Bug #17801: repeated read from PTY blocks all Fibers

• Status: Closed
• Assignee: ioquatix (Samuel Williams)
• Target version: -
• ruby -v: ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [x86_64-linux]
• Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN

[pebauer (peter bauer)]

Ruby version:
ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [x86_64-linux]

used Scheduler:
https://github.com/ruby/ruby/blob/master/test/fiber/scheduler.rb

Example Code which should work:

require 'pty'
require "./scheduler.rb"

line_event = TracePoint.new(:line) do |t|
  p [t.lineno, t.method_id, t.path]
end

Fiber.set_scheduler(Scheduler.new)

master, slave = PTY.open
read, write = IO.pipe
pid = spawn("bash", :in=>read, :out=>slave)
read.close     # we dont need the read
slave.close    # or the slave

line_event.enable do

Fiber.schedule do
  loop do
    puts "date"
    write.puts "date"
    sleep 1
  end
end

Fiber.schedule do
  loop do
    puts master.gets.chomp
    puts master.gets.chomp # this line blocks all fibers, why ?
    puts "test"
    sleep 1
  end
end

#write.close 
end

[ioquatix (Samuel Williams)]

• Description updated (diff)
• Assignee set to ioquatix (Samuel Williams)

Investigating.

[pebauer (peter bauer)]

expected output from script on cli:
date
Di 13 Apr 2021 21:51:20 CEST
test
date
Di 13 Apr 2021 21:51:21 CEST
test

[pebauer (peter bauer)]

ioquatix (Samuel Williams) wrote in #note-1:

Investigating.

Hello Samuel, if found the following:
A read on a PTY master which is not ready seems to block all fibers.
I think not ready - means 0 Bytes to read, which can be checked via IO.nread

`Workaround 1: # read line by line
Fiber.schedule do
loop do
puts master.gets if master.ready?
sleep 0.1
end
end

Workaround 2: # shows much better performance than Workraround 1
Fiber.schedule do # never try to read more bytes then available
loop do
puts master.read(master.nread) if master.nread > 0
sleep 0.1
end
end`

[ioquatix (Samuel Williams)]

This works for me if I set:

master.nonblock = true

Can you try it and report back?

We could consider making this the default.

[pebauer (peter bauer)]

Yes, this works. Thank you.

master.nonblock = true

[hsbt (Hiroshi SHIBATA)]

• Status changed from Open to Closed