Discussion with a sub-process, using Ruby with IO and threading - ruby

Question

I am trying to use IO.popen in order to put (with .puts method) and to get (with .gets method) messages from a process to its sub-process.
I am not very experimented and I have a question about. Having the following code, I have an error because it is not possible to write in a closed stream.
class Interface
def initialize(path)
#sub_process = IO.popen(path, 'w+')
end
def start!
if ok?
#sub_process.puts 'Hello', 'my name is ...'
# and more...
end
end
protected
def ok?
is_ready?(#sub_process) && is_cool?(#sub_process)
end
def is_ready?(sub_process)
reply = process_command(sub_process, 'are u ready?')
reply.chomp.match(/yes_i_am_ready$/)
end
def is_cool?(sub_process)
reply = process_command(sub_process, 'are u cool?')
reply.chomp.match(/yes_i_am_cool$/)
end
def process_command(sub_process, command)
rdr = Thread.new { sub_process.read } # alternative: io.readlines
sub_process.puts "#{command}"
sub_process.close_write
rdr.value # joins and fetches the result
end
end
a = Interface.new("./program")
a.start!
(...) in `write': not opened for writing (IOError)
As we can see, this error occur during is_cool? test (as explained at: http://ruby-doc.org/core/classes/IO.html#M002289).
But if I try to comment in process_command method the line:
# sub_process.close_write
the script seems to sleep... infinitely :s
I believe that it is not possible to open again a closed stream. And I can't create an other IO.popen instance of my program "./program" because it needs to be initialized with some command (like 'are u ready?' and 'are u cool?') at the beginning, before I use it (by sending and receiving messages like a simple discussion).
How changes can I do over the current code in order to solve this problem?
Edit: in other words, I would like to establish a such communication (according to a given protocol):
Parent message: Child answer:
-------------- ------------
'are u ready?' 'yes_i_am_ready'
'are u cool?' 'yes_i_am_cool'
'Hello' 'foo'
'my name is ...' 'bar'
Many thanks for any help.

Answer 1

Perhaps it will help to have a working example. Here's one, tested and known to work in MRI 1.8.7 on Linux.
bar.rb
#!/usr/bin/ruby1.8
begin
loop do
puts "You said: #{gets}"
$stdout.flush
end
rescue Errno::EPIPE
end
foo.rb
#!/usr/bin/ruby1.8
class Parent
def initialize
#pipe = IO.popen(CHILD_COMMAND, 'w+')
end
def talk(message)
#pipe.puts(message)
response = #pipe.gets
if response.nil?
$stderr.puts "Failed: #{CHILD_COMMAND}"
exit(1)
end
response.chomp
end
private
CHILD_COMMAND = './bar.rb'
end
parent = Parent.new
puts parent.talk('blah blah blah')
puts parent.talk('foo bar baz')
foo.rb output
You said: blah blah blah
You said: foo bar baz

Answer 2

A closed IO can not be used anymore. You should not close an IO if you intend on still using it.
If you remove the IO#close_write there still remains the problem with your code in the following line.
rdr = Thread.new { sub_process.read }
IO#read read's until EOF. So until the stream get's closed it never terminates. You mentioned IO#readline in your code, this would be the better alternative. Using IO#readline your program would only hang if the popend process never send's a newline.
Another problem with popen is the following. IO#popen create's a new process. Process's may be killed by you, other users, memory shortages, …. Don't expect your process to always run all the time. If the process is killed IO#readline will throw an EOFError, IO#read will return imidiatley. You can determine the termination reason with the following code.
Process::wait(io.pid)
status= $?
status.class # => Process::Status
status.signaled? # killed by signal?
status.stopsig # the signal which killed it
status.exited # terminated normal
status.exitstatus # the return value
status.ki

Answer 3

Does it help to use this form of Thread.new?
rdr = Thread.new(sub_process) {|x| x.readlines }