Lock the GC mode once something has queried it. All of the GNUstep code that

does run-time detection of whether to use GC calls objc_collecting_enabled()
(sometimes indirectly, via NSGarbageCollector) to see if we're in GC mode.
After any of these calls, it is not safe to switch modes.

abi_version.c

@@ -61,7 +61,7 @@ static int known_abi_count =
 	}\
 } while(0)
 
-static enum objc_gc_mode current_gc_mode = GC_Optional;
+PRIVATE enum objc_gc_mode current_gc_mode = GC_Optional;
 
 static BOOL endsWith(const char *string, const char *suffix)
 {

gc_boehm.c

@@ -531,6 +531,9 @@ static void init(void)
 
 BOOL objc_collecting_enabled(void)
 {
+	// Lock the GC in the current state once it's been queried.  This prevents
+	// the loading of any modules with an incompatible GC mode.
+	current_gc_mode = isGCEnabled ? GC_Required : GC_None;
 	return isGCEnabled;
 }
 

gc_ops.h

@@ -54,6 +54,10 @@ enum objc_gc_mode
 /**
  * The current Objective-C garbage collection mode.
  */
+extern enum objc_gc_mode current_gc_mode;
+/**
+ * Have we loaded any code that triggers the ObjC GC support?
+ */
 extern BOOL isGCEnabled;
 
 /**