Ensure we don't dereference fields_obj as Qundef

We rely on the GC to clear this when the GC is run on another EC than
the cache.

Author: jhawthorn

test/ruby/test_variable.rb

@@ -529,6 +529,22 @@ def test_genivar_cache_free
     assert_equal :new_value, str.instance_variable_get(:@x)
   end
 
+  def test_genivar_cache_invalidated_by_gc
+    str = +"hello"
+    str.instance_variable_set(:@x, :old_value)
+
+    str.instance_variable_get(:@x) # populate cache
+
+    Fiber.new {
+      str.remove_instance_variable(:@x)
+      str.instance_variable_set(:@x, :new_value)
+    }.resume
+
+    GC.start
+
+    assert_equal :new_value, str.instance_variable_get(:@x)
+  end
+
   private
   def with_kwargs_11(v1:, v2:, v3:, v4:, v5:, v6:, v7:, v8:, v9:, v10:, v11:)
     local_variables

variable.c

@@ -1273,7 +1273,7 @@ rb_obj_fields(VALUE obj, ID field_name)
           generic_fields:
             {
                 rb_execution_context_t *ec = GET_EC();
-                if (ec->gen_fields_cache.obj == obj && rb_imemo_fields_owner(ec->gen_fields_cache.fields_obj) == obj) {
+                if (ec->gen_fields_cache.obj == obj && !UNDEF_P(ec->gen_fields_cache.fields_obj) && rb_imemo_fields_owner(ec->gen_fields_cache.fields_obj) == obj) {
                     fields_obj = ec->gen_fields_cache.fields_obj;
                     RUBY_ASSERT(fields_obj == rb_obj_fields_generic_uncached(obj));
                 }
@@ -1309,6 +1309,8 @@ rb_free_generic_ivar(VALUE obj)
           default:
           generic_fields:
             {
+                // Other EC may have stale caches, so fields_obj should be
+                // invalidated and the GC will replace with Qundef
                 rb_execution_context_t *ec = GET_EC();
                 if (ec->gen_fields_cache.obj == obj) {
                     ec->gen_fields_cache.obj = Qundef;