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I'm refactoring a bit of concurrent processing in my Ruby on Rails server (running on Linux) to use Spawn. Spawn::fork_it documentation claims that forked processes can still be waited on after being detached: https://github.com/tra/spawn/blob/master/lib/spawn.rb (line 186):

# detach from child process (parent may still wait for detached process if they wish)
Process.detach(child)

However, the Ruby Process::detach documentation says you should not do this: http://www.ruby-doc.org/core/classes/Process.html

Some operating systems retain the status of terminated child processes until the parent collects that status (normally using some variant of wait(). If the parent never collects this status, the child stays around as a zombie process. Process::detach prevents this by setting up a separate Ruby thread whose sole job is to reap the status of the process pid when it terminates. Use detach only when you do not intent to explicitly wait for the child to terminate.

Yet Spawn::wait effectively allows you to do just that by wrapping Process::wait. On a side note, I specifically want to use the Process::waitpid2 method to wait on the child processes, instead of using the Spawn::wait method.

Will detach-and-wait not work correctly on Linux? I'm concerned that this may cause a race condition between the detached reaper thread and the waiting parent process, as to who collects the child status first.

Ben Simmons
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1 Answers1

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The answer to this question is there in the documentation. Are you writing code for your own use in a controlled environment? Or to be used widely by third parties? Ruby is written to be widely used by third parties, so their recommendation is to not do something that could fail on "some operating systems". Perhaps the Spawn library is designed primarily for use on Linux machines and tested only on a small subset thereof where this tactic works.

If you're distributing the code you're writing to be used by anyone and everyone, I would take Ruby's approach.

If you control the environment where this code will be run, I would write two tests:

  1. A test that spawns a process, detaches it and then waits for it.
  2. A test that spawns a process and then just waits for it.

Count the failure rate for both and if they are equal (within a margin that you feel is acceptable), go for it!

Lee
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