multiplayer: fix the black screen - a cab and is_ready waiting on each other

The renderer draws nothing at all while simulation::is_ready is false, and that
flag only goes up once the local player's cab is built.

The previous commit made walking into a cab a crew request, so the cab of a
claimed vehicle is created by Crews::reconcile() rather than by the command
itself. reconcile() runs from the network housekeeping, which was gated on
simulation::is_ready - so a host started with -v had its entervehicle turned
into a claim, and then the cab waited for is_ready while is_ready waited for
the cab. Black screen on both ends, since a client whose server never gets
going has nothing to show either.

- the housekeeping now waits for Global.simulation_loaded, which says the world
  exists, instead of simulation::is_ready, which says the player has a cab
- in a session, a player with no cab counts as ready: they are an observer in
  the lobby, not a game still loading, and the world has to be drawn for them
- entervehicle is only taken as a crew request when the session actually lists
  that vehicle; anything else stays an ordinary command, so a scenario that
  starts its host in something the roster does not cover still works

application/drivermode.cpp

@@ -29,6 +29,7 @@ http://mozilla.org/MPL/2.0/.
 #include "utilities/Timer.h"
 #include "rendering/renderer.h"
 #include "utilities/Logs.h"
+#include "network/entities.h"
 /*
 namespace input {
 
@@ -454,7 +455,11 @@ bool driver_mode::update()
 
 	GfxRenderer->Update(deltarealtime);
 
-	simulation::is_ready = simulation::is_ready || (simulation::Train != nullptr && simulation::Train->is_cab_initialized) || Global.local_start_vehicle == "ghostview";
+	simulation::is_ready = simulation::is_ready || (simulation::Train != nullptr && simulation::Train->is_cab_initialized) || Global.local_start_vehicle == "ghostview"
+	                       // in a session a player with no cab is an observer waiting in the
+	                       // lobby, not a game still loading: the world has to be drawn for
+	                       // them, or they are left staring at a black screen
+	                       || (network::is_multiplayer() && simulation::Train == nullptr);
 
 	return true;
 }

application/scenarioloadermode.cpp

@@ -62,6 +62,10 @@ bool scenarioloader_mode::update() {
 		Application.pop_mode();
 	}
 
+	// the world exists from here on. this is what the network housekeeping waits for;
+	// simulation::is_ready means something else, namely that the player has a cab
+	Global.simulation_loaded = true;
+
 	if( Application.is_server() ) {
 		// the server owns the vehicle numbering for the whole session and publishes it
 		network::Entities.build();

network/manager.cpp

@@ -194,7 +194,10 @@ void network::manager::update()
 	for (auto &backend : backend_list())
 		backend.second->update();
 
-	if (servers && simulation::is_ready) {
+	if (servers && Global.simulation_loaded) {
+		// note: not simulation::is_ready. that one waits for the local player's cab, and
+		// the cab of a claimed vehicle is built by update_crews() - waiting for it here
+		// would leave the two waiting on each other
 		servers->update_crews();
 		servers->publish_vehicle_list();
 		servers->publish_state();

network/session.cpp

@@ -120,10 +120,12 @@ void filter_commands(PeerId Peer, command_queue::commands_map &Commands)
 				continue;
 			}
 
-			if (command->command == user_command::entervehicle)
+			if ((command->command == user_command::entervehicle) && (Entities.id_of(command->payload) != ENTITY_NONE))
 			{
 				// the local player walked into a cab. that is a crew request; the cab
-				// itself is built by the authority once the seat is granted
+				// itself is built by the authority once the seat is granted.
+				// a vehicle the session does not list falls through to the ordinary path,
+				// so a scenario that starts its host in something unusual still works
 				NetworkEntityId entity{ENTITY_NONE};
 				auto const result = claim_by_name(Peer, command->payload, entity);
 				if (result != claim_result::granted && result != claim_result::already_member)

utilities/Globals.h

@@ -372,6 +372,9 @@ struct global_settings {
 	uint64_t network_session_token = 0;
 	// how far the worst vehicle was out of place when the last correction arrived, metres
 	float network_position_error = 0.0f;
+	// the scenario has finished loading and the world can be worked with. this is not the
+	// same as simulation::is_ready, which only goes up once the player has a cab
+	bool simulation_loaded = false;
 
 	std::unordered_map<int, std::string> trainset_overrides;