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[F52.1] mxaccess-codec: BytesMut output buffer for write encoder

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Adds `write_message::encode_to_bytes_mut` (and the timestamped variant)
returning a freshly-allocated `BytesMut`. Allocation count is identical
to `encode` (2 allocs/op for fixed-width scalars); the benefit is
downstream — consumers can `BytesMut::split_to` / `freeze` and forward
the body bytes to a wire-level sink without an intermediate copy.

The body builders (`encode_boolean` / `encode_fixed` / `encode_variable`
/ `encode_array`) were refactored to fill a pre-sized `&mut [u8]`
rather than each allocating their own `Vec<u8>`. The dispatcher
computes the body size up front via small `*_body_size` helpers and
resizes the destination buffer (Vec or BytesMut) once. This is also
the prerequisite refactor for F52.3.

Bench delta in `design/M6-bench-baseline.md` § F52.1; existing
`encode` row unchanged at 2 allocs/op. All 265 round-trip tests
unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Joseph Doherty
2026-05-06 22:46:02 -04:00
parent c7505f9570
commit 4e76b44391
6 changed files with 385 additions and 95 deletions
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design/followups.md
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@@ -64,7 +64,7 @@ Array tags (`TestIntArray`, `TestBoolArray`, etc.) read live as `type_id=0 lengt
**Source:** `design/M6-bench-baseline.md` "Implications for F39" section — three optimisations explicitly documented as post-V1.
**Scope.** Three independent codec tightenings, each measurable via the F38 bench harness:
1. **`bytes::BytesMut` output buffer** on the encoder side. Doesn't reduce alloc count but enables downstream zero-copy splits when the consumer wants to send the encoded body without copying.
1. **`bytes::BytesMut` output buffer** on the encoder side. Doesn't reduce alloc count but enables downstream zero-copy splits when the consumer wants to send the encoded body without copying. ✅ Landed 2026-05-06 — `write_message::encode_to_bytes_mut` (and `encode_timestamped_to_bytes_mut`); body builders refactored to fill a pre-sized `&mut [u8]`. Bench delta in `design/M6-bench-baseline.md` § F52.1.
2. **Per-handle name-signature cache** in `MxReferenceHandle::from_names`. Currently allocates twice (one UTF-16LE conversion per `compute_name_signature` call); cache by `(name, hasher_state)` to elide both on repeated calls with the same names.
3. **Session-level scratch pool** for the per-write encode buffer. Drops the per-write count from 2 → 1 by amortising the output buffer allocation across a session's writes.