From: Eric Wong <normalperson@...>
Date: 2012-09-07T02:32:59+09:00
Subject: Re: Passing Sockets (or other FDs) to target of Exec()

Sean Slavik <lists@ruby-forum.com> wrote:
> I've been playing around with an issue that is giving me a bit of a
> headache and I'm hoping someone can point me in a more useful
> direction...
> 
> On a standard startup, I have two variables:
> 
> $nCS = TCPServer.new(HOST, PORT)
> $connected = Array.new

You'll need:

$nCS.close_on_exec = false if $nCS.respond_to?(:close_on_exec=)

for compatibility with Ruby 2.0.0 in the future
Also, you probably don't need $ global variables

> As users connect to $nCS and login, their connection (and user data) is
> passed to $connected.
> 
> I have a command that triggers a function "copyover" that is intended to
> hot-reboot the server, hopefully maintaining the connections. While I've
> tried writing the connections to a file and reloading them that way, the
> closest I seem to have gotten is the following:
> 
> exec "./main.rb", "--copyover", *$nCS, *$connected

You can't just pass an object over the command-line, the
file descriptor number is all you can pass:

exec "./main.rb", "--copyover", $nCS.fileno.to_s, *$connected

> Then passing:
> 
> $nCS = ARGV[1]

You need to convert the file descriptor number back to an object:

$nCS = TCPServer.for_fd(ARGV[1].to_i)

> $connected = ARGV[2]

I think you need to do something similar for $connected, too,
you can't pass over a Ruby array, either, so maybe something
along these lines:

# in the original, stringify a list of integer file descriptors:
# (you'll get something like "4,5,6,7,8")
connected_args = $connected.map do |sock|
  sock.close_on_exec = false if sock.respond_to?(:close_on_exec=)
  sock.fileno.to_s
end.join(",")

exec "./main.rb", "--copyover", $nCS.fileno.to_s, connected_args

# in the exec-ed version:
# parse the stringified list of descriptors:
connected_args = ARGV[2]
connected = connected_args.split(/,/).map do |sockfd|
  # I'm assuming TCPSocket is the correct class based on your use of
  # TCPServer, but any subclass of IO will support for_fd.
  TCPSocket.for_fd(sock_fd)
end

Unrelated to your original problem, you should also check out the
optparse or similar libraries for parsing command-line options.  They
can even handle the split(/,/) part for you.

> Since Exec replaces the process, it should maintain the files until the
> process is destroyed, correct?

Yes.  However, Ruby 2.0.0 will set close_on_exec=true by default,
so you need to set close_on_exec=false to future-proof your code.

> However, this still tells me "ERROR: Caught error in Client Thread:
> Socket is not connected"
> 
> Can anyone point me in the right direction on this? I'm stumped!

Hopefully I didn't make any typos or mistakes in my examples :> But the
basic idea is you can't pass Ruby objects over the command-line (nor can
you marshall IO objects to strings).  Passing stringified file
descriptor numbers is required in your case.

You can also use UNIX sockets with send_io/recv_io, but you still
need to know the class you're recv_io-ing.