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

JSON::State.default_sort_keys_proc is registered at extension load time
so it's owned by the main Ractor.

When we implement the registration table on each Ractor storing another
Ractor's unshareable Proc into that slot leaves a dangling reference
when the owning Ractor runs a local GC.

This commit ensures that we won't use a shareable proc by raising a
Ractor::IsolationError.

Author: eightbitraptor

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)