1 files changed, 54 insertions, 7 deletions

diff --git a/activesupport/lib/active_support/descendants_tracker.rb b/activesupport/lib/active_support/descendants_tracker.rb
index a4cee788b6..2dca990712 100644
--- a/activesupport/lib/active_support/descendants_tracker.rb
+++ b/activesupport/lib/active_support/descendants_tracker.rb
@@ -1,5 +1,7 @@
 # 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 ActiveSupport
 
     class << self
       def direct_descendants(klass)
-        @@direct_descendants[klass] || []
+        descendants = @@direct_descendants[klass]
+        descendants ? descendants.to_a : []
       end
 
       def descendants(klass)
@@ -34,16 +37,19 @@ module ActiveSupport
       # 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) }
+
+        def accumulate_descendants(klass, acc)
+          if direct_descendants = @@direct_descendants[klass]
+            direct_descendants.each do |direct_descendant|
+              acc << direct_descendant
+              accumulate_descendants(direct_descendant, acc)
+            end
+          end
         end
-      end
     end
 
     def inherited(base)
@@ -58,5 +64,46 @@ module ActiveSupport
     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