story: DGR-033 Build a standalone fake C++ gRPC Shard worker

packages/node/native/worker/fake_engine.h

@@ -0,0 +1,164 @@
+// Deterministic, model-free fake ShardEngine for the native worker (DGR-033).
+//
+// This is the C++ analogue of `meshnet_node.fake_shard_engine.FakeShardEngine`
+// (DGR-032): a pure fixture that performs a *bounded real forward* over the
+// bytes it received off the socket and never links, loads, or dispatches to
+// llama.cpp. It exists to prove the standalone worker process, stream,
+// lifecycle, and supervision shape before any real engine is bound (DGR-037).
+//
+// The "forward" is deliberately transport-verifiable rather than semantic: it
+// reassembles a tensor's fragments, checks they tile exactly, and derives a
+// CRC32C over the uncompressed bytes — the same rule the schema's `Checksum`
+// declares and the same bounded forward the DGR-024 Python surface performs.
+// Feeding the same bytes back (echo) lets a client prove the payload truly
+// traversed the wire and returned unmodified; a direct hop and an opaque relay
+// of the identical frames therefore yield byte-identical responses.
+//
+// There is no arbitrary-graph entry point here and no llama.cpp RPC: the engine
+// only knows how to reassemble/checksum a bundle. That is the whole point of a
+// fixture worker (acceptance criterion 4).
+
+#ifndef MESHNET_NATIVE_WORKER_FAKE_ENGINE_H_
+#define MESHNET_NATIVE_WORKER_FAKE_ENGINE_H_
+
+#include <algorithm>
+#include <cstdint>
+#include <optional>
+#include <string>
+#include <vector>
+
+#include "shard_runtime.pb.h"
+
+namespace meshnet::worker {
+
+namespace sp = ::meshnet::shard::v1;
+
+// Standard CRC-32 (ISO-HDLC / zlib polynomial 0xEDB88320, reflected).
+//
+// The schema's `Checksum` field is labelled CRC32C, but the DGR-024 Python
+// runtime surface (`shard_runtime_server.py`) computes it with `zlib.crc32`
+// (standard CRC-32, not the Castagnoli CRC32C). This worker deliberately mirrors
+// that exact computation so its checksum acceptance is byte-for-byte identical
+// to the existing Python gRPC surface and to a relayed frame's expectations.
+inline uint32_t Crc32(const std::string& data, uint32_t seed = 0) {
+  static uint32_t table[256];
+  static bool built = false;
+  if (!built) {
+    for (uint32_t i = 0; i < 256; ++i) {
+      uint32_t c = i;
+      for (int k = 0; k < 8; ++k) {
+        c = (c & 1) ? (c >> 1) ^ 0xEDB88320u : (c >> 1);
+      }
+      table[i] = c;
+    }
+    built = true;
+  }
+  uint32_t crc = seed ^ 0xFFFFFFFFu;
+  for (unsigned char byte : data) {
+    crc = (crc >> 8) ^ table[(crc ^ byte) & 0xFF];
+  }
+  return crc ^ 0xFFFFFFFFu;
+}
+
+// Outcome of validating one bundle before the bounded forward runs.
+struct BundleCheck {
+  // Set when the bundle is malformed/corrupt (maps to PAYLOAD_CORRUPT).
+  std::optional<std::string> corrupt_detail;
+  // Set when the declared payload exceeds the negotiated per-chunk ceiling
+  // (maps to RESOURCE_EXHAUSTED) — the worker refuses unbounded messages.
+  std::optional<std::string> oversize_detail;
+};
+
+// The fake engine's only capability: verify a bundle tiles and checksums, and
+// that it stays within the negotiated byte ceiling. Mirrors `_validate_bundle`
+// in `shard_runtime_server.py` plus the bounded-message rule DGR-033 adds.
+class FakeShardEngine {
+ public:
+  // Marker mirroring `FakeShardEngine.EVIDENCE_CLASS` so a future parity check
+  // (DGR-036) can assert this is a fixture, not a real engine.
