merge revision(s) 82b57d7bfeefd717c10f7a5a3484aca6b3e708a3: [Backport…

[nurse]

… #20162]

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
---
 object.c                 |  3 +--
 test/ruby/test_shapes.rb | 13 +++++++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)