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

runtime/vm/object_id_ring.h

 // 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.
 
 #ifndef VM_OBJECT_ID_RING_H_
 #define VM_OBJECT_ID_RING_H_
 
 namespace dart {
 
 // Forward declarations.
 class RawObject;
 class Isolate;
 class ObjectPointerVisitor;
 
 // A ring buffer of object pointers that have been given an id. An object
 // may be pointed to by multiple ids. Objects contained in the ring will
 // be preserved across scavenges but not old space collections.
 // When the ring buffer wraps around older objects will be replaced and their
 // ids will be invalidated.
 class ObjectIdRing {
  public:
+  enum LookupResult {

[rmacnak, 2014/08/26 21:36:50]

Not thrilled about this name.

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

Sure.  I'm having trouble coming up with an improvement though.  Maybe if
GetObjectForId were called "Lookup" then the type name would feel better.  Not
necessarily suggesting that.

Maybe add one-line comments for kInvalid, kCollected, and kExpired.

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

Added one-liners.

+    kValid = 0,
+    kInvalid,
+    kCollected,
+    kExpired,
+  };
+
   static const int32_t kMaxId = 0x3FFFFFFF;
   static const int32_t kInvalidId = -1;
   static const int32_t kDefaultCapacity = 1024;
 
   static void Init(Isolate* isolate, int32_t capacity = kDefaultCapacity);
 
   ~ObjectIdRing();
 
   int32_t GetIdForObject(RawObject* raw_obj);
 
   // Returns Object::sentinel() when the id is not valid.

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

This comment is now out of date with the code... but I'm wondering if we should
change the code to return Object::sentinel() whenever the LookupResult is not
kValid?  Or something like that -- always the same result in those three cases.

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

Updated to say we return Object::null() when !kValid.

-  RawObject* GetObjectForId(int32_t id);
+  RawObject* GetObjectForId(int32_t id, LookupResult* kind);
 
   void VisitPointers(ObjectPointerVisitor* visitor);
 
  private:
   friend class ObjectIdRingTestHelper;
 
   void SetCapacityAndMaxSerial(int32_t capacity, int32_t max_serial);
 
   ObjectIdRing(Isolate* isolate, int32_t capacity);
   Isolate* isolate_;
@@ skipping 15 matching lines @@
   int32_t IndexOfId(int32_t id);
   bool IsValidContiguous(int32_t id);
   bool IsValidId(int32_t id);
 
   DISALLOW_COPY_AND_ASSIGN(ObjectIdRing);
 };
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_ID_RING_H_