Do not allow external strings in old pointer space.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 5978ed270e34d51d8a5de093310ba023d0810a55..3f5a0793b21a5e53d68070494f18e28d79c57cf2 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -5930,6 +5930,23 @@ i::Handle<i::String> NewExternalAsciiStringHandle(i::Isolate* isolate,
 }
 
 
+bool RedirectToExternalString(i::Isolate* isolate,
+                              i::Handle<i::String> parent,
+                              i::Handle<i::String> external) {
+  if (parent->IsConsString()) {
+    i::Handle<i::ConsString> cons = i::Handle<i::ConsString>::cast(parent);
+    cons->set_first(*external);
+    cons->set_second(isolate->heap()->empty_string());
+  } else {
+    ASSERT(parent->IsSlicedString());
+    i::Handle<i::SlicedString> slice = i::Handle<i::SlicedString>::cast(parent);
+    slice->set_parent(*external);
+    slice->set_offset(0);
+  }
+  return true;
+}
+
+
 Local<String> v8::String::NewExternal(
       v8::String::ExternalStringResource* resource) {
   i::Isolate* isolate = i::Isolate::Current();
@@ -5958,9 +5975,23 @@ bool v8::String::MakeExternal(v8::String::ExternalStringResource* resource) {
     return false;
   }
   CHECK(resource && resource->data());
-  bool result = obj->MakeExternal(resource);
-  if (result && !obj->IsInternalizedString()) {
-    isolate->heap()->external_string_table()->AddString(*obj);
+
+  bool result;
+  i::Handle<i::String> external;
+  if (isolate->heap()->old_pointer_space()->Contains(*obj)) {
+    // We do not allow external strings in the old pointer space.  Instead of
+    // converting the string in-place, we keep the cons/sliced string and
+    // point it to a newly-allocated external string.
+    external = NewExternalStringHandle(isolate, resource);
+    result = RedirectToExternalString(isolate, obj, external);
+  } else {
+    result = obj->MakeExternal(resource);
+    external = obj;
+  }
+
+  ASSERT(external->IsExternalString());
+  if (result && !external->IsInternalizedString()) {
+    isolate->heap()->external_string_table()->AddString(*external);
   }
   return result;
 }
@@ -5995,9 +6026,23 @@ bool v8::String::MakeExternal(
     return false;
   }
   CHECK(resource && resource->data());
-  bool result = obj->MakeExternal(resource);
-  if (result && !obj->IsInternalizedString()) {
-    isolate->heap()->external_string_table()->AddString(*obj);
+
+  bool result;
+  i::Handle<i::String> external;
+  if (isolate->heap()->old_pointer_space()->Contains(*obj)) {
+    // We do not allow external strings in the old pointer space.  Instead of
+    // converting the string in-place, we keep the cons/sliced string and
+    // point it to a newly-allocated external string.
+    external = NewExternalAsciiStringHandle(isolate, resource);
+    result = RedirectToExternalString(isolate, obj, external);
+  } else {
+    result = obj->MakeExternal(resource);
+    external = obj;
+  }
+
+  ASSERT(external->IsExternalString());
+  if (result && !external->IsInternalizedString()) {
+    isolate->heap()->external_string_table()->AddString(*external);
   }
   return result;
 }