[Bug libc/26371] [RFE] please add clone3() wrapper (in particular the CLONE_INTO_CGROUP feature of it)

["mztyvop at 0pointer dot net" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26371

--- Comment #7 from Lennart Poettering <mztyvop at 0pointer dot net> ---
So we do employ threads, but in very limited fashion. Primarily there are four
uses for threads in PID 1 right now: 

1. We need an async close() sometimes, for fds that people pass to us via D-Bus
and similar, since they can send us arbitrary stuff, i.e. dead NFS or FUSE fd
handles where close() takes ages to finish. For "dangerous" fds like that we
fork off small threads that do nothing but invoke close().

2. Some ioctls (in particular autofs related) are blocking, but we want to
integrate them into our usual event loop. So we run minimal threads for them.
This is used in the .automount support in systemd.

3. When we notice that a shutdown operation is initiated we fork off a thread
that just calls sync() in order to start flushing out buffers early on without
having to wait for it needlessly.

4. When a service shuts down that has RuntimeDirectory= set we do the
equivalent of "rm -rf" in a background thread to get rid of the directory
again, since apparently people sometimes have ridiculously large runtime dirs,
that take ages to clear out. Similar if people have PrivateTmp= set, we empty
out the private /tmp/ and /var/tmp/ subdirs when services go down, the same
way.

In general we try to avoid threads though, and when we have them we try to
minimize them, i.e. they do simple, ideally single operations only. The "rm
-rf" thing is by far the most complex.

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