2 files changed, 97 insertions, 3 deletions
diff --git a/ractor.c b/ractor.c
index 2b9d5b3d5b..bd26c7739d 100644
--- a/ractor.c
+++ b/ractor.c
@@ -1357,9 +1357,26 @@ make_shareable_check_shareable(VALUE obj)
         }
     }
 
-    if (RB_TYPE_P(obj, T_IMEMO)) {
-        return traverse_skip;
-    }
+    switch (TYPE(obj)) {
+        case T_IMEMO:
+          return traverse_skip;
+        case T_OBJECT:
+          {
+              // If a T_OBJECT is shared and has no free capacity, we can't safely store the object_id inline,
+              // as it would require to move the object content into an external buffer.
+              // This is only a problem for T_OBJECT, given other types have external fields and can do RCU.
+              // To avoid this issue, we proactively create the object_id.
+              shape_id_t shape_id = RBASIC_SHAPE_ID(obj);
+              attr_index_t capacity = RSHAPE_CAPACITY(shape_id);
+              attr_index_t free_capacity = capacity - RSHAPE_LEN(shape_id);
+              if (!rb_shape_has_object_id(shape_id) && capacity && !free_capacity) {
+                  rb_obj_id(obj);
+              }
+          }
+          break;
+        default:
+          break;
+    } 
 
     if (!RB_OBJ_FROZEN_RAW(obj)) {
         rb_funcall(obj, idFreeze, 0);
diff --git a/test/ruby/test_object_id.rb b/test/ruby/test_object_id.rb
index 44421ea256..018cc81496 100644
--- a/test/ruby/test_object_id.rb
+++ b/test/ruby/test_object_id.rb
@@ -198,3 +198,80 @@ class TestObjectIdTooComplexGeneric < TestObjectId
     end
   end
 end
+
+class TestObjectIdRactor < Test::Unit::TestCase
+  def test_object_id_race_free
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      Warning[:experimental] = false
+      class MyClass
+        attr_reader :a, :b, :c
+        def initialize
+          @a = @b = @c = nil
+        end
+      end
+      N = 10_000
+      objs = Ractor.make_shareable(N.times.map { MyClass.new })
+      results = 4.times.map{
+        Ractor.new(objs) { |objs|
+          vars = []
+          ids = []
+          objs.each do |obj|
+            vars << obj.a << obj.b << obj.c
+            ids << obj.object_id
+          end
+          [vars, ids]
+        }
+      }.map(&:value)
+      assert_equal 1, results.uniq.size
+    end;
+  end
+
+  def test_external_object_id_ractor_move
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      Warning[:experimental] = false
+      class MyClass
+        attr_reader :a, :b, :c
+        def initialize
+          @a = @b = @c = nil
+        end
+      end
+      obj = Ractor.make_shareable(MyClass.new)
+      object_id = obj.object_id
+      obj = Ractor.new { Ractor.receive }.send(obj, move: true).value
+      assert_equal object_id, obj.object_id
+    end;
+  end
+
+  def test_object_id_race_free_with_stress_compact
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      Warning[:experimental] = false
+      class MyClass
+        attr_reader :a, :b, :c
+        def initialize
+          @a = @b = @c = nil
+        end
+      end
+      N = 20
+      objs = Ractor.make_shareable(N.times.map { MyClass.new })
+
+      GC.stress = true
+      GC.auto_compact = true if GC.respond_to?(:auto_compact=)
+
+      results = 4.times.map{
+        Ractor.new(objs) { |objs|
+          vars = []
+          ids = []
+          objs.each do |obj|
+            vars << obj.a << obj.b << obj.c
+            ids << obj.object_id
+          end
+          [vars, ids]
+        }
+      }.map(&:value)
+      assert_equal 1, results.uniq.size
+    end;
+  end
+end