feat(3p/nix/nix-daemon): Implement Worker::IsValidPath handler

Change-Id: I741c2b9b58f234a21850640e2b0c071ff4441234
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1223
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/nix-daemon/nix-daemon-proto.cc

@@ -0,0 +1,29 @@
+#include "libproto/worker.grpc.pb.h"
+#include "libproto/worker.pb.h"
+#include "libstore/store-api.hh"
+
+namespace nix::daemon {
+
+using ::grpc::Status;
+using ::nix::proto::StorePath;
+using ::nix::proto::Worker;
+
+class WorkerServiceImpl final : public Worker::Service {
+ public:
+  WorkerServiceImpl(nix::Store* store) : store_(store) {}
+
+  Status IsValidPath(grpc::ServerContext* context, const StorePath* request,
+                     nix::proto::IsValidPathResponse* response) {
+    const auto& path = request->path();
+    store_->assertStorePath(path);
+    response->set_is_valid(store_->isValidPath(path));
+
+    return Status::OK;
+  }
+
+ private:
+  // TODO(tazjin): Who owns the store?
+  nix::Store* store_;
+};
+
+}  // namespace nix::daemon