Annoying REPL SIGSTOP Issue

readme.md

• Make sure you are running the bash shell.
• Run repl.rb which will echo what you print unless you enter "irb".
• Type "irb" to launch a new irb process.
• Hit Ctrl-Z to send SIGTSTP to stop the irb process.
• You'll be brought back to the main repl.
• Type "foo", hit enter. You'll be echoed "oo".
• Type "foo" again, hit enter. You'll see "foo".
• Now hit Ctrl-C to interrupt the process, it will die and the missing "f" will show up.

This seems to not be a problem with irb, but any child process that calls "gets". If you launch "irb" from bash though and do the same steps, bash doesn't exhibit this issue, just a ruby REPL.

Also, if you run tcshas the parent shell and do the above, the issue doesn't happen. What's going on?

2015-03-07 10:00pm Update

This problem happens every time when running Bash (3.x or 4.x) under OSX if bash is the parent shell that the process starts in. The issue doesn't seem to exist when running under Linux (at least not on the one Linux box I tested it with).

If bash is not the parent, but say tcsh is, then the problem no longer exists. However, is bash is in hte parent tree then it still fails. E.g. don't start tcsh from within bash to test.

A possibly related issue is exhibited if you run the "dash" shell from within bash, and then run "irb" inside of dash. Once irb is up hit Ctrl-Z to suspend. The dash shell will be frozen. However, if you load dash (not from within bash) then load "irb" and Ctrl-Z, there is no issue.

repl.rb

Signal.trap("SIGTSTP", "IGNORE")

pids = []
stty_save = `stty -g`
loop do
  print "#{$0}> "
  input = STDIN.gets.chomp
  if input == "exit"
    exit
  else
    command = nil
    if input == "irb"
      command = "irb"
    elsif input == "fg" && pid.last
      Process.kill "SIGCONT", pids.last
    else
      puts input
      next
    end
    if command
      pid = fork do
        Signal.trap("SIGTSTP", "DEFAULT")
        exec command
      end
      pids << pid
    end
    pid, status = Process.waitpid2 pids.last, Process::WUNTRACED if pids.last
    if status.stopsig == Signal.list["TSTP"]
      system "stty", stty_save
      puts "Stopped"
    else
      pids.pop
    end
  end
end