json: reject non-shareable default sort_keys proc from non-main Ractors

lib/json/common.rb

@@ -80,6 +80,24 @@ def parser=(parser) # :nodoc:
     def generator=(generator) # :nodoc:
       old, $VERBOSE = $VERBOSE, nil
 
+      unless generator::State.respond_to?(:default_sort_keys_proc_unchecked=, true)
+        generator::State.singleton_class.class_eval do
+          alias_method :default_sort_keys_proc_unchecked=, :default_sort_keys_proc=
+          private :default_sort_keys_proc_unchecked=
+
+          def default_sort_keys_proc=(proc)
+            unless ::Proc === proc
+              raise ::TypeError, "sort_key_proc must be a Proc"
+            end
+            if defined?(::Ractor) && !::Ractor.shareable?(proc) && !::Ractor.current.equal?(::Ractor.main)
+              raise ::Ractor::IsolationError,
+                    "can not set a non-shareable Proc as the default sort_keys proc from a non-main Ractor"
+            end
+            self.default_sort_keys_proc_unchecked = proc
+          end
+        end
+      end
+
       # The default proc used when the +sort_keys+ generation option is +true+.
       # It returns a new hash with the entries sorted by their keys.
       sort_keys_proc = ->(hash) { hash.sort.to_h }

test/json/json_generator_test.rb

@@ -248,6 +248,18 @@ def test_generate_sort_keys_with_proc
     assert_instance_of Proc, state.sort_keys
   end
 
+  def test_default_sort_keys_proc_setter_survives_generator_reassignment
+    omit "fork not supported" unless Process.respond_to?(:fork)
+    pid = fork do
+      JSON.generator = JSON::Ext::Generator
+      JSON.generator = JSON::Ext::Generator
+      JSON::State.default_sort_keys_proc = ->(hash) { hash.sort.to_h }
+      exit!(JSON.generate({b: 1, a: 2}, sort_keys: true) == '{"a":2,"b":1}' ? 0 : 1)
+    end
+    _, status = Process.wait2(pid)
+    assert_predicate status, :success?
+  end
+
   def test_generate_custom
     state = State.new(space_before: " ", space: "   ", indent: "<i>", object_nl: "\n", array_nl: "<a_nl>")
     json = generate({1=>{2=>3,4=>[5,6]}}, state)

test/json/ractor_test.rb

@@ -123,4 +123,33 @@ def test_coder_proc
     _, status = Process.waitpid2(pid)
     assert_predicate status, :success?
   end if Ractor.respond_to?(:shareable_proc)
+
+  def test_default_sort_keys_proc_ractor_safety
+    pid = fork do
+      Warning[:experimental] = false
+      results = Ractor.new do
+        outcomes = []
+
+        begin
+          JSON::State.default_sort_keys_proc = ->(hash) { hash.sort.to_h }
+          outcomes << :accepted
+        rescue Ractor::IsolationError
+          outcomes << :rejected
+        end
+
+        # A shareable Proc is allowed from any Ractor.
+        JSON::State.default_sort_keys_proc = Ractor.shareable_lambda { |hash| hash.sort.reverse.to_h }
+        outcomes << JSON.generate({b: 1, a: 2}, sort_keys: true)
+        outcomes
+      end.value
+
+      # The main Ractor owns the slot, so it may install any Proc.
+      JSON::State.default_sort_keys_proc = ->(hash) { hash.sort.to_h }
+      main_result = JSON.generate({b: 1, a: 2}, sort_keys: true)
+
+      exit(results == [:rejected, '{"b":1,"a":2}'] && main_result == '{"a":2,"b":1}' ? 0 : 1)
+    end
+    _, status = Process.waitpid2(pid)
+    assert_predicate status, :success?
+  end if Ractor.respond_to?(:shareable_lambda)
 end if defined?(Ractor) && Process.respond_to?(:fork)