Fix generational GC for weak references

Fixes issue pointed out in https://bugs.ruby-lang.org/issues/21084#note-7.
The following script crashes:

    wmap = ObjectSpace::WeakMap.new

    GC.disable # only manual GCs
    GC.start
    GC.start

    retain = []
    50.times do
      k = Object.new
      wmap[k] = true
      retain << k
    end

    GC.start # wmap promoted, other objects still young

    retain.clear

    GC.start(full_mark: false)

    wmap.keys.each(&:itself) # call method on keys to cause crash

Author: peterzhu2118

gc/default/default.c

@@ -5329,15 +5329,23 @@ rb_gc_impl_handle_weak_references_alive_p(void *objspace_ptr, VALUE obj)
 {
     rb_objspace_t *objspace = objspace_ptr;
 
-    return RVALUE_MARKED(objspace, obj);
+    bool marked = RVALUE_MARKED(objspace, obj);
+
+    if (marked) {
+        rgengc_check_relation(objspace, obj);
+    }
+
+    return marked;
 }
 
 static void
 gc_update_weak_references(rb_objspace_t *objspace)
 {
     VALUE *obj_ptr;
     rb_darray_foreach(objspace->weak_references, i, obj_ptr) {
+        gc_mark_set_parent(objspace, *obj_ptr);
         rb_gc_handle_weak_references(*obj_ptr);
+        gc_mark_set_parent_invalid(objspace);
     }
 
     size_t capa = rb_darray_capa(objspace->weak_references);

test/ruby/test_weakmap.rb

@@ -265,4 +265,27 @@ def test_use_after_free_bug_20688
       10_000.times { weakmap[Object.new] = Object.new }
     RUBY
   end
+
+  def test_generational_gc
+    EnvUtil.without_gc do
+      wmap = ObjectSpace::WeakMap.new
+
+      (GC::INTERNAL_CONSTANTS[:RVALUE_OLD_AGE] - 1).times { GC.start }
+
+      retain = []
+      50.times do
+        k = Object.new
+        wmap[k] = true
+        retain << k
+      end
+
+      GC.start # WeakMap promoted, other objects still young
+
+      retain.clear
+
+      GC.start(full_mark: false)
+
+      wmap.keys.each(&:itself) # call method on keys to cause crash
+    end
+  end
 end