From: Caleb Clausen <vikkous@...>
Date: 2010-06-04T04:03:25+09:00
Subject: Re: redirecting stdin for 3. party program

On 6/3/10, Thomas Secret <martin.gerdes@directbox.com> wrote:
> OS: Debian Lenny
> Ruby version: 1.8.7.72-3lenny1
>
> ==Code==
>
> stdin_read,stdin_write=IO.pipe
> pid=fork
> if !pid
>     #$stdin.reopen("/home/admin/.bashrc")
>     stdin_write.close
>     $stdin.reopen(stdin_read)
>
>     a=gets
>     puts "ruby child: got '#{a}'"
>     system("echo bash; mawk '{print \"bash child: got \"$0;exit}'")
>     exit!
> end
>
> stdin_read.close
> sleep 1
> stdin_write.puts "word1"
> sleep 1
> stdin_write.puts "word2"
> sleep 1
> puts "parent: before wait"
> Process.wait(pid)
> puts "parent: after wait"
>
> ==/Code==
>
> =Expected result:=
>
> ruby child: got 'ruby word1
> '
> bash
> bash child: got 'ruby word2
> '
> parent: before wait
> parent: after wait
>
> =Actual result:=
> ruby child: got 'word1
> '
> bash
> parent: before wait
>
> ->after the line 'parent: before wait' has been printed, the program
> hangs.

I've deliberately forgotten all the awk I ever knew, so I modified
your program to use ruby where you had mawk, and it appears to work. I
trust that I have correctly translated the intention of your original
mawk script... Basically, I replaced the system line with:
   system("echo bash; ruby -e 'print \"bash child: got \"+readline;exit'")

And I see this output:
ruby child: got 'word1
'
bash
bash child: got word2
parent: before wait
parent: after wait

With your original mawk line, I see the same behavior you do. I'd
suspect a buffering problem, but inserting a flush after puts of word2
doesn't change anything.

However, when I close the handle stdin_write after writing word2, it
does work, even with mawk. Perhaps mawk is waiting for its stdin to
close before it does anything?