Test case for `UDPSocket#close` hanging

This is the behaviour I can observe on my machine:

Running on macOS with Ruby 3.3.0

Running statsd.timing only in sync blocks works fine:

bundle && bundle exec ruby test.rb 0 # returns after a second or so

Running 50/50 sync and async will hang:

bundle && bundle exec ruby test.rb 0.5 # hangs

So does running with only async, no sync:

bundle && bundle exec ruby test.rb 1 # hangs

Running only sync blocks works fine:

docker compose run --rm -e ASYNC_PERCENT=0 test # returns after a second or so

Unlike running on the host, so does running only async blocks:

docker compose run --rm -e ASYNC_PERCENT=1 test # returns after a second or so

But, running 50/50 sync and async will hang again:

docker compose run --rm -e ASYNC_PERCENT=0.5 test # hangs

In one terminal:

docker compose up test # hangs

In another terminal:

docker compose exec test bundle exec pry-remote

In the pry console:

$tasks.each(&:print_hierarchy)

... should show one fiber hanging in IO#close, the others waiting to get the Mutex.