# Replay — route C8 outbound encoder bytes through MavlinkTransport seam (closes AZ-401 AC-9)

**Task**: AZ-558_mavlink_transport_routing
**Name**: Retrofit `PymavlinkArdupilotAdapter`, `Msp2InavAdapter`, and the replay FC adapter to write through `MavlinkTransport.write(bytes)` instead of calling pymavlink's `mav.*_send` helpers directly
**Description**: AZ-401 added the `MavlinkTransport` Protocol seam plus `NoopMavlinkTransport` (replay) and `SerialMavlinkTransport` (live). Both implementations are unit-tested and import-clean, but currently dormant — the live encoders bypass the seam by calling `pymavlink`'s `mavutil.mavlink_connection.mav.gps_input_send(...)` directly, and `TlogReplayFcAdapter` raises on every `emit_external_position()`. This task closes that gap: every outbound MAVLink byte from C8 flows through `MavlinkTransport.write()`, and AZ-401 AC-9 (`NoopMavlinkTransport.bytes_written() > 0`) becomes assertable.
**Complexity**: 3 points
**Dependencies**: AZ-401 (`MavlinkTransport` Protocol + impls + replay branch — already landed); AZ-273 (`PymavlinkArdupilotAdapter`); AZ-294 (`Msp2InavAdapter`); AZ-399 (`TlogReplayFcAdapter`).
**Component**: c8_fc_adapter (epic AZ-265 / E-DEMO-REPLAY)
**Tracker**: AZ-558
**Epic**: AZ-265 (E-DEMO-REPLAY)

### Document Dependencies

- `_docs/02_document/contracts/replay/replay_protocol.md` (v2.0.0) — Invariant 5 (encoders produce identical byte streams in both modes; only the transport differs).
- `_docs/02_document/contracts/c8_fc_adapter/fc_adapter_protocol.md` — `MavlinkTransport` Protocol shape.
- `_docs/03_implementation/reviews/batch_61_review.md` — F1 / F2 (the open spec gap this task closes).

## Problem

AZ-401 wired `NoopMavlinkTransport` into `compose_root`'s replay-mode branch as a `mavlink_transport` slot. The slot is constructed; the value is plumbed into the `RuntimeRoot.components` dict. But no encoder consumes it. `PymavlinkArdupilotAdapter.__init__` accepts a pymavlink `mavutil.mavlink_connection` (not a `MavlinkTransport`) and calls `connection.mav.gps_input_send(...)` directly inside `emit_external_position()`. The same shape holds for `Msp2InavAdapter`. The result: AZ-401 AC-9 (`NoopMavlinkTransport.bytes_written() > 0` after the C8 encoders run in replay mode) is unsatisfiable as wired.

## Outcome

- `PymavlinkArdupilotAdapter.__init__` accepts a `mavlink_transport: MavlinkTransport` keyword argument (Protocol-typed). Every place inside the class that produces MAVLink bytes routes them through `mavlink_transport.write(payload)` instead of calling `connection.mav.gps_input_send(...)` (or any other `mav.*_send` helper).
- `Msp2InavAdapter` adopts the same retrofit.
- `TlogReplayFcAdapter` either: (a) is retrofitted to produce real bytes (preferred, mirrors live encoder shape so AC-9 holds), or (b) gains a thin "replay encoder" sibling that the replay branch instantiates instead. Decide during implementation; the AC-9 outcome is the same.
- `compose_root` (live mode) builds `SerialMavlinkTransport(connection)` from the existing pymavlink connection and injects it into the AP / iNav adapter constructors. Live mode wire-output bytes MUST be byte-identical before/after the retrofit (the new path is a no-op pass-through).
- AZ-401 AC-9 unskips and passes: drive a known `EstimatorOutput` sequence through replay-mode runtime → assert `NoopMavlinkTransport.bytes_written() > 0` AND no serial-port descriptor activity.
- INFO log on every `MavlinkTransport.write()` is **NOT** emitted (would flood the FDR at 10 Hz). The `MavlinkTransport.bytes_written()` counter is the diagnostic surface.

## Acceptance Criteria

**AC-1: AP / iNav adapter constructors accept `mavlink_transport`** — `PymavlinkArdupilotAdapter.__init__` and `Msp2InavAdapter.__init__` accept a `mavlink_transport: MavlinkTransport` kwarg. Every outbound `mav.*_send(...)` call in those classes is replaced by `mavlink_transport.write(payload)` where `payload` is the bytes the helper would have sent. (Implementation hint: use `mav.gps_input_encode(...)` to get the message object, then `msg.pack(...)` → bytes, then `mavlink_transport.write(bytes)`. Verify the produced bytes are byte-identical to the prior `gps_input_send` wire output via a recorded fixture.)

