Offer an objc_enumerationMutation hook for platforms without weak imp…

…orts.

mutation.m

@@ -2,11 +2,19 @@
 #include <stdlib.h>
 #include "objc/runtime.h"
 
-// This function is exported as a weak symbol to enable GNUstep or some other
-// framework to replace it trivially
-void __attribute__((weak)) objc_enumerationMutation(id obj)
+static void objc_enumeration_mutation_fallback(id object)
 {
 	fprintf(stderr, "Mutation occured during enumeration.");
 	abort();
 }
 
+void (*_objc_enumeration_mutation)(id object) = &objc_enumeration_mutation_fallback;
+
+// This function is exported as a weak symbol to enable GNUstep or some other
+// framework to replace it trivially. On platforms with linkers that cannot handle weak exports,
+// the objc_enumeration_mutation hook is the preferred override point.
+void __attribute__((weak)) objc_enumerationMutation(id obj)
+{
+	_objc_enumeration_mutation(obj);
+}
+

objc/hooks.h

@@ -110,6 +110,11 @@ typedef IMP (*objc_tracing_hook)(id, SEL, IMP, int, void*);
  */
 int objc_registerTracingHook(SEL, objc_tracing_hook);
 
+/**
+ * Hook called when a collection is mutated while it is being enumerated.
+ */
+OBJC_HOOK void(*_objc_enumeration_mutation)(id object);
+
 #ifdef __cplusplus
 }
 #endif