+  static constexpr const char* kEvidenceClass = "fixture";
+
+  explicit FakeShardEngine(uint64_t max_chunk_bytes) : max_chunk_bytes_(max_chunk_bytes) {}
+
+  BundleCheck Validate(const sp::TensorBundle& bundle) const {
+    BundleCheck result;
+    for (const auto& tensor : bundle.tensors()) {
+      // Bounded message: a declared payload larger than the ceiling is refused
+      // before any reassembly work is done.
+      if (max_chunk_bytes_ != 0 && tensor.total_bytes() > max_chunk_bytes_) {
+        result.oversize_detail =
+            "tensor '" + tensor.name() + "': declared total_bytes " +
+            std::to_string(tensor.total_bytes()) + " exceeds max_chunk_bytes " +
+            std::to_string(max_chunk_bytes_);
+        return result;
+      }
+
+      // Fragments must tile the wire body exactly: no hole, no overlap.
+      std::vector<const sp::TensorFragment*> ordered;
+      ordered.reserve(tensor.fragments_size());
+      for (const auto& fragment : tensor.fragments()) {
+        ordered.push_back(&fragment);
+      }
+      std::sort(ordered.begin(), ordered.end(),
+                [](const sp::TensorFragment* a, const sp::TensorFragment* b) {
+                  return a->byte_offset() < b->byte_offset();
+                });
+      uint64_t expected_offset = 0;
+      std::string payload;
+      for (const auto* fragment : ordered) {
+        if (fragment->byte_offset() != expected_offset) {
+          result.corrupt_detail =
+              "tensor '" + tensor.name() + "': fragment at offset " +
+              std::to_string(fragment->byte_offset()) +
+              " does not tile the preceding " + std::to_string(expected_offset) +
+              " bytes (gap or overlap)";
+          return result;
+        }
+        payload.append(fragment->payload());
+        expected_offset += fragment->payload().size();
+      }
+      if (tensor.compression() == sp::COMPRESSION_NONE &&
+          expected_offset != tensor.total_bytes()) {
+        result.corrupt_detail =
+            "tensor '" + tensor.name() + "': fragments cover " +
+            std::to_string(expected_offset) + " bytes, declared total_bytes is " +
+            std::to_string(tensor.total_bytes());
+        return result;
+      }
+      if (tensor.compression() == sp::COMPRESSION_NONE &&
+          tensor.checksum().algorithm() == sp::CHECKSUM_ALGORITHM_CRC32C) {
+        const uint32_t actual = Crc32(payload);
+        const std::string& declared = tensor.checksum().value();
+        std::string actual_be(4, '\0');
+        actual_be[0] = static_cast<char>((actual >> 24) & 0xFF);
+        actual_be[1] = static_cast<char>((actual >> 16) & 0xFF);
+        actual_be[2] = static_cast<char>((actual >> 8) & 0xFF);
+        actual_be[3] = static_cast<char>(actual & 0xFF);
+        if (declared != actual_be) {
+          result.corrupt_detail = "tensor '" + tensor.name() + "': checksum mismatch";
+          return result;
+        }
+      }
+    }
+    return result;
+  }
+
+  // Bounded real forward: fold every fragment's payload through CRC32C so the
+  // digest is only reproducible if the payload really traversed the wire.
+  uint32_t BoundedForward(const sp::TensorBundle& bundle) const {
+    uint32_t digest = 0;
+    for (const auto& tensor : bundle.tensors()) {
+      for (const auto& fragment : tensor.fragments()) {
+        digest = Crc32(fragment.payload(), digest);
+      }
+    }
+    return digest;
+  }
+
+ private:
+  uint64_t max_chunk_bytes_;
+};
+
+}  // namespace meshnet::worker
+
+#endif  // MESHNET_NATIVE_WORKER_FAKE_ENGINE_H_