Bug #20461: Unreadable pipe included in the readable IO of IO.select

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 3.4.0dev (2024-04-27T12:55:28Z master c844968b72) [x86_64-linux]
• Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN

[yamam (Masanari Yamamoto)]

When executing the following script, pipe_r is not supposed to be readable because no writing is done to pipe_w, but pipe_r is included in the return value rs of IO.select. Since it is not possible to read from pipe_r, the IO::EAGAINWaitReadable exception is raised.

pipe_r, pipe_w = IO.pipe
1000.times do |i|
    IO.popen(['seq', '1', '1000']) do |popen_r|
        j = 0
        while true
            rs, ws, = IO.select([popen_r, pipe_r])
            if rs.include?(popen_r)
                unless popen_r.gets
                    break
                end
            end
            if rs.include?(pipe_r)
                puts "IO.select BUG pipe_r is not readable! i = #{i} j = #{j}"
                p pipe_r.read_nonblock(1)
            end
            j += 1
        end
    end
end

$ ruby select.rb
IO.select BUG pipe_r is not readable! i = 0 j = 20
<internal:io>:63:in `read_nonblock': Resource temporarily unavailable - read would block (IO::EAGAINWaitReadable)
        from select.rb:14:in `block (2 levels) in <main>'
        from select.rb:3:in `popen'
        from select.rb:3:in `block in <main>'
        from select.rb:2:in `times'
        from select.rb:2:in `<main>'
[1]    73732 exit 1     ruby select.rb

[yamam (Masanari Yamamoto)]

• Description updated (diff)

[yamam (Masanari Yamamoto)]

Using git bisect, I found that the following commit is the cause of the bug:
https://github.com/ruby/ruby/commit/49a55f28abdcf3dea1f84ec5b59cfbedc37f44bf