* ext/objspace/objspace.c: add ObjectSpace#reachable_objects_from.

This method returns an array of objects referenced by given object.
  If given object is special objects such as true/false/nil/Fixnum etc
  then it returns nil. See rdoc for details.
  [ruby-core:39772]
* test/objspace/test_objspace.rb: add a test for this method.
* gc.c: add rb_objspace_reachable_objects_from().
  To make this function, add several member `mark_func_data'
  to rb_objspace_t.  If mark_func_data is not null, then
  gc_mark() calls mark_func_data::mark_func.
* gc.h: export rb_objspace_reachable_objects_from().



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37094 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/objspace/test_objspace.rb

@@ -68,4 +68,30 @@ class TestObjSpace < Test::Unit::TestCase
     ObjectSpace.count_tdata_objects(arg)
     assert_equal(false, arg.empty?)
   end
+
+  def test_reachable_objects_from
+    assert_equal(nil, ObjectSpace.reachable_objects_from(nil))
+    assert_equal([Array, 'a', 'b', 'c'], ObjectSpace.reachable_objects_from(['a', 'b', 'c']))
+
+    assert_equal([Array, 'a', 'a', 'a'], ObjectSpace.reachable_objects_from(['a', 'a', 'a']))
+    assert_equal([Array, 'a', 'a'], ObjectSpace.reachable_objects_from(['a', v = 'a', v]))
+    assert_equal([Array, 'a'], ObjectSpace.reachable_objects_from([v = 'a', v, v]))
+
+    long_ary = Array.new(1_000){''}
+    max = 0
+
+    ObjectSpace.each_object{|o|
+      refs = ObjectSpace.reachable_objects_from(o)
+      max = [refs.size, max].max
+
+      unless refs.nil?
+        refs.each{|ro|
+          # check this referenced object is not internal object
+          assert_equal(false, ro.nil?)
+        }
+      end
+    }
+    STDERR.puts max
+    assert_equal(true, max >= 1_001) # 1000 elems + Array class
+  end
 end