Include free_immediately: false in heap dumps

This adds "free_immediately: false" to the heap dump for any T_DATA
which doesn't specify RUBY_TYPED_FREE_IMMEDIATELY. These are the objects
which end up as T_ZOMBIE and will have their dfree callback deferred to
later when the program is running holding the GVL.

This is useful to include because there is more overhead to freeing
these objects, and it's worth knowing how many there are.

Author: jhawthorn

ext/objspace/objspace_dump.c

@@ -574,9 +574,14 @@ dump_object(VALUE obj, struct dump_config *dc)
 
       case T_DATA:
         if (RTYPEDDATA_P(obj)) {
+            const rb_data_type_t *type = RTYPEDDATA_TYPE(obj);
             dump_append(dc, ", \"struct\":\"");
-            dump_append(dc, RTYPEDDATA_TYPE(obj)->wrap_struct_name);
+            dump_append(dc, type->wrap_struct_name);
             dump_append(dc, "\"");
+            if (!(type->flags & RUBY_TYPED_FREE_IMMEDIATELY) &&
+                    type->function.dfree != RUBY_DEFAULT_FREE) {
+                dump_append(dc, ", \"free_immediately\":false");
+            }
         }
         break;
 

test/objspace/test_objspace.rb

@@ -1021,6 +1021,20 @@ def test_escape_class_name
     assert_equal class_name, JSON.parse(json)["name"]
   end
 
+  def test_dump_free_immediately
+    require '-test-/typeddata'
+
+    # Bug::TypedData has flags=0 (no FREE_IMMEDIATELY)
+    info = ObjectSpace.dump(Bug::TypedData.new)
+    assert_include(info, '"struct":"typed_data"')
+    assert_include(info, '"free_immediately":false')
+
+    # Most typed data objects have FREE_IMMEDIATELY, so the field should be absent
+    info = ObjectSpace.dump(Thread.current.group)
+    assert_include(info, '"struct":"thgroup"')
+    assert_not_include(info, '"free_immediately"')
+  end
+
   def test_dump_include_shareable
     omit 'Not provided by mmtk' if RUBY_DESCRIPTION.include?("+GC[mmtk]")