improve protection against multiple instantiation

original commit: bcbe42f4ffb06877d01846f7e6bf4d959db07a63

collects/mred/private/wx/common/once.rkt

@@ -0,0 +1,14 @@
+#lang racket/base
+(require ffi/unsafe)
+
+(provide scheme_register_process_global)
+
+;; This module must be instantiated only once:
+
+(define scheme_register_process_global
+  (get-ffi-obj 'scheme_register_process_global #f (_fun _string _pointer -> _pointer)))
+
+(let ([v (scheme_register_process_global "GRacket-support-initialized"
+                                         (cast 1 _scheme _pointer))])
+  (when v
+    (error "cannot instantiate `racket/gui/base' a second time in the same process")))

collects/mred/private/wx/common/queue.rkt

@@ -5,7 +5,8 @@
          racket/class
          "rbtree.rkt"
          "../../lock.rkt"
-         "handlers.rkt")
+         "handlers.rkt"
+         "once.rkt")
 
 (provide queue-evt
          set-check-queue!
@@ -54,15 +55,6 @@
 
          scheme_register_process_global)
 
-;; ------------------------------------------------------------
-;; This module must be instantiated only once:
-
-(define-mz scheme_register_process_global (_fun _string _pointer -> _pointer))
-(let ([v (scheme_register_process_global "GRacket-support-initialized"
-                                         (cast 1 _scheme _pointer))])
-  (when v
-    (error "cannot start GRacket a second time in the same process")))
-
 ;; ------------------------------------------------------------
 ;; Create a Scheme evt that is ready when a queue is nonempty
 

collects/mred/private/wx/common/utils.rkt

@@ -1,6 +1,7 @@
 #lang racket/base
 (require ffi/unsafe
-         ffi/unsafe/define)
+         ffi/unsafe/define
+         "once.rkt")
 
 (provide define-mz)