2 files changed, 55 insertions, 8 deletions

diff --git a/activesupport/lib/active_support/descendants_tracker.rb b/activesupport/lib/active_support/descendants_tracker.rb
index 05236d3162..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)
@@ -34,15 +37,17 @@ 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) }
+            direct_descendants.each do |direct_descendant|
+              acc << direct_descendant
+              accumulate_descendants(direct_descendant, acc)
+            end
           end
         end
     end
@@ -59,5 +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
diff --git a/activesupport/lib/active_support/notifications/instrumenter.rb b/activesupport/lib/active_support/notifications/instrumenter.rb
index 00a57c38c9..a03e7e483e 100644
--- a/activesupport/lib/active_support/notifications/instrumenter.rb
+++ b/activesupport/lib/active_support/notifications/instrumenter.rb
@@ -13,14 +13,15 @@ module ActiveSupport
         @notifier = notifier
       end
 
-      # Instrument the given block by measuring the time taken to execute it
-      # and publish it. Notice that events get sent even if an error occurs
-      # in the passed-in block.
+      # Given a block, instrument it by measuring the time taken to execute
+      # and publish it. Without a block, simply send a message via the
+      # notifier. Notice that events get sent even if an error occurs in the
+      # passed-in block.
       def instrument(name, payload = {})
         # some of the listeners might have state
         listeners_state = start name, payload
         begin
-          yield payload
+          yield payload if block_given?
         rescue Exception => e
           payload[:exception] = [e.class.name, e.message]
           payload[:exception_object] = e