Avoid crash on Cmd-Q exit of the app on macOS

On this code path, AppKit uses exit, rather than letting main run as
normal. We handle most things via C++ destructors, which do run, but
Qt is happier if we clean it up explicitly.

src/GUI/CocoaHelpers.h

@@ -41,4 +41,10 @@ SGPath platformDefaultDataPath();
  */
 void transformToForegroundApp();
 
+/**
+ * AppKit shuts us down via exit(), the code in main to cleanup is not run
+ * in that scenario. Do some cleanup manually to avoid crashes on exit.
+ */
+void cocoaRegisterTerminateHandler();
+
 #endif // of FG_GUI_COCOA_HELPERS_H

src/GUI/CocoaHelpers.mm

@@ -17,6 +17,8 @@
 // Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 //
 
+#include <config.h>
+
 #include "CocoaHelpers.h"
 #include "CocoaHelpers_private.h"
 
@@ -34,11 +36,18 @@
 
 // simgear
 #include <simgear/misc/sg_path.hxx>
+#include <simgear/debug/logstream.hxx>
+#include <simgear/structure/commands.hxx>
 
 // flightgear
 #include <GUI/MessageBox.hxx>
 #include <Main/options.hxx>
 #include <Main/locale.hxx>
+#include <Main/globals.hxx>
+
+#if defined(HAVE_QT)
+#  include <GUI/QtLauncher.hxx>
+#endif
 
 NSString* stdStringToCocoa(const std::string& s)
 {
@@ -165,3 +174,51 @@ void transformToForegroundApp()
     
     [[NSApplication sharedApplication] activateIgnoringOtherApps: YES];
 }
+
+@interface FlightGearNSAppDelegate  : NSObject <NSApplicationDelegate>
+@end
+
+@implementation FlightGearNSAppDelegate
+
+
+// it would be lovely to do the following, but the return code doesn't
+// seem to work as documented - maybe it's ignored due to newer process
+// management
+#if 0
+- (NSApplicationTerminateReply) applicationShouldTerminate:(NSApplication *)sender
+{    
+    SGCommandMgr* mgr = SGCommandMgr::instance();
+    SGPropertyNode_ptr propArgs(new SGPropertyNode);
+    propArgs->setStringValue("dialog-name", "exit");
+    mgr->execute("dialog-show", propArgs, globals->get_props());
+    
+    return NSTerminateCancel;
+}
+#endif
+
+- (void) applicationWillTerminate:(NSNotification *)notification
+{
+    SG_LOG(SG_GENERAL, SG_INFO, "macOS quit occuring");
+#if defined(HAVE_QT)
+    flightgear::shutdownQtApp();
+#endif
+}
+
+@end
+
+void cocoaRegisterTerminateHandler()
+{
+    FlightGearNSAppDelegate* delegate = [[FlightGearNSAppDelegate alloc] init];
+    
+    auto *app = [NSApplication sharedApplication];
+    [[NSNotificationCenter defaultCenter]
+     addObserver:delegate
+     selector:@selector(applicationWillTerminate:)
+     name:NSApplicationWillTerminateNotification object:app];
+#if 9
+    [[NSNotificationCenter defaultCenter]
+     addObserver:delegate
+     selector:@selector(applicationShouldTerminate:)
+     name:NSApplicationWillTerminateNotification object:app];
+#endif
+}

src/Viewer/fg_os_osgviewer.cxx

@@ -61,6 +61,10 @@
 #include <QCoreApplication>
 #endif
 
+#if defined(SG_MAC)
+#  include <GUI/CocoaHelpers.h>
+#endif
+
 // Static linking of OSG needs special macros
 #ifdef OSG_LIBRARY_STATIC
 #include <osgDB/Registry>
@@ -366,6 +370,10 @@ void fgOSInit(int* argc, char** argv)
         SG_LOG(SG_GL, SG_INFO, "Using stock OSG implementation of GraphicsWindow");
     }
 #endif
+#if defined(SG_MAC)
+    cocoaRegisterTerminateHandler();
+#endif
+    
     globals->get_renderer()->init();
     WindowSystemAdapter::setWSA(new WindowSystemAdapter);
 }