This is happened for me on both 1.7.20.1 and 1.7.22. (Running OS X 10.10.5 FWIW)

Fortunately, I don't see it with my repro case with JRuby 9.0.0.0. ✨

This is a problem due to our using the JDK process APIs. All processes started via java.lang.ProcessBuilder or java.lang.Runtime work the following way (at least in OpenJDK-based impls):

1. A native call is made to fork+exec the new process
2. All streams are closed in the child. Stdio streams are connected to the other end of pipes from parent.
3. The parent immediately starts a new thread that waits on the child pid.

It's this last step that interferes with use of wait* on JRuby 1.7. Most of these functions can be called exactly once by the parent process (exception for WNOHANG), and most of the time the JDK thread gets to it before we do...often before we even finish the spawn logic and return a pid. This causes your subsequent call to wait* to raise ECHILD.

As documented in the man page:

ERRORS
     The wait() system call will fail and return immediately if:

     [ECHILD]           The calling process has no existing unwaited-for child processes.

The "unwaited-for" bit is what I'm talking about here.

It works on 9k because 9k reimplemented process spawning entirely using native calls, so the pid you get back is not touched by any other threads. This also fixes many issues with inherited streams, redirected streams, and so on.

Unfortunately the 9k logic is extensive and we probably will not be backporting it into the 9k codebase, so I'm going to close this as fixed in 9.0.0.0.

FWIW, a blog post I wrote about this JDK problem a few years ago: http://blog.headius.com/2013/06/the-pain-of-broken-subprocess.html

Great material for a Halloween horror story, where ECHILD popping up out of
nowhere is still not as terrifying as the Java implemention behind it.

(Oversimplifying a POSIX layer will always force the rest of the world to
implement a truckload of ridiculous hacks on top of hacks at every corner -
I'm thrilled to know JRuby went native with this).

Answers a ton of questions - thanks for what you do.
On Sep 4, 2015 2:09 AM, "Charles Oliver Nutter" notifications@github.com
wrote:

FWIW, a blog post I wrote about this JDK problem a few years ago:
http://blog.headius.com/2013/06/the-pain-of-broken-subprocess.html

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#3274 (comment).

Thanks again @headius for the detailed explanation! I learned a lot from it, and echo @e2 that I hugely appreciate what you do with JRuby! ✨