Singleton classes of unshareable objects are not shareable

Currently, all RClasses are created as shareable in `class_alloc0`,
which causes singleton classes of unshareable objects to be shareable
which allows passing unshareable objects by passing their singleton
class and getting their attached object.

This changes `singleton_class_of` to mark the singleton class as shareable
when the object is shareable, and inversely, to mark the singleton class as
unshareable if the object is unshareable.

We also need to not forcibly mark the class as shareable in
`shareable_p_enter`.

Additionally, if the object is not yet marked as shareable, but is
frozen (and `allow_frozen_shareable_p`), it becomes marked as shareable
through `rb_ractor_shareable_p`, and we need to also mark its singleton
class as shareable at that point.

Author: etiennebarrie

class.c

@@ -29,6 +29,7 @@
 #include "internal/string.h"
 #include "internal/variable.h"
 #include "ruby/st.h"
+#include "ruby/ractor.h"
 #include "vm_core.h"
 #include "ruby/ractor.h"
 #include "yjit.h"
@@ -2864,6 +2865,12 @@ singleton_class_of(VALUE obj, bool ensure_eigenclass)
             klass = rb_make_metaclass(obj, klass);
         }
         RB_FL_SET_RAW(klass, RB_OBJ_FROZEN_RAW(obj));
+        if (!RB_OBJ_SHAREABLE_P(obj) && RB_OBJ_SHAREABLE_P(klass)) {
+            RB_FL_UNSET_RAW(klass, RUBY_FL_SHAREABLE);
+        }
+        else if (RB_OBJ_SHAREABLE_P(obj) && !RB_OBJ_SHAREABLE_P(klass)) {
+            RB_FL_SET_RAW(klass, RUBY_FL_SHAREABLE);
+        }
         if (ensure_eigenclass && RB_TYPE_P(obj, T_CLASS)) {
             /* ensures an exposed class belongs to its own eigenclass */
             (void)ENSURE_EIGENCLASS(klass);

depend

@@ -12523,6 +12523,7 @@ ractor.$(OBJEXT): $(top_srcdir)/internal/basic_operators.h
 ractor.$(OBJEXT): $(top_srcdir)/internal/bignum.h
 ractor.$(OBJEXT): $(top_srcdir)/internal/bits.h
 ractor.$(OBJEXT): $(top_srcdir)/internal/box.h
+ractor.$(OBJEXT): $(top_srcdir)/internal/class.h
 ractor.$(OBJEXT): $(top_srcdir)/internal/compilers.h
 ractor.$(OBJEXT): $(top_srcdir)/internal/complex.h
 ractor.$(OBJEXT): $(top_srcdir)/internal/error.h

ractor.c

@@ -8,6 +8,7 @@
 #include "vm_core.h"
 #include "vm_sync.h"
 #include "ractor_core.h"
+#include "internal/class.h"
 #include "internal/complex.h"
 #include "internal/error.h"
 #include "internal/gc.h"
@@ -1540,6 +1541,9 @@ mark_shareable(VALUE obj)
     }
 
     rb_obj_set_shareable_no_assert(obj);
+    if (RCLASS_SINGLETON_P(RBASIC(obj)->klass)) {
+        rb_obj_set_shareable_no_assert(RBASIC(obj)->klass);
+    }
     return traverse_cont;
 }
 
@@ -1587,9 +1591,14 @@ shareable_p_enter(VALUE obj)
     else if (RB_TYPE_P(obj, T_CLASS)  ||
              RB_TYPE_P(obj, T_MODULE) ||
              RB_TYPE_P(obj, T_ICLASS)) {
-        // TODO: remove it
-        mark_shareable(obj);
-        return traverse_skip;
+        if (RCLASS_SINGLETON_P(obj) && !RB_OBJ_SHAREABLE_P(RCLASS_ATTACHED_OBJECT(obj))) {
+            return traverse_stop;
+        }
+        else {
+            // TODO: remove it
+            mark_shareable(obj);
+            return traverse_skip;
+        }
     }
     else if (RB_OBJ_FROZEN_RAW(obj) &&
              allow_frozen_shareable_p(obj)) {