WEBrick has an unsafe shutdown process it tries to concurrently write and close the @shutdown_pipe

When WEBrick shutdowns, it tries to concurrently write and close a file descriptor, and even tries to close it from multiple threads:

closing it from the main webrick thread:
https://github.com/ruby/webrick/blob/841b7dadd5d2dc7cd5f5cceca2258ad53023f8dd/lib/webrick/server.rb#L207

closing it from an arbitrary thread:
https://github.com/ruby/webrick/blob/841b7dadd5d2dc7cd5f5cceca2258ad53023f8dd/lib/webrick/server.rb#L237

writing to it (from an arbitrary thread):
https://github.com/ruby/webrick/blob/841b7dadd5d2dc7cd5f5cceca2258ad53023f8dd/lib/webrick/server.rb#L227

A hack:
https://github.com/ruby/webrick/blob/841b7dadd5d2dc7cd5f5cceca2258ad53023f8dd/lib/webrick/server.rb#L353

The problem is if the write_nonblock which calls write(2) ends up happening once the fd is close(2)d then it's EBADF, or worse writing to the wrong file descriptor.
This became such an issue that ruby/spec stopped using WEBrick and rewrote to make its own HTTP server to avoid this issue. Also the commit message of https://github.com/ruby/spec/commit/d8ead5da7158d31660b87f2c9db7d80755a57b27 may be interesting.

Only one thread (e.g. the main webrick thread) should close it, and it should wait all sub-threads before closing it so there are concurrent writes to the close.

CRuby has some very complex logic in IO#close which avoids the issue in most cases but it's not clear if it's fully reliable: https://ruby.slack.com/archives/C02A3SL0S/p1636604027275700?thread_ts=1636592668.266300&cid=C02A3SL0S
IIRC I've seen it fail for ruby/spec too on CRuby.

cc @ioquatix

Start by tracing the shutdown path in lib/webrick/server.rb, especially the referenced lines around closing, writing, and the shutdown hack. Compare the behavior with ruby/spec's library/net/http/http/fixtures/http_server.rb and verify that one thread closes @shutdown_pipe only after sub-threads finish, with no concurrent writes or closes.