From d82f9d609285ce6d8fbe1e2aaebcb0ac04e4398e Mon Sep 17 00:00:00 2001
From: Edouard CHIN <chin.edouard@gmail.com>
Date: Wed, 8 Jul 2026 17:30:05 +0200
Subject: [PATCH] Allow to use Concurrent::Map#compute_if_absent in a Ractor

- ### Problem

  It is currently not possible to use the `compute_if_absent` method
  in a Ractor because there is a NULL object used solely as a sentinel
  that isn't shareable.

  ```ruby
  Ractor.new do
    map = Concurrent::Map.new
    map.compute_if_absent(3) do |map, key|
    end
  end

  # Ractor::IsolationError:
  #   can not access non-shareable objects in constant Concurrent::NULL by non-main ractor.
  ```

  ### Solution

  I'd like to simply freeze the object being hold in the `NULL`
  constant.
---
 lib/concurrent-ruby/concurrent/constants.rb |  2 +-
 spec/concurrent/map_spec.rb                 | 22 +++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/lib/concurrent-ruby/concurrent/constants.rb b/lib/concurrent-ruby/concurrent/constants.rb
index 676c2afb9..38e01e6b9 100644
--- a/lib/concurrent-ruby/concurrent/constants.rb
+++ b/lib/concurrent-ruby/concurrent/constants.rb
@@ -3,6 +3,6 @@ module Concurrent
   # Various classes within allows for +nil+ values to be stored,
   # so a special +NULL+ token is required to indicate the "nil-ness".
   # @!visibility private
-  NULL = ::Object.new
+  NULL = ::Object.new.freeze
 
 end
diff --git a/spec/concurrent/map_spec.rb b/spec/concurrent/map_spec.rb
index 4f1ff2bff..54c57b7cb 100644
--- a/spec/concurrent/map_spec.rb
+++ b/spec/concurrent/map_spec.rb
@@ -70,6 +70,28 @@ module Concurrent
         expect(map.keys).to eq [3]
       end
 
+      if defined?(Ractor)
+        it 'works in a Ractor' do
+          ractor = Ractor.new do
+            inner_map = Concurrent::Map.new do |map, key|
+              map.compute_if_absent(key) { key * 2 }
+            end
+
+            inner_map[3]
+            inner_map
+          end
+
+          map = if ractor.respond_to?(:take)
+            ractor.take
+          else
+            ractor.value # Ruby 4.0
+          end
+
+          expect(map[3]).to eq 6
+          expect(map.keys).to eq [3]
+        end
+      end
+
       it 'common' do
         with_or_without_default_proc do
           expect_size_change(3) do