merge revision(s) 6c252912af4981f016a9abdb4c1689307a4f1d2f: [Backport #20145]

	Memory leak when duplicating identhash

	[Bug #20145]

	Before this commit, both copy_compare_by_id and hash_copy will create a
	copy of the ST table, so the ST table created in copy_compare_by_id will
	be leaked.

	    h = { 1 => 2 }.compare_by_identity

	    10.times do
	      1_000_000.times do
	        h.select { false }
	      end

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

	Before:

	    110736
	    204352
	    300272
	    395520
	    460704
	    476736
	    542000
	    604704
	    682624
	    770528

	After:

	    15504
	    16048
	    16144
	    16256
	    16320
	    16320
	    16752
	    16752
	    16752
	    16752
	---
	 hash.c                 | 10 +++++++++-
	 test/ruby/test_hash.rb | 10 ++++++++++
	 2 files changed, 19 insertions(+), 1 deletion(-)

1 files changed, 10 insertions, 0 deletions

diff --git a/test/ruby/test_hash.rb b/test/ruby/test_hash.rb
index 70daea0ef1..c72b256bab 100644
--- a/test/ruby/test_hash.rb
+++ b/test/ruby/test_hash.rb
@@ -1458,6 +1458,16 @@ class TestHash < Test::Unit::TestCase
     assert_predicate(h.dup, :compare_by_identity?, bug8703)
   end
 
+  def test_compare_by_identy_memory_leak
+    assert_no_memory_leak([], "", "#{<<~"begin;"}\n#{<<~'end;'}", "[Bug #20145]", rss: true)
+    begin;
+      h = { 1 => 2 }.compare_by_identity
+      1_000_000.times do
+        h.select { false }
+      end
+    end;
+  end
+
   def test_same_key
     bug9646 = '[ruby-dev:48047] [Bug #9646] Infinite loop at Hash#each'
     h = @cls[a=[], 1]