message: CHECK that tpu_name looks like a TPU name
diff: #include <thread>
  #include "absl/memory/memory.h"
  +#include "absl/strings/match.h"
  #include "absl/strings/numbers.h"
  #include "absl/strings/str_cat.h"
  #include "absl/strings/string_view.h"
  @@ -130,6 +131,9 @@ void TpuDualNet::Worker::Reserve(size_t capacity) {
  TpuDualNet::TpuDualNet(const std::string& tpu_name,
  const std::string& graph_path)
  : graph_path_(graph_path) {
  + // Make sure tpu_name looks like a valid name.
  + MG_CHECK(absl::StartsWith(tpu_name, "grpc://"));
  +
  // If we can't find the specified graph, try adding a .pb extension.
  auto* env = Env::Default();
  if (!env->FileExists(graph_path_).ok()) {
  