Let skip messages in async, and block on sync mode

CHANGELOG.md

@@ -1,3 +1,7 @@
+## Latest
+
+  * Fixed random dead-lock on synchronous mode at high frame rate
+  
 ## CARLA 0.9.10
 
   * Added retrieval of bounding boxes for all the elements of the level

LibCarla/source/carla/streaming/Server.h

@@ -60,6 +60,10 @@ namespace streaming {
       _pool.AsyncRun(worker_threads);
     }
 
+    void SetSynchronousMode(bool is_synchro) {
+      _server.SetSynchronousMode(is_synchro);
+    }
+
   private:
 
     // The order of these two arguments is very important.

LibCarla/source/carla/streaming/detail/tcp/Server.cpp

@@ -20,7 +20,8 @@ namespace tcp {
   Server::Server(boost::asio::io_context &io_context, endpoint ep)
     : _io_context(io_context),
       _acceptor(_io_context, std::move(ep)),
-      _timeout(time_duration::seconds(10u)) {}
+      _timeout(time_duration::seconds(10u)),
+      _synchronous(false) {}
 
   void Server::OpenSession(
       time_duration timeout,
@@ -28,7 +29,7 @@ namespace tcp {
       ServerSession::callback_function_type on_closed) {
     using boost::system::error_code;
 
-    auto session = std::make_shared<ServerSession>(_io_context, timeout);
+    auto session = std::make_shared<ServerSession>(_io_context, timeout, *this);
 
     auto handle_query = [on_opened, on_closed, session](const error_code &ec) {
       if (!ec) {

LibCarla/source/carla/streaming/detail/tcp/Server.h

@@ -54,6 +54,14 @@ namespace tcp {
       });
     }
 
+    void SetSynchronousMode(bool is_synchro) {
+      _synchronous = is_synchro;
+    }
+
+    bool IsSynchronousMode(void) {
+      return _synchronous;
+    }
+
   private:
 
     void OpenSession(
@@ -66,6 +74,8 @@ namespace tcp {
     boost::asio::ip::tcp::acceptor _acceptor;
 
     std::atomic<time_duration> _timeout;
+
+    bool _synchronous;
   };
 
 } // namespace tcp

LibCarla/source/carla/streaming/detail/tcp/ServerSession.cpp

@@ -5,6 +5,7 @@
 // For a copy, see <https://opensource.org/licenses/MIT>.
 
 #include "carla/streaming/detail/tcp/ServerSession.h"
+#include "carla/streaming/detail/tcp/Server.h"
 
 #include "carla/Debug.h"
 #include "carla/Logging.h"
@@ -15,6 +16,7 @@
 #include <boost/asio/post.hpp>
 
 #include <atomic>
+#include <thread>
 
 namespace carla {
 namespace streaming {
@@ -25,9 +27,11 @@ namespace tcp {
 
   ServerSession::ServerSession(
       boost::asio::io_context &io_context,
-      const time_duration timeout)
+      const time_duration timeout,
+      Server &server)
     : LIBCARLA_INITIALIZE_LIFETIME_PROFILER(
           std::string("tcp server session ") + std::to_string(SESSION_COUNTER)),
+      _server(server),
       _session_id(SESSION_COUNTER++),
       _socket(io_context),
       _timeout(timeout),
@@ -79,8 +83,17 @@ namespace tcp {
       if (!_socket.is_open()) {
         return;
       }
-      while (_is_writing) {
+      if (_is_writing) {
-        std::this_thread::yield();
+        if (_server.IsSynchronousMode()) {
+          // wait until previous message has been sent
+          while (_is_writing) {
+            std::this_thread::yield();
+          }
+        } else {
+          // ignore this message
+          log_debug("session", _session_id, ": connection too slow: message discarded");
+          return;
+        }      
       }
       _is_writing = true;
 

LibCarla/source/carla/streaming/detail/tcp/ServerSession.h

@@ -26,6 +26,8 @@ namespace streaming {
 namespace detail {
 namespace tcp {
 
+  class Server;
+
   /// A TCP server session. When a session opens, it reads from the socket a
   /// stream id object and passes itself to the callback functor. The session
   /// closes itself after @a timeout of inactivity is met.
@@ -40,7 +42,8 @@ namespace tcp {
 
     explicit ServerSession(
         boost::asio::io_context &io_context,
-        time_duration timeout);
+        time_duration timeout,
+        Server &server);
 
     /// Starts the session and calls @a on_opened after successfully reading the
     /// stream id, and @a on_closed once the session is closed.
@@ -82,6 +85,8 @@ namespace tcp {
 
     friend class Server;
 
+    Server &_server;
+
     const size_t _session_id;
 
     stream_id_type _stream_id = 0u;

LibCarla/source/carla/streaming/low_level/Server.h

@@ -64,6 +64,10 @@ namespace low_level {
       return _dispatcher.MakeStream();
     }
 
+    void SetSynchronousMode(bool is_synchro) {
+      _server.SetSynchronousMode(is_synchro);
+    }
+
   private:
 
     void StartServer() {

Unreal/CarlaUE4/Plugins/Carla/Source/Carla/Server/CarlaServer.cpp

@@ -287,6 +287,7 @@ void FCarlaServer::FPimpl::BindActions()
   {
     REQUIRE_CARLA_EPISODE();
     Episode->ApplySettings(settings);
+    StreamingServer.SetSynchronousMode(settings.synchronous_mode);
     return GFrameCounter;
   };