Treat null like the object it is in the Observatory and Service.

runtime/vm/object_id_ring.cc

 // Copyright (c) 2013, 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 "platform/assert.h"
 #include "vm/dart_api_state.h"
 #include "vm/object_id_ring.h"
 
 namespace dart {
 
 void ObjectIdRing::Init(Isolate* isolate, int32_t capacity) {
   ObjectIdRing* ring = new ObjectIdRing(isolate, capacity);
   isolate->set_object_id_ring(ring);
 }
 
 
 ObjectIdRing::~ObjectIdRing() {
   ASSERT(table_ != NULL);
   free(table_);
   table_ = NULL;
   if (isolate_ != NULL) {
     isolate_->set_object_id_ring(NULL);
     isolate_ = NULL;
   }
 }
 
 
 int32_t ObjectIdRing::GetIdForObject(RawObject* object) {
+  // We do not allow inserting null or any of the psuedo-nulls because the are
+  // used as sentinel values for expired and reclaimed entries.
+  ASSERT(!object->IsNullOrPseudoNull());

[turnidge, 2014/08/27 16:13:56]

Technically we could allow null to be inserted now that we have LookupResult...
but it's still better not to let it in.

[rmacnak, 2014/08/27 18:09:59]

No, the regular null is how we know an entry was collected because the GC places
a regular null.

Narrowed this to disallowing only regular null based on returning regular null
for !kValid lookups.

   return AllocateNewId(object);
 }
 
 
-RawObject* ObjectIdRing::GetObjectForId(int32_t id) {
+RawObject* ObjectIdRing::GetObjectForId(int32_t id, LookupResult* kind) {
+  *kind = kValid;
   int32_t index = IndexOfId(id);
   if (index == kInvalidId) {
-    // Return sentinel to allow caller to distinguish expired ids.
-    return Object::sentinel().raw();
+    *kind = kExpired;
+    return Object::null();
   }
   ASSERT(index >= 0);
   ASSERT(index < capacity_);
+  if (table_[index] == Object::null()) {
+    *kind = kCollected;
+  }
   return table_[index];
 }
 
 
 void ObjectIdRing::VisitPointers(ObjectPointerVisitor* visitor) {
   ASSERT(table_ != NULL);
   visitor->VisitPointers(table_, capacity_);
 }
 
 
@@ skipping 108 matching lines @@
       const int32_t max_serial_num = max_serial_;
       const int32_t bottom = max_serial_num - (capacity_ - serial_num_);
       return id >= bottom && bottom < max_serial_num;
     }
   }
   ASSERT(wrapped_ == false);
   return IsValidContiguous(id);
 }
 
 }  // namespace dart