WEBrick::HTTPProxyServer hangs when process exits

I have a proxy server running in a thread:

@server = WEBrick::HTTPProxyServer.new(ServerType: Thread, BindAddress: host, Port: port)

when process exists I call @server.shutdown but it hangs forever so that I have to send Ctrl+C. While debugging this issue I figured that def do_CONNECT(req, res) which binds a socket os = TCPSocket.new(host, port) sometimes in ensure block has os set to nil. Which leads me to a question if https://github.com/ruby/ruby/blob/55c771c302f94f1d1d95bf41b42459b4d2d1c337/ext/socket/ipsocket.c#L187 socket can be nil if we take into account inetsock_cleanup(VALUE v) which returns return Qnil;

I fixed the issue by adding raise HTTPStatus::EOFError unless os in ensure block so that we don't interact with the rest of descriptors. Other note is that none of the rescue blocks were called in do_CONNECT.

Update: I still assume that TCPSocket.new returns a socket or raises an error, but in my case I have no idea how I end up with nil as a socket even though we call thgroup.list.each{|th| th.join if th[:WEBrickThread] } in shutdown. Maybe because ruby starts to terminate all threads because main thread exits?

Start with WEBrick::HTTPProxyServer#do_CONNECT and its shutdown path, then review the referenced ext/socket/ipsocket.c cleanup code. Reproduce the threaded proxy exit and trace whether the CONNECT socket or thread join causes the hang. Done means the shutdown behavior and nil socket path are explained and covered by a regression test.