class.c: descendants must not cause GC until the result array is created

Follow up of 428227472fc6563046d8138aab17f07bef6af753. The previous fix
uses `rb_ary_new_from_values` to create the result array, but it may
trigger the GC.

This second try is to create the result array by `rb_ary_new_capa`
before the second iteration, and assume that `rb_ary_push` does not
trigger GC. This assumption is very fragile, so should be improved in
future.

[Bug #18282] [Feature #14394]

1 files changed, 8 insertions, 0 deletions

diff --git a/test/ruby/test_class.rb b/test/ruby/test_class.rb
index 034f4c6d20..4ae230f91e 100644
--- a/test/ruby/test_class.rb
+++ b/test/ruby/test_class.rb
@@ -761,4 +761,12 @@ class TestClass < Test::Unit::TestCase
     100000.times { Class.new(c) }
     assert(c.descendants.size <= 100000)
   end
+
+  def test_descendants_gc_stress
+    10000.times do
+      c = Class.new
+      100.times { Class.new(c) }
+      assert(c.descendants.size <= 100)
+    end
+  end
 end