Fix Objective C exception handling instability

native/macosx/run.m

@@ -3,19 +3,30 @@
 #include "../factor.h"
 #import "Foundation/NSException.h"
 
+/* This code is convoluted because Cocoa places restrictions on longjmp and
+exception handling. In particular, a longjmp can never cross an NS_DURING,
+NS_HANDLER or NS_ENDHANDLER. */
 void platform_run()
 {
+	CELL error = F;
+
 	for(;;)
 	{
+NS_DURING
 		SETJMP(stack_chain->toplevel);
 		handle_error();
-NS_DURING
+
+		if(error != F)
+		{
+			CELL e = error;
+			error = F;
+			general_error(ERROR_OBJECTIVE_C,error,true);
+		}
+
 		run();
 		NS_VOIDRETURN;
 NS_HANDLER
-        	general_error(ERROR_OBJECTIVE_C,
-			tag_object(make_alien(F,(CELL)localException)),
-			true);
+		error = tag_object(make_alien(F,(CELL)localException));
 NS_ENDHANDLER
 	}
 }