Add `GC.compact` again.

🙏

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67620 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 97 insertions, 0 deletions

diff --git a/test/ruby/test_gc_compact.rb b/test/ruby/test_gc_compact.rb
new file mode 100644
index 0000000000..16c62915fd
--- /dev/null
+++ b/test/ruby/test_gc_compact.rb
@@ -0,0 +1,97 @@
+# frozen_string_literal: true
+require 'test/unit'
+require 'fiddle'
+
+class TestGCCompact < Test::Unit::TestCase
+  def memory_location(obj)
+    (Fiddle.dlwrap(obj) >> 1)
+  end
+
+  def assert_object_ids(list)
+    same_count = list.find_all { |obj|
+      memory_location(obj) == obj.object_id
+    }.count
+    list.count - same_count
+  end
+
+  def big_list
+    1000.times.map {
+      # try to make some empty slots by allocating an object and discarding
+      Object.new
+      Object.new
+    } # likely next to each other
+  end
+
+  # Find an object that's allocated in a slot that had a previous
+  # tenant, and that tenant moved and is still alive
+  def find_object_in_recycled_slot(addresses)
+    new_object = nil
+
+    100_000.times do
+      new_object = Object.new
+      if addresses.include? memory_location(new_object)
+        break
+      end
+    end
+
+    new_object
+  end
+
+  def test_find_collided_object
+    list_of_objects = big_list
+
+    ids       = list_of_objects.map(&:object_id) # store id in map
+    addresses = list_of_objects.map(&self.:memory_location)
+
+    assert_equal ids, addresses
+
+    # All object ids should be equal
+    assert_equal 0, assert_object_ids(list_of_objects) # should be 0
+
+    GC.verify_compaction_references
+
+    # Some should have moved
+    id_count = assert_object_ids(list_of_objects)
+    skip "couldn't get objects to move" if id_count == 0
+    assert_operator id_count, :>, 0
+
+    new_ids = list_of_objects.map(&:object_id)
+
+    # Object ids should not change after compaction
+    assert_equal ids, new_ids
+
+    new_tenant = find_object_in_recycled_slot(addresses)
+    assert new_tenant
+
+    # This is the object that used to be in new_object's position
+    previous_tenant = list_of_objects[addresses.index(memory_location(new_tenant))]
+
+    assert_not_equal previous_tenant.object_id, new_tenant.object_id
+
+    # Should be able to look up object by object_id
+    assert_equal new_tenant, ObjectSpace._id2ref(new_tenant.object_id)
+
+    # Should be able to look up object by object_id
+    assert_equal previous_tenant, ObjectSpace._id2ref(previous_tenant.object_id)
+
+    int = (new_tenant.object_id >> 1)
+    # These two should be the same! but they are not :(
+    assert_equal int, ObjectSpace._id2ref(int.object_id)
+  end
+
+  def test_many_collisions
+    list_of_objects = big_list
+    ids       = list_of_objects.map(&:object_id)
+    addresses = list_of_objects.map(&self.:memory_location)
+
+    GC.verify_compaction_references
+
+    new_tenants = 10.times.map {
+      find_object_in_recycled_slot(addresses)
+    }
+
+    collisions = GC.stat(:object_id_collisions)
+    skip "couldn't get objects to collide" if collisions == 0
+    assert_operator collisions, :>, 0
+  end
+end