[grpc-io] multiple async clients for route_guide_callback_server example

[Dmitry Gorelov]

Hi All,

please help to modify this peace of server code for bidirectional stream in 
order to make it work correclty with *multiple clients* at one time. 
Currently it crashes with segmentation fault in the proto_utils.h.

class RouteGuideImpl final : public RouteGuide::CallbackService {
 public:
  explicit RouteGuideImpl(const std::string& db) {
    routeguide::ParseDb(db, &feature_list_);
  }  

  grpc::ServerBidiReactor<RouteNote, RouteNote>* RouteChat(
      CallbackServerContext* context) override {
    class Chatter : public grpc::ServerBidiReactor<RouteNote, RouteNote> {
     public:
      Chatter(absl::Mutex* mu, std::vector<RouteNote>* received_notes)
          : mu_(mu), received_notes_(received_notes) {
        StartRead(&note_);
      }

      void OnDone() override { delete this; }
      void OnReadDone(bool ok) override {
        if (ok) {
          // Unlike the other example in this directory that's not using
          // the reactor pattern, we can't grab a local lock to secure the
          // access to the notes vector, because the reactor will most 
likely
          // make us jump threads, so we'll have to use a different locking
          // strategy. We'll grab the lock locally to build a copy of the
          // list of nodes we're going to send, then we'll grab the lock
          // again to append the received note to the existing vector.
          mu_->Lock();
          std::copy_if(received_notes_->begin(), received_notes_->end(),
                       std::back_inserter(to_send_notes_),
                       [this](const RouteNote& note) {
                         return note.location().latitude() ==
                                    note_.location().latitude() &&
                                note.location().longitude() ==
                                    note_.location().longitude();
                       });
          mu_->Unlock();
          notes_iterator_ = to_send_notes_.begin();
          NextWrite();
        } else {
          std::cout << "some client finished" << std::endl;
          Finish(Status::OK);
        }
      }
      void OnWriteDone(bool /*ok*/) override { NextWrite(); }

     private:
      void NextWrite() {
        if (notes_iterator_ != to_send_notes_.end()) {
          StartWrite(&*notes_iterator_);
          notes_iterator_++;
        } else {
          mu_->Lock();
          received_notes_->push_back(note_);
          mu_->Unlock();
          StartRead(&note_);
        }
      }
      RouteNote note_;
      absl::Mutex* mu_;
      std::vector<RouteNote>* received_notes_;
      std::vector<RouteNote> to_send_notes_;
      std::vector<RouteNote>::iterator notes_iterator_;
    };
    return new Chatter(&mu_, &received_notes_);
  }

 private:
  std::vector<Feature> feature_list_;
  absl::Mutex mu_;
  std::vector<RouteNote> received_notes_ ABSL_GUARDED_BY(mu_);
};

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