[Bug #18117] Fix Ractor race condition with GC

rb_objspace_reachable_objects_from requires that the GC not be active.
Since the Ractor barrier is not executed for incremental sweeping,
Ractor may call rb_objspace_reachable_objects_from after sweeping
has started to share objects. This causes a crash that looks like
the following:

```
<internal:ractor>:627: [BUG] rb_objspace_reachable_objects_from() is not supported while during_gc == true
```

Co-authored-by: Vinicius Stock <vinicius.stock@shopify.com>

1 files changed, 10 insertions, 2 deletions

diff --git a/ractor.c b/ractor.c
index 7996d1c275..bf19aa62d3 100644
--- a/ractor.c
+++ b/ractor.c
@@ -2368,7 +2368,11 @@ obj_traverse_i(VALUE obj, struct obj_traverse_data *data)
                 .stop = false,
                 .data = data,
             };
-            rb_objspace_reachable_objects_from(obj, obj_traverse_reachable_i, &d);
+            RB_VM_LOCK_ENTER_NO_BARRIER();
+            {
+                rb_objspace_reachable_objects_from(obj, obj_traverse_reachable_i, &d);
+            }
+            RB_VM_LOCK_LEAVE_NO_BARRIER();
             if (d.stop) return 1;
         }
         break;
@@ -2678,7 +2682,11 @@ static int
 obj_refer_only_shareables_p(VALUE obj)
 {
     int cnt = 0;
-    rb_objspace_reachable_objects_from(obj, obj_refer_only_shareables_p_i, &cnt);
+    RB_VM_LOCK_ENTER_NO_BARRIER();
+    {
+        rb_objspace_reachable_objects_from(obj, obj_refer_only_shareables_p_i, &cnt);
+    }
+    RB_VM_LOCK_LEAVE_NO_BARRIER();
     return cnt == 0;
 }