Implement a simpler regression test for #341

What is happening is that when we call into a recursive packing
proc, we first save the packer buffer state onto the stack
and then reset the buffer.

Once we return from the proc, the original buffer state is copied
back.

The problem with this is that if any of the chunk has a mapped string
then they are not reachable by any Ruby object and may be garbage collected
at any moment.

bin/rspec

@@ -0,0 +1,27 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+#
+# This file was generated by Bundler.
+#
+# The application 'rspec' is installed as part of a gem, and
+# this file is here to facilitate running it.
+#
+
+ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../Gemfile", __dir__)
+
+bundle_binstub = File.expand_path("bundle", __dir__)
+
+if File.file?(bundle_binstub)
+  if File.read(bundle_binstub, 300).include?("This file was generated by Bundler")
+    load(bundle_binstub)
+  else
+    abort("Your `bin/bundle` was not generated by Bundler, so this binstub cannot run.
+Replace `bin/bundle` by running `bundle binstubs bundler --force`, then run this command again.")
+  end
+end
+
+require "rubygems"
+require "bundler/setup"
+
+load Gem.bin_path("rspec-core", "rspec")

spec/packer_spec.rb

@@ -589,4 +589,31 @@ def to_msgpack_ext
       GC.stress = stress
     end
   end
+
+  it "doesn't crash when using recursive packers concurrently" do
+    hash_with_indifferent_access = Class.new(Hash)
+    msgpack = MessagePack::Factory.new
+    msgpack.register_type(
+      0x02,
+      hash_with_indifferent_access,
+      packer: ->(value, packer) do
+        packer.write("a".b * 600_0000) # Over MSGPACK_BUFFER_STRING_WRITE_REFERENCE_DEFAULT
+        packer.write(value.to_h)
+        GC.start(full_mark: true, immediate_mark: true, immediate_sweep: true)
+      end,
+      unpacker: ->(unpacker) { hash_with_indifferent_access.new(unpacker.read) },
+      recursive: true
+    )
+
+    packer = msgpack.packer
+
+    top_hash = hash = hash_with_indifferent_access.new
+    10.times do
+      hash["a"] = new_hash = hash_with_indifferent_access.new
+      hash = new_hash
+    end
+    packer.write(top_hash)
+    packer.write(top_hash)
+    packer.full_pack
+  end
 end