From baec5bbfffb38083b4180240a0319d20c9afc2a7 Mon Sep 17 00:00:00 2001
From: Peter Zhu <peter@peterzhu.ca>
Date: Mon, 11 May 2026 19:50:07 -0400
Subject: Fix GC compaction for compare-by-identity sets

[Bug #22064]

Compare-by-identity sets use the address for hashing, so we must pin it
so the object does not move in GC compaction. Objects in a compare-by-identity
set is not currently pinned, causing the set to be broken if the object
is moved.

For example:

    set = Set.new.compare_by_identity

    o = Object.new
    set.add(o)

    puts set.include?(o)

    GC.verify_compaction_references(expand_heap: true, toward: :empty)

    puts set.include?(o)

It should output true twice, but it outputs true and false.
---
 set.c                 | 17 ++++++++++++++++-
 test/ruby/test_set.rb | 19 +++++++++++++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/set.c b/set.c
index 4d2bc997fa..1a5d0e26fa 100644
--- a/set.c
+++ b/set.c
@@ -123,6 +123,14 @@ struct set_object {
     set_table table;
 };
 
+static int
+mark_and_pin_key(st_data_t key, st_data_t data)
+{
+    rb_gc_mark((VALUE)key);
+
+    return ST_CONTINUE;
+}
+
 static int
 mark_key(st_data_t key, st_data_t data)
 {
@@ -135,7 +143,14 @@ static void
 set_mark(void *ptr)
 {
     struct set_object *sobj = ptr;
-    if (sobj->table.entries) set_table_foreach(&sobj->table, mark_key, 0);
+    if (sobj->table.entries) {
+        if (sobj->table.type == &identhash) {
+            set_table_foreach(&sobj->table, mark_and_pin_key, 0);
+        }
+        else {
+            set_table_foreach(&sobj->table, mark_key, 0);
+        }
+    }
 }
 
 static void
diff --git a/test/ruby/test_set.rb b/test/ruby/test_set.rb
index 46d649ee73..427dd4b6b0 100644
--- a/test/ruby/test_set.rb
+++ b/test/ruby/test_set.rb
@@ -902,6 +902,25 @@ class TC_Set < Test::Unit::TestCase
     assert_equal(array.uniq.sort, set.sort)
   end
 
+  def test_compare_by_identity_compact
+    omit "compaction is not supported on this platform" unless GC.respond_to?(:compact)
+
+    # [Bug #22064]
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      set = Set.new.compare_by_identity
+
+      o = Object.new
+      set.add(o)
+
+      assert_include(set, o)
+
+      GC.verify_compaction_references(expand_heap: true, toward: :empty)
+
+      assert_include(set, o)
+    end;
+  end
+
   def test_reset
     [Set, Class.new(Set)].each { |klass|
       a = [1, 2]
-- 
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