Ensure PrintToJSONStream can be called on all heap objects without crashing.

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 34253)
+++ runtime/vm/object.cc	(working copy)
@@ -4037,7 +4037,7 @@
                                         NameVisibility name_visibility) const {
   ASSERT(from_index + len <= Length());
   String& name = String::Handle();
-  const intptr_t num_strings = 2*len + 1;  // "<""T"", ""T"">".
+  const intptr_t num_strings = (len == 0) ? 2 : 2*len + 1;  // "<""T"", ""T"">".
   const Array& strings = Array::Handle(Array::New(num_strings));
   intptr_t s = 0;
   strings.SetAt(s++, Symbols::LAngleBracket());
@@ -6326,7 +6326,12 @@
     id = cls.FindFunctionIndex(*this);
     selector = "functions";
   }

[turnidge, 2014/03/23 22:24:20]

Did you consider grabbing an id from the object id ring but not delegating up to
the Object::PrintToJSONStream.  Something like:

if (id >= 0) {
  jsobj.AddPropertyF("id", "classes/%" Pd " ... etc.
} else {
  jsobj.AddPropertyF("id", "objects/%" Pd "", <get a number from the obj id
ring>);
}

That way this will have the expected format of a Function and only the id will
be different.  Otherwise we will have to guard against these odd functions in
the observatory.

[koda, 2014/03/24 17:18:34]

I wasn't sure whether all fields still work in all the special cases (e.g., will
things like "parent_function" work?), but I suppose it makes sense to try.

I'll send a follow-up CL.

-  ASSERT(id >= 0);
+  // TODO(17697): Oddball functions are treated as plain objects and use the
+  // object id ring. Current known examples are signature functions of closures
+  // and stubs like 'megamorphic_miss'.
+  if (id < 0) {
+    return Object::PrintToJSONStream(stream, ref);
+  }
   intptr_t cid = cls.id();
   JSONObject jsobj(stream);
   jsobj.AddProperty("type", JSONType(ref));
@@ -13481,6 +13486,10 @@
 
 
 void Type::PrintToJSONStream(JSONStream* stream, bool ref) const {
+  // TODO(koda): Decide whether to assign stable ids to non-canonical types.
+  if (!IsCanonical()) {
+    return Object::PrintToJSONStream(stream, ref);
+  }

[turnidge, 2014/03/23 22:24:20]

Ditto previous comment.  You could print an id from the object id ring but print
the rest of the expected properties for a Type.

[koda, 2014/03/24 17:18:34]

Also, here: not sure whether all printed properties are even defined for a
non-finalized type, for instance.

   ASSERT(IsCanonical());
   JSONObject jsobj(stream);
   jsobj.AddProperty("type", JSONType(ref));
@@ -16756,9 +16765,8 @@
       JSONObject jselement(&jsarr);
       jselement.AddProperty("index", index);
 
-      Instance& instance = Instance::Handle();
-      instance ^= At(index);
-      jselement.AddProperty("value", instance);
+      Object& element = Object::Handle(At(index));
+      jselement.AddProperty("value", element);
     }
   }
 }
@@ -17096,9 +17104,8 @@
       JSONObject jselement(&jsarr);
       jselement.AddProperty("index", index);
 
-      Instance& instance = Instance::Handle();
-      instance ^= At(index);
-      jselement.AddProperty("value", instance);
+      Object& element = Object::Handle(At(index));
+      jselement.AddProperty("value", element);
     }
   }
 }