ATC: fix crashes on shutdown

src/ATC/GroundController.cxx

@@ -80,6 +80,7 @@ FGGroundController::FGGroundController() :
 
 FGGroundController::~FGGroundController()
 {
+    _isDestroying = true;
 }
 
 void FGGroundController::init(FGAirportDynamics* aDynamics)
@@ -153,6 +154,9 @@ Search for and erase an aircraft with a certain id from the activeTraffic vector
 */
 void FGGroundController::signOff(int id)
 {
+    if (_isDestroying)
+        return;
+
     // Search the activeTraffic vector to find a traffic vector with our id
     TrafficVectorIterator i = searchActiveTraffic(id);
     

src/ATC/trafficcontrol.cxx

@@ -916,10 +916,7 @@ FGTowerController::FGTowerController(FGAirportDynamics *par) :
 
 FGTowerController::~FGTowerController()
 {
-    // to avoid the exception described in:
-    // https://sourceforge.net/p/flightgear/codetickets/1864/
-    // we want to ensure AI aircraft signing-off is a no-op now
-
+    _isDestroying = true;
     clearTrafficControllers(activeTraffic);
 }
 
@@ -1057,6 +1054,10 @@ void FGTowerController::updateAircraftInformation(int id, double lat, double lon
 
 void FGTowerController::signOff(int id)
 {
+    // ensure we don't modify activeTraffic during destruction
+    if (_isDestroying)
+        return;
+
     // Search activeTraffic for a record matching our id
     TrafficVectorIterator i = searchActiveTraffic(activeTraffic, id);
     if (i == activeTraffic.end() || (activeTraffic.empty())) {
@@ -1147,6 +1148,7 @@ FGStartupController::FGStartupController(FGAirportDynamics *par):
 
 FGStartupController::~FGStartupController()
 {
+    _isDestroying = true;
     clearTrafficControllers(activeTraffic);
 }
 
@@ -1219,6 +1221,10 @@ FGATCInstruction FGStartupController::getInstruction(int id)
 
 void FGStartupController::signOff(int id)
 {
+    // ensure we don't modify activeTraffic during destruction
+    if (_isDestroying)
+        return;
+
     // Search activeTraffic for a record matching our id
     TrafficVectorIterator i = searchActiveTraffic(activeTraffic, id);
     
@@ -1574,6 +1580,7 @@ FGApproachController::FGApproachController(FGAirportDynamics *par):
 
 FGApproachController::~FGApproachController()
 {
+    _isDestroying = true;
     clearTrafficControllers(activeTraffic);
 }
 
@@ -1653,6 +1660,10 @@ void FGApproachController::updateAircraftInformation(int id, double lat, double
 /* Search for and erase traffic record with a specific id */
 void FGApproachController::signOff(int id)
 {
+    // ensure we don't modify activeTraffic during destruction
+    if (_isDestroying)
+        return;
+
     // Search activeTraffic for a record matching our id
     TrafficVectorIterator i = searchActiveTraffic(activeTraffic, id);
     

src/ATC/trafficcontrol.hxx

@@ -448,6 +448,10 @@ public:
     virtual void update(double) = 0;
 
 
+protected:
+    // guard variable to avoid modifying state during destruction
+    bool _isDestroying = false;
+
 private:
 
     AtcMsgDir lastTransmissionDirection;