Use weak references in descendants tracker

It allows anonymous subclasses to be garbage collected.

activesupport/CHANGELOG.md

+*   Use weak references in descendants tracker to allow anonymous subclasses to
+    be garbage collected.
+
+    *Edgars Beigarts*
+
 *   Update `ActiveSupport::Notifications::Instrumenter#instrument` to make
     passing a block optional. This will let users use
     `ActiveSupport::Notifications` messaging features outside of

activesupport/lib/active_support/descendants_tracker.rb

 # frozen_string_literal: true
 
+require "weakref"
+
 module ActiveSupport
   # This module provides an internal implementation to track descendants
   # which is faster than iterating through ObjectSpace.
@@ -8,7 +10,8 @@ module DescendantsTracker
 
     class << self
       def direct_descendants(klass)
-        @@direct_descendants[klass] || []
+        descendants = @@direct_descendants[klass]
+        descendants ? descendants.to_a : []
       end
 
       def descendants(klass)
@@ -34,15 +37,17 @@ def clear
       # This is the only method that is not thread safe, but is only ever called
       # during the eager loading phase.
       def store_inherited(klass, descendant)
-        (@@direct_descendants[klass] ||= []) << descendant
+        (@@direct_descendants[klass] ||= DescendantsArray.new) << descendant
       end
 
       private
 
         def accumulate_descendants(klass, acc)
           if direct_descendants = @@direct_descendants[klass]
-            acc.concat(direct_descendants)
-            direct_descendants.each { |direct_descendant| accumulate_descendants(direct_descendant, acc) }
+            direct_descendants.each do |direct_descendant|
+              acc << direct_descendant
+              accumulate_descendants(direct_descendant, acc)
+            end
           end
         end
     end
@@ -59,5 +64,46 @@ def direct_descendants
     def descendants
       DescendantsTracker.descendants(self)
     end
+
+    # DescendantsArray is an array that contains weak references to classes.
+    class DescendantsArray # :nodoc:
+      include Enumerable
+
+      def initialize
+        @refs = []
+      end
+
+      def initialize_copy(orig)
+        @refs = @refs.dup
+      end
+
+      def <<(klass)
+        cleanup!
+        @refs << WeakRef.new(klass)
+      end
+
+      def each
+        @refs.each do |ref|
+          yield ref.__getobj__
+        rescue WeakRef::RefError
+        end
+      end
+
+      def refs_size
+        @refs.size
+      end
+
+      def cleanup!
+        @refs.delete_if { |ref| !ref.weakref_alive? }
+      end
+
+      def reject!
+        @refs.reject! do |ref|
+          yield ref.__getobj__
+        rescue WeakRef::RefError
+          true
+        end
+      end
+    end
   end
 end

activesupport/test/descendants_tracker_test_cases.rb

@@ -27,6 +27,15 @@ def test_descendants
     assert_equal_sets [], Child2.descendants
   end
 
+  def test_descendants_with_garbage_collected_classes
+    1.times do
+      child_klass = Class.new(Parent)
+      assert_equal_sets [Child1, Grandchild1, Grandchild2, Child2, child_klass], Parent.descendants
+    end
+    GC.start
+    assert_equal_sets [Child1, Grandchild1, Grandchild2, Child2], Parent.descendants
+  end
+
   def test_direct_descendants
     assert_equal_sets [Child1, Child2], Parent.direct_descendants
     assert_equal_sets [Grandchild1, Grandchild2], Child1.direct_descendants