Environment

jruby 10.1.0.0 (4.0.0) 2026-04-20 32f988b78c OpenJDK 64-Bit Server VM 21.0.11 on 21.0.11 +indy +jit [arm64-darwin]

Compared with:

ruby 4.0.5 (2026-05-20 revision 64336ffd0e) +PRISM [arm64-darwin24]

Reproduction

# frozen_string_literal: true

require "socket"

server = TCPServer.new("127.0.0.1", 0)
client = TCPSocket.new("127.0.0.1", server.addr[1])
socket = server.accept

begin
  client.send("!", Socket::MSG_OOB)

  readable, writable, priority = IO.select(nil, nil, [socket], 1)

  puts RUBY_DESCRIPTION
  puts "priority: #{priority&.include?(socket).inspect}"

  unless priority&.include?(socket)
    warn "Expected socket to be reported as priority-ready."
    exit 1
  end
ensure
  socket&.close
  client&.close
  server&.close
end

Expected behavior

After sending TCP out-of-band data with Socket::MSG_OOB, IO.select(nil, nil, [socket], timeout) should report the socket in the priority result array.

CRuby reports:

ruby 4.0.5 (2026-05-20 revision 64336ffd0e) +PRISM [arm64-darwin24]
priority: true

Actual behavior

JRuby times out without reporting priority readiness:

jruby 10.1.0.0 (4.0.0) 2026-04-20 32f988b78c OpenJDK 64-Bit Server VM 21.0.11 on 21.0.11 +indy +jit [arm64-darwin]
priority: nil
Expected socket to be reported as priority-ready.

Context

This came up while enabling JRuby support in io-event. IO::Event::Selector::Select supports IO::PRIORITY using Ruby's IO.select priority array. The corresponding test passes on CRuby but has to be skipped on JRuby because the priority event is not reported.