Fix memory leak when duplicating too complex object

[Bug #20162]

Creating a ST table then calling st_replace leaks memory because the
st_replace overwrites the ST table without freeing any of the existing
memory. This commit changes it to use st_copy instead.

For example:

    RubyVM::Shape.exhaust_shapes

    o = Object.new
    o.instance_variable_set(:@a, 0)

    10.times do
      100_000.times { o.dup }

      puts `ps -o rss= -p #{$$}`
    end

Before:

    23264
    33600
    42672
    52160
    61600
    71728
    81056
    90528
    100560
    109840

After:

    14752
    14816
    15584
    15584
    15664
    15664
    15664
    15664
    15664
    15664

1 files changed, 13 insertions, 0 deletions

diff --git a/test/ruby/test_shapes.rb b/test/ruby/test_shapes.rb
index 885b762eb9..ee99fbad6d 100644
--- a/test/ruby/test_shapes.rb
+++ b/test/ruby/test_shapes.rb
@@ -942,6 +942,19 @@ class TestShapes < Test::Unit::TestCase
     assert_shape_equal(RubyVM::Shape.of(obj), RubyVM::Shape.of(obj2))
   end
 
+  def test_duplicating_too_complex_objects_memory_leak
+    assert_no_memory_leak([], "#{<<~'begin;'}", "#{<<~'end;'}", "[Bug #20162]", rss: true)
+      RubyVM::Shape.exhaust_shapes
+
+      o = Object.new
+      o.instance_variable_set(:@a, 0)
+    begin;
+      1_000_000.times do
+        o.dup
+      end
+    end;
+  end
+
   def test_freezing_and_duplicating_object
     obj = Object.new.freeze
     obj2 = obj.dup