**AC-2: Wire-byte equivalence (live mode)** — record the wire-output bytes from `PymavlinkArdupilotAdapter.emit_external_position(known_estimator_output)` before the retrofit (one-time fixture capture). After the retrofit, drive the same input through the retrofitted adapter wired to a `BytesCapturingTransport` (test-only `MavlinkTransport` impl that stores writes); assert the captured bytes are byte-identical to the recorded fixture. Same for `Msp2InavAdapter`.

**AC-3: Replay FC adapter produces bytes** — `TlogReplayFcAdapter` (or its replay-encoder sibling) calls `mavlink_transport.write(payload)` from `emit_external_position()`. The exact byte content is whatever `pymavlink.mavutil.gps_input_encode(...).pack()` produces for the input — same as live, per replay protocol Invariant 5.

**AC-4: AZ-401 AC-9 unskips** — `tests/unit/test_az401_compose_root_replay.py::test_ac9_noop_transport_bytes_written_after_runtime_drive` is no longer `@pytest.mark.skip`. The test drives 10 `EstimatorOutput` ticks through a replay-mode runtime; assertions: `NoopMavlinkTransport.bytes_written() > 0`, no serial descriptor opened, no `mav.gps_input_send` calls (mock-spec assertion).

**AC-5: `mav.*_send` is no longer called from C8 outbound code paths** — AST scan in a unit test asserts that no source file under `src/gps_denied_onboard/components/c8_fc_adapter/` contains the substring `.gps_input_send(` or `.mav.` (the latter scoped to method-call AST nodes, not type annotations). The Protocol seam is the only egress.

**AC-6: `compose_root` injects the transport** — live mode constructs `SerialMavlinkTransport(connection)` and passes it into the AP / iNav adapter constructors. Replay mode reuses the existing `NoopMavlinkTransport` slot. Unit test asserts the constructor kwargs match.

## Non-Functional Requirements

- Live mode wire-output bytes MUST be byte-identical before and after this retrofit (`SerialMavlinkTransport` is a no-op pass-through). AC-2 is the gate.
- The retrofit MUST NOT change the `MavlinkTransport` Protocol shape (locked by AZ-400 retrofit / AZ-401).
- `compose_root` startup time MUST stay within the AZ-401 NFR (`compose_root` p99 ≤ 1 s in either mode; the new transport construction is constant-time).

## Constraints

- `mavlink_transport` is a constructor kwarg, not a setter. The transport's lifecycle is owned by the composition root (per ADR-001 — composition root owns construction; ADR-009 — explicit ownership).
- `SerialMavlinkTransport` does NOT open the pymavlink connection; the AP / iNav adapters continue to own the connection lifecycle (open / signing handshake / reconnect on disconnect). The transport just wraps `connection.write` — it's a thin adapter.
- Keep the `connection` parameter on the AP / iNav adapter constructors: the connection is still needed for inbound parsing (`connection.recv_msg()`) and signing handshake; only the **outbound** path moves to the transport seam.
- `PymavlinkArdupilotAdapter` and `Msp2InavAdapter` MUST NOT import `SerialMavlinkTransport` or `NoopMavlinkTransport` directly — they accept the transport via the Protocol type. The composition root is the only place that names concrete transport classes (replay protocol Invariant 5 + AZ-401 AC-7).

## Risks & Mitigation

- **Risk: live MAVLink wire output drifts** — *Mitigation*: AC-2 (byte-equivalence fixture). Recorded once before the retrofit; checked once after.
- **Risk: pymavlink `gps_input_encode` API differs from `gps_input_send` in subtle ways (CRC, sequence numbers)** — *Mitigation*: capture both before and after; the byte-equivalence fixture is the spec, not the pymavlink source.
- **Risk: signing handshake is performed by `mavutil.mavlink_connection`, not by `connection.write`; bypassing `mav.*_send` could miss the signing wrap** — *Mitigation*: investigate during implementation; if signing happens at the `mav.*_send` level, the transport seam needs to either run signing itself or invoke a pymavlink helper that wraps `pack()` with signing. Scoped here as a known-unknown.

## Runtime Completeness

- **Named capability**: byte-routing seam for outbound MAVLink — every C8 outbound byte goes through `MavlinkTransport.write()`.
- **Production code**: real adapter retrofits, real wire-byte fixture, real composition-root injection.
- **Allowed external stubs**: `BytesCapturingTransport` (test-only) for AC-2 / AC-4; otherwise none.
- **Unacceptable substitutes**: leaving `mav.*_send` in place "for compatibility" — defeats the seam and re-opens AZ-401 AC-9.

## Contract

Implements `_docs/02_document/contracts/replay/replay_protocol.md` (v2.0.0) Invariant 5 and `_docs/02_document/contracts/c8_fc_adapter/fc_adapter_protocol.md` `MavlinkTransport` shape. Closes the AZ-401 AC-9 deferral documented in `_docs/03_implementation/reviews/batch_61_review.md`.
