Question

I’m trying to run thin start 2>&1 in a subprocess by running fork/exec, and I’m reassigning STDOUT to a pipe so I can feed output through.

However, I don’t get any output when Thin starts successfully. When it errors, I get the output in the wrong order—error text first, then the standard Thin startup messages.

I copied the code from Foreman, which handles this correctly. Any idea what’s going on?

Code:

reader, writer = IO.pipe
pid = fork do
  Process.setpgrp
  trap("INT", "IGNORE")
  $stdout.reopen writer
  reader.close
  exec("thin start 2>&1")
end
Process.detach pid
until reader.eof?
  puts reader.gets
end

Answer 1

Data sent to $stdout isn’t printed immediately — it’s buffered. On the other hand, $stderr isn’t buffered and when you write to it you see results immediately. Let’s see a minimal example.

STDOUT.puts :stdout
STDERR.puts :stderr

Save it as test.rb and replace thin start with ruby test.rb. Most probably stdout will be printed after stderr. In order to fix it we’ll have to use the IO#sync= method

STDOUT.sync = true
STDOUT.puts :stdout
STDERR.puts :stderr

Now stdout will be printed synchronously, just as stderr is, and the resulting order of printed strings should be the intuitive one.

Initial lack of output from your child Thin process might be caused by the fact that the child isn’t flushing data to STDOUT. First piece of data written to STDERR causes STDOUT to flush. Try adding STDOUT.sync = true somewhere in your application’s or Thin’s sources and see whether it helped.

See also a discussion on ruby-forum.com titled capture output in real time.