Show chain of references for unshareable class variables

Author: etiennebarrie

test/ruby/test_ractor.rb

@@ -310,6 +310,22 @@ def test_error_for_module_instance_variable
     RUBY
   end
 
+  def test_error_for_class_variable
+    assert_ractor(<<~'RUBY')
+      module ModuleWithUnshareableClassVariable
+        def self.unshareable = @@unshareable
+        @@unshareable = Hash.new {}.freeze
+      end
+      e = Ractor.new do |mod|
+        ModuleWithUnshareableClassVariable.unshareable
+      rescue
+        $!
+      end.value
+      assert_kind_of Ractor::IsolationError, e
+      assert_match(/from Hash default proc/, e.detailed_message)
+    RUBY
+  end
+
   def test_error_for_module_constant
     assert_ractor(<<~'RUBY')
       module ModuleWithUnshareableConstant

variable.c

@@ -1206,10 +1206,13 @@ CVAR_ACCESSOR_SHOULD_BE_MAIN_RACTOR(VALUE klass, ID id)
 static void
 cvar_read_ractor_check(VALUE klass, ID id, VALUE val)
 {
-    if (UNLIKELY(!rb_ractor_main_p()) && !rb_ractor_shareable_p(val)) {
-        rb_raise(rb_eRactorIsolationError,
-                 "can not read non-shareable class variable %"PRIsVALUE" from non-main Ractors (%"PRIsVALUE")",
-                 rb_id2str(id), klass);
+    if (UNLIKELY(!rb_ractor_main_p())) {
+        VALUE chain = Qnil;
+        if (!rb_ractor_shareable_p_continue(val, &chain)) {
+            rb_raise(rb_eRactorIsolationError,
+                    "can not read non-shareable class variable %"PRIsVALUE" from non-main Ractors (%"PRIsVALUE")%"PRIsVALUE,
+                    rb_id2str(id), klass, chain);
+        }
     }
 }