Adds the encoding layer that turns the raw WAL (opaque bytes)
into a typed queue of client-mode mutations, ready for Phase 4e
to wire actual RPCs through.
Frame (self-describing, forward-compatible):
version : u8 = 0x01
kind : u8 = one of the Kind discriminants
body : [u8] kind-specific
Body encodings mirror the existing on-wire shapes so a future
replay path can splice a WAL record straight into an RPC payload.
Variants (Kinds 0x01–0x06):
PutRef, PutRefVersioned, PutTag, PutTagVersioned,
DeleteTag, SetTagExpiry
Blob-put mutations are deliberately NOT modeled — blob data is
too large to keep in the WAL. The roaming-client design stages
blobs on local disk and records a reference to them once the
local BlobPutStream completes.
Public helpers:
append_mutation(&mut wal, &m) -> Result<seq>
replay_mutations(&wal, start_seq)
-> Vec<(seq, Result<WalMutation, WalMutationError>)>
Unknown-kind records surface as `Err(UnknownKind(byte))`, not
a panic — forward-compat when a newer writer wrote a record
this reader doesn't understand. Malformed records also surface
as Err so the caller can decide (log-and-skip vs abort replay).
Tests (9, all green): kind-byte stability, roundtrip every
variant, rejects empty/short/bad-version/unknown-kind,
malformed bodies (wrong length, over-declared key_len, trailing
garbage on DeleteTag), non-UTF-8 keys, append+replay through a
real on-disk WAL, and replay-survives-unknown-kind mid-stream.
No new deps — hand-rolled error type in-tree (no thiserror).
515 lines, well under the 1300 ceiling.
Co-Authored-By: Claude Opus 4.7 <[email protected]>
Omar Sobh