Ruby on Linux PTY goes away without EOF, raises Errno::EIO

So I had to go as far as reading the C source for the PTY library to get really satisfied with what is going on here.

The Ruby PTY doc doesn't really say what the comments in the source code say.

My solution was to put together a wrapper method and to call that from my script where needed. I've also boxed into the method waiting on the process to for sure exit and the accessing of the exit status from $?:

# file: lib/safe_pty.rbrequire 'pty'module SafePty  def self.spawn command, &block    PTY.spawn(command) do |r,w,p|      begin        yield r,w,p      rescue Errno::EIO      ensure        Process.wait p      end    end    $?.exitstatus  endend

This is used basically the same as PTY.spawn:

require 'safe_pty'exit_status = SafePty.spawn(command) do |r,w,pid|  until r.eof? do    logger.debug r.readline  endend#test exit_status for zeroness

I was more than a little frustrated to find out that this is a valid response, as it was completely undocumented on ruby-doc.

It seems valid for Errno::EIO to be raised here (it simply means the child process has finished and closed the stream), so you should expect that and catch it.

For example, see the selected answer in Continuously read from STDOUT of external process in Ruby and http://www.shanison.com/2010/09/11/ptychildexited-exception-and-ptys-exit-status/

BTW, I did some testing. On Ruby 1.8.7 on Ubuntu 10.04, I don't get a error. With Ruby 1.9.3, I do. With JRuby 1.6.4 on Ubuntu in both 1.8 and 1.9 modes, I don't get an error. On OS X, with 1.8.7, 1.9.2 and 1.9.3, I don't get an error. The behavior is obviously dependent on your Ruby version and platform.

ruby-doc.org says this since ruby 1.9:

# The result of read operation when pty slave is closed is platform# dependent.ret = begin        m.gets          # FreeBSD returns nil.      rescue Errno::EIO # GNU/Linux raises EIO.        nil      end

Ok, so now I get this behavior is "normal" on Linux, but that means it's a little tricky to get the output of a PTY. If you do m.read it reads everything and then throws it away and raises Errno::EIO. You really need to read the content chunk by chunk with m.readline. And even then you risk losing the last line if it doesn't end with "\n" for whatever reason. To be extra safe you need to read the content byte by byte with m.read(1)

Additional note about the effect of tty and pty on buffering: it's not the same as STDOUT.sync = true (unbuffered output) in the child process, but rather it triggers line buffering, where output is flushed on "\n"