[ruby/json] Deprecate all *_default_options

Globally changing the behavior of the library is a bad idea, as
many different libraries may rely on `json` and may not expect it
and likely never tested that a different default config works for them.

If you need to change the behavior of JSON, it's best to do it only
locally, and not globally.

In addition the new `JSON::Coder` interface is much more suited for
that.

Another reason for the deprecation is that it's impossible to
make `JSON.load` and `JSON.dump` Ractor-safe with such API.

https://github.com/ruby/json/commit/172762c6e4

ext/json/lib/json/common.rb

@@ -101,6 +101,29 @@ module JSON
 
     # Sets or Returns the JSON generator state class that is used by JSON.
     attr_accessor :state
+
+    private
+
+    def deprecated_singleton_attr_accessor(*attrs)
+      args = RUBY_VERSION >= "3.0" ? ", category: :deprecated" : ""
+      attrs.each do |attr|
+        singleton_class.class_eval <<~RUBY
+          def #{attr}
+            warn "JSON.#{attr} is deprecated and will be removed in json 3.0.0", uplevel: 1 #{args}
+            @#{attr}
+          end
+
+          def #{attr}=(val)
+            warn "JSON.#{attr}= is deprecated and will be removed in json 3.0.0", uplevel: 1 #{args}
+            @#{attr} = val
+          end
+
+          def _#{attr}
+            @#{attr}
+          end
+        RUBY
+      end
+    end
   end
 
   # Sets create identifier, which is used to decide if the _json_create_
@@ -424,28 +447,26 @@ module JSON
   module_function :pretty_unparse
   # :startdoc:
 
-  class << self
   # Sets or returns default options for the JSON.unsafe_load method.
   # Initially:
   #   opts = JSON.load_default_options
   #   opts # => {:max_nesting=>false, :allow_nan=>true, :allow_blank=>true, :create_additions=>true}
-    attr_accessor :unsafe_load_default_options
+  deprecated_singleton_attr_accessor :unsafe_load_default_options
-  end
+
-  self.unsafe_load_default_options = {
+  @unsafe_load_default_options = {
     :max_nesting      => false,
     :allow_nan        => true,
     :allow_blank      => true,
     :create_additions => true,
   }
 
-  class << self
   # Sets or returns default options for the JSON.load method.
   # Initially:
   #   opts = JSON.load_default_options
   #   opts # => {:max_nesting=>false, :allow_nan=>true, :allow_blank=>true, :create_additions=>true}
-    attr_accessor :load_default_options
+  deprecated_singleton_attr_accessor :load_default_options
-  end
+
-  self.load_default_options = {
+  @load_default_options = {
     :allow_nan        => true,
     :allow_blank      => true,
     :create_additions => nil,
@@ -581,9 +602,9 @@ module JSON
   #
   def unsafe_load(source, proc = nil, options = nil)
     opts = if options.nil?
-      unsafe_load_default_options
+      _unsafe_load_default_options
     else
-      unsafe_load_default_options.merge(options)
+      _unsafe_load_default_options.merge(options)
     end
 
     unless source.is_a?(String)
@@ -741,9 +762,9 @@ module JSON
   #
   def load(source, proc = nil, options = nil)
     opts = if options.nil?
-      load_default_options
+      _load_default_options
     else
-      load_default_options.merge(options)
+      _load_default_options.merge(options)
     end
 
     unless source.is_a?(String)
@@ -781,14 +802,12 @@ module JSON
   alias restore load
   module_function :restore
 
-  class << self
   # Sets or returns the default options for the JSON.dump method.
   # Initially:
   #   opts = JSON.dump_default_options
   #   opts # => {:max_nesting=>false, :allow_nan=>true}
-    attr_accessor :dump_default_options
+  deprecated_singleton_attr_accessor :dump_default_options
-  end
+  @dump_default_options = {
-  self.dump_default_options = {
     :max_nesting => false,
     :allow_nan   => true,
   }
@@ -841,7 +860,7 @@ module JSON
       end
     end
 
-    opts = JSON.dump_default_options
+    opts = JSON._dump_default_options
     opts = opts.merge(:max_nesting => limit) if limit
     opts = opts.merge(kwargs) if kwargs
 

test/json/json_common_interface_test.rb

@@ -174,9 +174,9 @@ class JSONCommonInterfaceTest < Test::Unit::TestCase
   end
 
   def test_dump_should_modify_defaults
-    max_nesting = JSON.dump_default_options[:max_nesting]
+    max_nesting = JSON._dump_default_options[:max_nesting]
     dump([], StringIO.new, 10)
-    assert_equal max_nesting, JSON.dump_default_options[:max_nesting]
+    assert_equal max_nesting, JSON._dump_default_options[:max_nesting]
   end
 
   def test_JSON
@@ -211,6 +211,10 @@ class JSONCommonInterfaceTest < Test::Unit::TestCase
     end
   end
 
+  def test_deprecated_dump_default_options
+    assert JSON.dump_default_options
+  end
+
   private
 
   def with_external_encoding(encoding)