Each path has exactly one live interpretation | duplicate exact paths → IndexError::DuplicatePath (§5) |
| No staged path escapes the worktree or names repo metadata | path grammar: non-empty, no leading /, no ./../empty segments, no NUL or backslash, not .mkit/.git → IndexError::InvalidPath / Corrupt (§2) |
| Every entry has a defined kind, and removals carry no object | status whitelist → IndexError::BadStatus (§3); removed entries MUST carry [0;32], enforced at read time → IndexError::RemovedHasHash (§2, §3) |
| The stat cache never changes an observable result | cache is an optimization only — all five match conditions must hold, and racy entries (not safely older than the index file's mtime) are re-hashed (§4) |
| A cache hit reflects the bytes that were actually hashed | cache fields recorded from the opened descriptor used for hashing, never a stat taken after verification (§4) |
| A reader never sees a torn index | tempfile + fsync + rename + parent-dir fsync (§5); only an absent file reads as empty; present empty/corrupt state fails closed (§5) |
The index is local-only, never transported or signed. Its staged selections are durable user state and GC retention roots, even though the stat cache is reconstructible.