When externalizing a string ensure that the basic tag bits are preserved.

runtime/vm/dart_api_impl.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "include/dart_api.h"
 
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
 #include "vm/compiler.h"
 #include "vm/dart.h"
@@ skipping 1731 matching lines @@
   const String& str_obj = Api::UnwrapStringHandle(isolate, str);
   if (str_obj.IsExternal()) {
     return str;  // String is already an external string.
   }
   if (str_obj.IsNull()) {
     RETURN_TYPE_ERROR(isolate, str, String);
   }
   if (array == NULL) {
     RETURN_NULL_ERROR(array);
   }
+  Heap* vm_isolate_heap = Dart::vm_isolate()->heap();
+  if (vm_isolate_heap->Contains(reinterpret_cast<uword>(str_obj.raw()))) {

[cshapiro, 2012/11/29 21:03:12]

Is this the most efficient check?  Can we use the canonical bit instead?

[siva, 2012/11/29 22:37:39]

My initial thought was strings in the isolate can also be canonical and checking
the canonical bit would essentially mean that all canonical strings cannot be
externalized.

As discussed off-line I will start conservatively by checking the canonical bit
and if it turns out to be
too restrictive we can re-evaluate.

On 2012/11/29 21:03:12, cshapiro wrote:

+    return Api::NewError("Dart_MakeExternalString "
+                         "cannot externalize a read only string.");

[cshapiro, 2012/11/29 21:03:12]

s/read only/read-only/

[siva, 2012/11/29 22:37:39]

Done.

+  }
   intptr_t str_size = (str_obj.Length() * str_obj.CharSize());
   if ((length < str_size) || (length > String::kMaxElements)) {
     return Api::NewError("Dart_MakeExternalString "
                          "expects argument length to be in the range"
                          "[%"Pd"..%"Pd"].",
                          str_size, String::kMaxElements);
   }
   return Api::NewHandle(isolate,
                         str_obj.MakeExternal(array, length, peer, cback));
 }
@@ skipping 2689 matching lines @@
   }
   {
     NoGCScope no_gc;
     RawObject* raw_obj = obj.raw();
     isolate->heap()->SetPeer(raw_obj, peer);
   }
   return Api::Success(isolate);
 }
 
 }  // namespace dart