I have some logging code that looks like this:

def log(line)
  io.puts(line)
  io.fsync
end

This code is called from multiple threads.

In production, I've noticed lines that there are lines that look like this: "#{line1}#{line2}\n\n".

I looked into it, and it looks like puts might have a race condition between writing the line and writing the subsequent newline. See

Some repro code:

require 'thread'
8.times.map do
  Thread.new do
    1000.times do
      $stdout.puts "line"
      $stdout.fsync
    end
  end
end.map(&:join)

To test:

ruby puts.rb | grep lineline

/cc @cheald