Ensure strings are never externalized as a side-effect of doing a GC.

src/api.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4246 matching lines @@
   i::Handle<i::String> obj = Utils::OpenHandle(this);
   i::Isolate* isolate = obj->GetIsolate();
   if (IsDeadCheck(isolate, "v8::String::MakeExternal()")) return false;
   if (i::StringShape(*obj).IsExternalTwoByte()) {
     return false;  // Already an external string.
   }
   ENTER_V8(isolate);
   if (isolate->string_tracker()->IsFreshUnusedString(obj)) {
     return false;
   }
+  if (isolate->heap()->gc_post_process_count() > 0) {
+    return false;
+  }
   bool result = obj->MakeExternal(resource);
   if (result && !obj->IsSymbol()) {
     isolate->heap()->external_string_table()->AddString(*obj);
   }
   return result;
 }
 
 
 Local<String> v8::String::NewExternal(
       v8::String::ExternalAsciiStringResource* resource) {
@@ skipping 12 matching lines @@
   i::Handle<i::String> obj = Utils::OpenHandle(this);
   i::Isolate* isolate = obj->GetIsolate();
   if (IsDeadCheck(isolate, "v8::String::MakeExternal()")) return false;
   if (i::StringShape(*obj).IsExternalTwoByte()) {
     return false;  // Already an external string.
   }
   ENTER_V8(isolate);
   if (isolate->string_tracker()->IsFreshUnusedString(obj)) {
     return false;
   }
+  if (isolate->heap()->gc_post_process_count() > 0) {
+    return false;
+  }
   bool result = obj->MakeExternal(resource);
   if (result && !obj->IsSymbol()) {
     isolate->heap()->external_string_table()->AddString(*obj);
   }
   return result;
 }
 
 
 bool v8::String::CanMakeExternal() {
   i::Handle<i::String> obj = Utils::OpenHandle(this);
@@ skipping 1529 matching lines @@
 
 
 char* HandleScopeImplementer::Iterate(ObjectVisitor* v, char* storage) {
   HandleScopeImplementer* scope_implementer =
       reinterpret_cast<HandleScopeImplementer*>(storage);
   scope_implementer->IterateThis(v);
   return storage + ArchiveSpacePerThread();
 }
 
 } }  // namespace v8::internal