Add thread safety to at_exit

src/crystal/at_exit_handlers.cr

@@ -1,21 +1,24 @@
 # :nodoc:
 module Crystal::AtExitHandlers
-  private class_getter(handlers) { [] of Int32, ::Exception? -> }
+  @@mutex = ::Thread::Mutex.new
 
   def self.add(handler)
-    handlers << handler
+    @@mutex.synchronize do
+      handlers = @@handlers ||= [] of Int32, ::Exception? ->
+      handlers << handler
+    end
   end
 
   def self.run(status, exception = nil)
-    if handlers = @@handlers
-      # Run the registered handlers in reverse order
-      while handler = handlers.pop?
-        begin
-          handler.call status, exception
-        rescue handler_ex
-          Crystal::System.print_error "Error running at_exit handler: %s\n", handler_ex.message || ""
-          status = 1 if status.zero?
-        end
+    return status unless @@handlers
+
+    # Run the registered handlers in reverse order
+    while handler = @@mutex.synchronize { @@handlers.try(&.pop?) }
+      begin
+        handler.call status, exception
+      rescue handler_ex
+        Crystal::System.print_error "Error running at_exit handler: %s\n", handler_ex.message || ""
+        status = 1 if status.zero?
       end
     end