class.c: calculate the length of Class.descendants in advance

GC must not be triggered during callback of rb_class_foreach_subclass.
To prevent GC, we can not use rb_ary_push. Instead, this changeset calls
rb_class_foreach_subclass twice: first counts the subclasses, then
allocates a buffer (which may cause GC and reduce subclasses, but not
increase), and finally stores the subclasses to the buffer.

[Bug #18282] [Feature #14394]

1 files changed, 6 insertions, 0 deletions

diff --git a/test/ruby/test_class.rb b/test/ruby/test_class.rb
index 96bca08601..034f4c6d20 100644
--- a/test/ruby/test_class.rb
+++ b/test/ruby/test_class.rb
@@ -755,4 +755,10 @@ class TestClass < Test::Unit::TestCase
     assert_include(object_descendants, sc)
     assert_include(object_descendants, ssc)
   end
+
+  def test_descendants_gc
+    c = Class.new
+    100000.times { Class.new(c) }
+    assert(c.descendants.size <= 100000)
+  end
 end