Singleton Classes Aren't round-trippable

使用 YAML.unsafe_load 重现 Singleton 示例，然后阅读 psych/visitors/to_ruby.rb 中 revive 和报告的 klass.allocate 调用附近的代码。将其行为与 Singleton 对 Marshal 的处理进行比较，并确定分配自定义应如何工作。完成标准是 Singleton 实例在不序列化实例变量的情况下完成往返，并且反序列化会重新使用现有实例。

require "psych"
require "singleton"

class A
  include Singleton
end

YAML.unsafe_load(A.instance.to_yaml

result:

/Users/cody/.gem/ruby/3.1.3/gems/psych-5.1.1/lib/psych/visitors/to_ruby.rb:408:in `revive': private method `allocate' called for A:Class (NoMethodError)

        s = register(node, klass.allocate)
                                ^^^^^^^^^
	from /Users/cody/.gem/ruby/3.1.3/gems/psych-5.1.1/lib/psych/visitors/to_ruby.rb:215:in `visit_Psych_Nodes_Mapping'
	from /Users/cody/.gem/ruby/3.1.3/gems/psych-5.1.1/lib/psych/visitors/visitor.rb:30:in `visit'
	from /Users/cody/.gem/ruby/3.1.3/gems/psych-5.1.1/lib/psych/visitors/visitor.rb:6:in `accept'
	from /Users/cody/.gem/ruby/3.1.3/gems/psych-5.1.1/lib/psych/visitors/to_ruby.rb:35:in `accept'
	from /Users/cody/.gem/ruby/3.1.3/gems/psych-5.1.1/lib/psych/visitors/to_ruby.rb:320:in `visit_Psych_Nodes_Document'
	from /Users/cody/.gem/ruby/3.1.3/gems/psych-5.1.1/lib/psych/visitors/visitor.rb:30:in `visit'
	from /Users/cody/.gem/ruby/3.1.3/gems/psych-5.1.1/lib/psych/visitors/visitor.rb:6:in `accept'
	from /Users/cody/.gem/ruby/3.1.3/gems/psych-5.1.1/lib/psych/visitors/to_ruby.rb:35:in `accept'
	from /Users/cody/.gem/ruby/3.1.3/gems/psych-5.1.1/lib/psych/nodes/node.rb:50:in `to_ruby'
	from /Users/cody/.gem/ruby/3.1.3/gems/psych-5.1.1/lib/psych.rb:274:in `unsafe_load'

I would expect a Singleton class to both a) not actually serialize any instance variables, and b) upon deserialization, just use the existing instance. Singleton does this when using Marshal, but Marshal is part of Ruby core. And Psych does not provide any way for a class to control allocation when coming from YAML the way that Marshal can, so even if the argument is that Singleton should control its own destiny (by implementing encode_with), Psych needs to provide a way to customize object allocation (without having to override allocate, which might make things complicated for allocating objects "normally").