message: tfdbg core: fix data race
diff: @@ -927,8 +927,11 @@ DebugGrpcIO::GetEnabledDebugOpStates() {
  // static
  DebugGrpcIO::DebugNodeName2State* DebugGrpcIO::GetEnabledDebugOpStatesAtUrl(
  const string& grpc_debug_url) {
  + static mutex* debug_ops_state_mu = new mutex();
  std::unordered_map<string, DebugNodeName2State>* states =
  GetEnabledDebugOpStates();
  +
  + mutex_lock l(*debug_ops_state_mu);
  if (states->find(grpc_debug_url) == states->end()) {
  DebugNodeName2State url_enabled_debug_op_states;
  (*states)[grpc_debug_url] = url_enabled_debug_op_states;
  