Jørgen pointed out that there’s no real
compelling reason to use
IO#popen over a combo of
IO#select. That said,
Open3#popen3 is practically as brain-dead in 1.8 as
IO#popen is. Regardless, I’m not going to let that get in the way of a good cathartic rant (the changes in 1.9 are pretty sweet though).
For better or worse, Lookout has
standardized on Ruby 1.8 in which I’ve discovered one absolutely infuriating
“quirk” with how
IO#popen handles pipe redirection and spawning processes. A
“quirk” that has managed to screw me in more than two projects thus far. While
the issue affects both Ruby 1.8 and Ruby 1.9, there is no work-around in Ruby
1.8, which is especially frustrating.
When you execute
IO#popen, you receive an
IO object which implements some
methods which allow you to read
stdout from the child process. Pretty
reasonable, sort of I guess, except that the new child process inherits the
stderr pipe. From the process table point of view, you will see
exactly what you expect:
3046 pts/7 Ss 0:00 ruby demo.rb 9521 pts/7 R+ 0:00 \_ run_daemon -f some.conf --verbose
Naturally you might want to redirect all the pipes in order to segment
the output of the spawned child process from the parent process. If you want to
separate them, i.e. a
child_stdout and a
child_stderr pipe, then the
has you covered.
What if you want to combine the two pipes (!) for reading inside of your parent process? If for example, you’re wrapping a process and running particular code depending on the collective output of the subprocess.
IO#popen can do this with the magical “2>&1” token in your
def spawn_and_watch @child = IO.popen("run_daemon -f some.conf --verbose 2>&1") Thread.new do while @child.gets |line| do # do magic end end end
So that’s pretty disgusting just from the code standpoint. What’s even more disgusting about this is what happens underneath the hood when you add the “2>&1” token into the command:
3046 pts/7 Ss 0:00 ruby demo.rb 9521 pts/7 R+ 0:00 \_ sh -c run_daemon -f some.conf --verbose 2>&1 9522 pts/7 R+ 0:00 \_ run_daemon -f some.conf --verbose
Subtle isn’t it?
This has wide-reaching consequences for the caller of
IO#popen, the returned
IO object inside of my make believe
spawn_and_watch method has the
pid of the
sh executable. On top of that,
to the best of my knowledge there is no way to actually get a handle on the
run_daemon process ID that is now a grand-child process of
ruby demo.rb. If
I do something as foolish as call:
Process.kill("TERM", @child.pid) then I
will orphan my actual process and not get anything close to my desired
Basically, using “2>&1” with
IO#popen is only good if you want to orphan
processes, and prevent yourself from ever being able to send any signals to the
process you want. This handling of commands passed to
IO#popen with the
“2>&1” token is consistent(ly bad) in both Ruby 1.8 and Ruby 1.9.
Fortunately for those using Ruby 1.9, you can work-around this behavior by changing the above method to look like this:
def spawn_and_watch @child = IO.popen(["run_daemon", "-f", "some.conf", "--verbose", :err => [:child, :out]]) # ... end
It is because of this behavior alone that I have switched some personal projects to default to Ruby 1.9.2 and above, which is nuts.