How to synchronize puts output?

[postmodern]

I've noticed that if I have a lot of async sub-tasks which pass data to a shared callback, which then prints the data using puts, sometimes the newlines are doubled up as if Ruby tries to write both values at once then write the newline characters.

Example

require 'async'

def test(&block)
  Async do |task|
    task.async do
      loop do
        block.call("foooooooooooooooooooooooo")
        sleep(0.1)
      end
    end

    task.async do
      loop do
        block.call("baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaar")
        sleep(0.1)
      end
    end
  end
end

test do |word|
  puts word
end

$ ruby test_printing.rb 
foooooooooooooooooooooooobaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaar

foooooooooooooooooooooooo
baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaar
foooooooooooooooooooooooo
baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaar
foooooooooooooooooooooooo
baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaar
foooooooooooooooooooooooo
baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaar
foooooooooooooooooooooooo

How do I prevent this from happening?

[ioquatix]

IIRC, this should be fixed in the latest version of CRuby. What version did you test on?

[postmodern]

Can confirm this doesn't happen under ruby-3.2.0, but does happen under ruby-3.1.4.

[ioquatix]

Awesome, yeah there were some odd code in io.c w.r.t. locking. It was too fine grained, so because puts is actually two writes, the string itself followed by a newline, before the lock was taken per write, now it's taken for the entire puts.

Here are the commits that made more operations atomic: https://github.com/ruby/ruby/commits/6b0131db6f029f3e472d00deb2134d45ad4dde3a