[lldb] Use the non-locking variant of objc_copyRealizedClassList

[JDevlieghere]

Summary

Avoid standing the Objective-C runtime lock by calling objc_copyRealizedClassList_nolock instead of objc_copyRealizedClassList. We already guarantee that no other threads can run while we're running this utility expression, similar to when we parse the data ourself from the gdb_objc_realized_classes struct.

[jingham]

That seems fine to me. We already only allow one thread to run when we make this call, and copyRealizedClassList is read only, so at worst this will crash if the list is getting edited, but it won't do any harm and we'll just try again later. That's a much better failure mode than potentially stranding the ObjC runtime lock.

lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCRuntimeV2.cpp

 static const char *g_get_dynamic_class_info2_name =
     "__lldb_apple_objc_v2_get_dynamic_class_info2";
 
 static const char *g_get_dynamic_class_info2_body = R"(
 
 extern "C" {
     int printf(const char * format, ...);
     void free(void *ptr);
-    Class* objc_copyRealizedClassList(unsigned int *outCount);
+    Class* objc_copyRealizedClassList_nolock(unsigned int *outCount);
     const char* objc_debug_class_getNameRaw(Class cls);
 }
 
 #define DEBUG_PRINTF(fmt, ...) if (should_log) printf(fmt, ## __VA_ARGS__)
 
 struct ClassInfo
 {
     Class isa;
__lldb_apple_objc_v2_get_dynamic_class_info2(void *gdb_objc_realized_classes_ptr,
     DEBUG_PRINTF ("class_infos_byte_size = %u\n", class_infos_byte_size);
 
     const size_t max_class_infos = class_infos_byte_size/sizeof(ClassInfo);
     DEBUG_PRINTF ("max_class_infos = %u\n", max_class_infos);
 
     ClassInfo *class_infos = (ClassInfo *)class_infos_ptr;
 
     uint32_t count = 0;
-    Class* realized_class_list = objc_copyRealizedClassList(&count);
+    Class* realized_class_list = objc_copyRealizedClassList_nolock(&count);
     DEBUG_PRINTF ("count = %u\n", count);
 
     uint32_t idx = 0;
     for (uint32_t i=0; i<=count; ++i)
     {
         if (idx < max_class_infos)
         {
             Class isa = realized_class_list[i];
: AppleObjCRuntime(process), m_objc_module_sp(objc_module_sp),
       m_loaded_objc_opt(false), m_non_pointer_isa_cache_up(),
       m_tagged_pointer_vendor_up(
           TaggedPointerVendorV2::CreateInstance(*this, objc_module_sp)),
       m_encoding_to_type_sp(), m_noclasses_warning_emitted(false),
       m_CFBoolean_values(), m_realized_class_generation_count(0) {
   static const ConstString g_gdb_object_getClass("gdb_object_getClass");
   m_has_object_getClass = HasSymbol(g_gdb_object_getClass);
   static const ConstString g_objc_copyRealizedClassList(
-      "objc_copyRealizedClassList");
+      "objc_copyRealizedClassList_nolock");
   m_has_objc_copyRealizedClassList = HasSymbol(g_objc_copyRealizedClassList);
 
   RegisterObjCExceptionRecognizer(process);
 }
 
 bool AppleObjCRuntimeV2::GetDynamicTypeAndAddress(
     ValueObject &in_value, lldb::DynamicValueType use_dynamic,
     TypeAndOrName &class_type_or_name, Address &address,