Ensure the thread local state is always set up.

[ioquatix]

I was running irb over a drb-like system implemented on Async.

The problem is, some calls may be transparently sent over the network, including obj.pp.

In such cases, push_inspect_key could be called without Thread.current[:__recursive_key__][:inspect] being set up correctly, resulting in NoMethodError.

This change ensures that if we are printing an object, the state is set correctly by introducing guard_inspect(object) do ... end. This is an alternative to push_inspect_key and pop_inspect_key that (1) ensures the thread state is set up correctly and (2) can be implemented slightly more efficiently because we don't need to pop the state if the :inspect key is going to be discarded.

This change retains compatibility with all the existing methods.

[ioquatix]

I was able to create a test that reproduces the issue, but it's A/B as in, it was too tricky to create an isolated test using a distributed object system.

However, here is the same error I'm experiencing:

> ruby ./test.rb
/Users/samuel/.rubies/ruby-3.4.2/lib/ruby/3.4.0/pp.rb:183:in 'PP::PPMethods#pop_inspect_key': undefined method 'delete' for nil (NoMethodError)

      Thread.current[:__recursive_key__][:inspect].delete id
                                                  ^^^^^^^
	from /Users/samuel/.rubies/ruby-3.4.2/lib/ruby/3.4.0/pp.rb:209:in 'PP::PPMethods#pp'
	from ./test.rb:7:in '<main>'
/Users/samuel/.rubies/ruby-3.4.2/lib/ruby/3.4.0/pp.rb:178:in 'PP::PPMethods#push_inspect_key': undefined method '[]=' for nil (NoMethodError)

      Thread.current[:__recursive_key__][:inspect][id] = true
                                                  ^^^^^^
	from /Users/samuel/.rubies/ruby-3.4.2/lib/ruby/3.4.0/pp.rb:202:in 'PP::PPMethods#pp'
	from ./test.rb:7:in '<main>'

It happens from the following test:

require "pp"

a = []
a << a

q = PP::SingleLine.new($stderr)
q.pp(a)
q.flush

Now the current design expects users to write this:

q.guard_inspect_key{q.pp(a)}

But it's possible that even if you execute that code, q.pp(a) may be executed on a different thread or even a different process (remote object), in which Thread.current[:__recursive_key__][:inspect] isn't yet set up resulting in NoMethodError. By ensuring it's set up in guard_key, we avoid this problem.