Expose stats about weak references

[Feature #19783]

This commit adds stats about weak references to `GC.latest_gc_info`.
It adds the following two keys:

- `weak_references_count`: number of weak references registered during
  the last GC.
- `retained_weak_references_count`: number of weak references that
  survived the last GC.

1 files changed, 48 insertions, 0 deletions

diff --git a/test/ruby/test_gc.rb b/test/ruby/test_gc.rb
index 8d77e65bbe..bfcfbbb72f 100644
--- a/test/ruby/test_gc.rb
+++ b/test/ruby/test_gc.rb
@@ -286,6 +286,54 @@ class TestGc < Test::Unit::TestCase
     assert_not_nil(major_by)
   end
 
+  def test_latest_gc_info_weak_references_count
+    assert_separately([], __FILE__, __LINE__, <<~RUBY)
+      count = 10_000
+      # Some weak references may be created, so allow some margin of error
+      error_tolerance = 100
+
+      # Run full GC to clear out weak references
+      GC.start
+      # Run full GC again to collect stats about weak references
+      GC.start
+
+      before_weak_references_count = GC.latest_gc_info(:weak_references_count)
+      before_retained_weak_references_count = GC.latest_gc_info(:retained_weak_references_count)
+
+      # Create some objects and place it in a WeakMap
+      wmap = ObjectSpace::WeakMap.new
+      ary = Array.new(count)
+      count.times do |i|
+        obj = Object.new
+        ary[i] = obj
+        wmap[obj] = nil
+      end
+
+      # Run full GC to collect stats about weak references
+      GC.start
+
+      assert_operator(GC.latest_gc_info(:weak_references_count), :>=, before_weak_references_count + count - error_tolerance)
+      assert_operator(GC.latest_gc_info(:retained_weak_references_count), :>=, before_retained_weak_references_count + count - error_tolerance)
+      assert_operator(GC.latest_gc_info(:retained_weak_references_count), :<=, GC.latest_gc_info(:weak_references_count))
+
+      before_weak_references_count = GC.latest_gc_info(:weak_references_count)
+      before_retained_weak_references_count = GC.latest_gc_info(:retained_weak_references_count)
+
+      ary = nil
+
+      # Free ary, which should empty out the wmap
+      GC.start
+      # Run full GC again to collect stats about weak references
+      GC.start
+
+      assert_equal(0, wmap.size)
+
+      assert_operator(GC.latest_gc_info(:weak_references_count), :<=, before_weak_references_count - count + error_tolerance)
+      assert_operator(GC.latest_gc_info(:retained_weak_references_count), :<=, before_retained_weak_references_count - count + error_tolerance)
+      assert_operator(GC.latest_gc_info(:retained_weak_references_count), :<=, GC.latest_gc_info(:weak_references_count))
+    RUBY
+  end
+
   def test_stress_compile_send
     assert_in_out_err(%w[--disable-gems], <<-EOS, [], [], "")
       GC.stress = true