https://git.ruby-lang.org/ruby.git/commit/?id=72f1c43584

From 72f1c43584df714a011864ca9fafc6e15ace392c Mon Sep 17 00:00:00 2001
From: Koichi Sasada <ko1@a...>
Date: Thu, 10 Dec 2020 13:27:08 +0900
Subject: ObjectSpace._id2ref should not support unshareable

ObjectSpace._id2ref(id) can return any objects even if they are
unshareable, so this patch raises RangeError if it runs on multi-ractor
mode and the found object is unshareable.

diff --git a/bootstraptest/test_ractor.rb b/bootstraptest/test_ractor.rb
index 125b51b..2bbf823 100644
--- a/bootstraptest/test_ractor.rb
+++ b/bootstraptest/test_ractor.rb
@@ -919,6 +919,19 @@ assert_equal '0', %q{ https://github.com/ruby/ruby/blob/trunk/bootstraptest/test_ractor.rb#L919
   }.take
 }
 
+# ObjectSpace._id2ref can not handle unshareable objects with Ractors
+assert_equal 'ok', %q{
+  s = 'hello'
+
+  Ractor.new s.object_id do |id ;s|
+    begin
+      s = ObjectSpace._id2ref(id)
+    rescue => e
+      :ok
+    end
+  end.take
+}
+
 # Ractor.make_shareable(obj)
 assert_equal 'true', %q{
   class C
diff --git a/gc.c b/gc.c
index c583ed2..75255bd 100644
--- a/gc.c
+++ b/gc.c
@@ -3987,6 +3987,8 @@ id2ref_obj_tbl(rb_objspace_t *objspace, VALUE objid) https://github.com/ruby/ruby/blob/trunk/gc.c#L3987
  *     r = ObjectSpace._id2ref(s.object_id)   #=> "I am a string"
  *     r == s                                 #=> true
  *
+ *  On multi-ractor mode, if the object is not sharable, it raises
+ *  RangeError.
  */
 
 static VALUE
@@ -4023,7 +4025,13 @@ id2ref(VALUE objid) https://github.com/ruby/ruby/blob/trunk/gc.c#L4025
 
     if ((orig = id2ref_obj_tbl(objspace, objid)) != Qundef &&
         is_live_object(objspace, orig)) {
-        return orig;
+
+        if (!rb_multi_ractor_p() || rb_ractor_shareable_p(orig)) {
+            return orig;
+        }
+        else {
+            rb_raise(rb_eRangeError, "%+"PRIsVALUE" is id of the unshareable object on multi-ractor", rb_int2str(objid, 10));
+        }
     }
 
     if (rb_int_ge(objid, objspace->next_object_id)) {
-- 
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