By now I have spent several days trying to reproduce the behaviour in production environment with debugger attached. Unfortunately, no success. On the other hand yesterday the application froze, again, and colleague today experienced the problem in his script, too. (talking about RHEL)

Dealing with this kind of problem is always very frustrating.

By now, I agree with @pitrou that OSX/RHEL could be two different problems. In advance, I tried the approach by @calimeroteknik and this would actually make sense.

If the child process receives a signal (SIGTERM or SIGSEGV), parent waits forever. We do call 3rd party libraries and segfault is indeed possible. I've tried to send signal to a child and script really froze. By now, it seems to be the most probable explanation.

OSX debugger may also be buggy, yesterday I completely broke my system just by trying my original script, leading to a regular segfaults and system restart (never happened before).

Since I can't reproduce the problem under controlled conditions, I am ok with closing this bug. The script by @calimeroteknik seems to be pointing in the same direction and I think this may solve our problem, too.