Example of deadlocking by executing require in multithread

a.rb

# ruby 3.2.3 (2024-01-18 revision 52bb2ac0a6) [arm64-darwin23]
$LOAD_PATH << "#{__dir__}/lib"

# # lib/first.rb
# module First
#   require 'second'
# end

# # lib/second.rb
# module Second
#   require 'first'
# end

[
  Thread.new { require 'second' },
  Thread.new { require 'first' }
].each(&:join)
# ./a.rb:16:in `join': No live threads left. Deadlock? (fatal)
# 3 threads, 3 sleeps current:0x000000011f715450 main thread:0x00000001201048d0
# * #<Thread:0x0000000100eab108 sleep_forever>
#    rb_thread_t:0x00000001201048d0 native:0x00000001d6f2d000 int:0
#
# * #<Thread:0x0000000101224780 ./a.rb:14 sleep_forever>
#    rb_thread_t:0x000000011f713700 native:0x000000016f26b000 int:0 mutex:0x000000011f717720 cond:1
#     depended by: tb_thread_id:0x00000001201048d0
#
# * #<Thread:0x0000000101224640 ./a.rb:15 sleep_forever>
#    rb_thread_t:0x000000011f715450 native:0x000000016f477000 int:0
#
#
#         from ./a.rb:16:in `each'
#         from ./a.rb:16:in `<main>'