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welcome to the docker pid 1 zombie reaping problem

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Joey Hess 2015-12-02 12:55:04 -04:00
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[[!comment format=mdwn
username="joey"
subject="""comment 1"""
date="2015-12-02T16:34:03Z"
content="""
This seems to be the same problem discussed here in the context of
git-annex <https://github.com/datalad/datalad/issues/132> and here in the
general context of docker being broken
<blog.phusion.nl/2015/01/20/docker-and-the-pid-1-zombie-reaping-problem/>.
So, it's a docker problem; since docker containers lack
any proper init, nothing waits on orphaned processes. While git-annex
normally waits on all child processes it starts, if a git-annex process
itself exits for some reason (eg an error) while a child process is
running, this results in an orphaned process.
It is the responsibility of init to wait on such orphaned processes. So, eg
systemd-nspawn containers don't have this problem (in addition to their
many, many other benefits over docker). And if you're stuck running docker
all it takes to solve the problem is to make the top-level process in the
container do a `wait()` loop. As is done by eg
<http://github.com/phusion/baseimage-docker>.
It might be possible for git-annex to have a top-level exception
handler that waits on any child processes that might be running. However,
this seems difficult to arrange; when git-annex was talking to a child
process over a handle, the child process may keep running as long as that
handle remains open, so the exception handler would block waiting for the
child process to exit. This kind of difficulty is probably why unix
delegates this job to init in the first place.
"""]]