# Native Shard protocol

`proto/shard_runtime.proto` is the semantic contract between a Meshnet node and
a Shard worker: Protocol Buffers over gRPC/HTTP2 (ADR-0020). It is the source of
truth. The Python and C++ types are generated from it; neither is the contract.

## What lives here

| Path | Purpose |
|---|---|
| `proto/shard_runtime.proto` | The schema: capability, health, session stream, release, cancel |
| `testdata/*.binpb` | Committed conformance vectors both languages assert against |
| `tests/test_shard_protocol_conformance.cpp` | C++ conformance test |
| `CMakeLists.txt` | C++ generation, build wiring, and `ctest` registration |

The Python stubs are generated into
`packages/node/meshnet_node/native_protocol/generated/` and are committed, so
installing a node needs no protoc. The C++ stubs are generated into the build
tree and are never committed — a C++ consumer already has a toolchain, and a
committed copy could only rot.

## Regenerating

```bash
pip install grpcio-tools==1.82.1          # bundles protoc; no system protoc needed
python scripts/generate_native_protocol.py           # rewrite the Python stubs
python scripts/generate_native_protocol.py --check   # fail if they drifted
python scripts/generate_protocol_goldens.py --check  # fail if the vectors drifted
```

Both `--check` modes run in CI via `tests/test_native_shard_protocol.py`, so a
schema edit that is not accompanied by regenerated output fails the suite rather
than shipping stubs that disagree with the schema they claim to implement.

## Building and running the C++ conformance test

If the machine has no protobuf C++ toolchain:

```bash
scripts/bootstrap_native_toolchain.sh build/native-toolchain
```

Then:

```bash
cmake -S packages/node/native -B build/native \
      -DCMAKE_PREFIX_PATH="$PWD/build/native-toolchain"
cmake --build build/native -j
ctest --test-dir build/native --output-on-failure
```

gRPC C++ is optional: without it, CMake builds the message types only, which is
all the conformance test needs. When gRPC C++ *is* found, the `ShardRuntime`
service stubs are built too and exported as `shard_runtime_grpc` for the worker
(DGR-008) to link.

## How the cross-language check actually proves something

Two codecs that each round-trip their own output prove only that each is
self-consistent. Instead:

1. Python builds the canonical message and commits its bytes to `testdata/`.
2. The C++ test parses *those* bytes, asserts every field, independently
   recomputes the CRC32C from the polynomial, and re-serializes to
   `cpp_roundtrip.binpb` in the build tree.
3. `test_cpp_and_python_agree_byte_for_byte` compares that file to the golden.

Byte equality across the two implementations is the claim; anything less is two
parallel test suites that can drift apart.